+ Guide · Immunoassays

A few molecules in a drop.

How antibodies, labels and analyzers turn a protein present at a few nanograms per liter into a number a physician can act on, why each new way of reading a bound antibody moved that limit lower, and what a reagent and its analyzer have to demonstrate before they are sold in the US, the EU and India.

+ The central question

How little of one protein can a test find in a drop of blood, and what sets that floor: the antibody, the label, or the instrument that reads it?

Starts at:school chemistry and biology: a protein is a folded chain with a shape, and a concentration is an amount in a volume. No immunology, optics, statistics or regulatory background is assumed.Ends at:working out from a package insert or a datasheet what limits a test's sensitivity and precision, telling a real result from an interference, and planning the evidence a reagent and its analyzer need together. Built around:the high-sensitivity troponin assays that let an emergency department rule out a heart attack within an hour, from a Swiss study published in 2009 to the first US clearance in January 2017; and a second case, the first blood tests cleared to aid the diagnosis of Alzheimer's disease.

A test that could not see troponin in the first three hours left physicians waiting for a second sample, and a test that could see it changed the decision at the door. The difference lay below a few nanograms per liter, a concentration at which a drop of blood holds a few million troponin molecules among roughly ten million billion molecules of albumin alone. That raises the question this guide answers: how little of one protein can a test find in a drop of blood, and what sets that floor: the antibody, the label, or the instrument that reads it?

Some words first. An assay is a procedure that measures how much of a particular substance a sample contains. An immunoassay is an assay in which the part that recognizes the substance is an antibody, a protein the immune system makes to bind one molecular shape. The antibody's binding cannot be seen, so the antibody carries, or calls in, a label: an enzyme that makes color, a molecule that gives off light, a particle that shows as a colored line. Most immunoassays then wash: they rinse away everything that did not bind, while the surface that holds the antibodies keeps what did, so that only label held in place by the target protein is left to be measured.

The short answer runs as follows. The antibody decides what is measured, and how much of it can be caught in the minutes a test allows. The label and the way it is read decide how much signal each caught molecule gives. Color from an enzyme, light from a chemical reaction, fluorescence read after a delay and light switched on at an electrode each raised that signal and cut the light that reaches the detector for other reasons, and between them they took the floor for troponin from about a hundred nanograms per liter in the early 1990s to a few nanograms per liter. Past that point the detector is rarely the limit. The floor becomes the label that stays behind when there is no target to hold it, plus the scatter in counting very few molecules. Assays that count labels one at a time instead of measuring a glow lower the floor again, but they still pay for every label that sticks where it should not. The analyzer's washing, timing, temperature and calibration decide whether that floor holds from run to run and from laboratory to laboratory, which is what made a sensitive assay usable for a decision made within an hour.

Under all of this sits one physical fact. Binding is a balance, not a latch. At every moment some antibody lets go of its target and some other antibody catches it, so the fraction bound depends on concentration and on time. And the label that reports a bond cannot tell the bond it was meant to report from any other surface or molecule it has stuck to. Every chapter that follows is about one side or the other of that fact.

How to read this guide

The 26 chapters form one sequence in eight parts. The first five parts build the answer: how an antibody catches its target, four ways of making a caught molecule visible, the analyzer that runs those chemistries as a machine, the strip that runs them with no machine at all, and the assays that count single molecules. The last three read results, survey the field as of October 2026, and set out what a test system must show regulators. The troponin T assay of the opening scene recurs throughout as the thread's assay, and the blood tests for Alzheimer's disease as a second case. Each chapter opens with the question left by the one before, and each works one example with real numbers. Plates are numbered on their own, so any one can be cited by itself. Four kinds of box recur:

The mathematics stays at arithmetic: multiplying and dividing, percentages, ratios and powers of ten, as in 10⁹ for a billion. Concentrations appear in two forms, as a mass in a volume (nanograms per liter) and as a count of molecules, and Chapter 1 shows how to move between them. Binding, counting statistics and calibration curves are described in words and worked once with numbers; the plates draw the curves. Where a figure is a company's own claim about its product, the text says so. The guide uses US spelling.

Inside the plates, color is a legend and never decoration. The protein being measured is violet; antibodies and other binders are teal; the label, and the color, light or current it produces, is gold; whatever makes the label give its signal, such as exciting light, a voltage or a substrate, is red; and anything that binds or reads as signal without the target is magenta. The measured readout and every number computed from it are always blue, and one orange mark in each plate points at the detail that matters most. A key strip under every plate lists only the colors that plate uses.

The color key

+ Part I · Binding

What an antibody catches, and how fast.

An antibody and its target, mixed in a tube, look exactly like the same tube without them: no color appears, nothing settles, and no instrument pointed at the liquid can tell that a single bond has formed. The five chapters of this part work out what happens out of sight. They put a number on how little protein there is to find, describe the antibody that finds it, show why catching takes time, set out the two ways of turning a catch into a measurement, and end on the signal that appears when there is nothing to catch at all, which is where every immunoassay's floor begins.

+ The questionHow much troponin is in a healthy person's blood, and how does that compare with everything else in it?

Four million molecules in a sample

Five nanograms per liter is the limit of detection reported for the European version of the high-sensitivity troponin T test in the opening scene. A nanogram is a billionth of a gram, so 5 ng/L means that a liter of plasma, the liquid part of blood, holds five billionths of a gram of the protein. A 2021 review by scientists at an antibody supplier puts cardiac troponin T and troponin I in healthy people between about 1 and 50 ng/L, with men's values 1.2 to 2.4 times women's and values rising with age, particularly after 60. Many healthy women sit below 5 ng/L. A test that has to tell a slightly raised value from a normal one therefore works at the very bottom of that range.

A mass per liter hides how few molecules that is. Every protein has a molecular weight, the mass of one mole of it, and a mole is about 6.02 × 10²³ molecules. Human cardiac troponin T is a chain of 298 amino acids weighing 35,924 grams per mole, according to the UniProt protein database. Dividing a mass by the molecular weight gives moles, and multiplying by 6.02 × 10²³ gives molecules. The arithmetic below follows that route for a sample of 50 microliters, a little more than a drop, chosen here as a round example and not taken from any one test's instructions.

Four million sounds like plenty. It is not, for two reasons the rest of the guide develops. The molecules are spread through the whole sample, and only those that meet an antibody in the minutes the test allows can be counted. And the instrument never sees a troponin molecule itself; it sees a label that the antibody brings, and every label that sticks in the wrong place counts as well.

A needle in forty grams of albumin

The troponin shares its liquid with far more of everything else. Albumin, the most abundant protein in plasma, makes up about half of all plasma protein at 35 to 50 grams per liter. Immunoglobulin G, the most common class of antibody in the blood, adds roughly another 12 grams per liter. Fats, sugars, salts and hundreds of other proteins share the same volume.

Set side by side, the gap is wider than any everyday comparison. At 40 g/L of albumin and 5 ng/L of troponin T, there are 8 billion grams of albumin for every gram of troponin. Albumin molecules are heavier, 66,438 grams per mole, so the gap in molecules is a little smaller, about 4.3 billion to one; the same 50 µL that holds 4.2 million troponin molecules holds about 18 million billion albumin molecules. A test that mistook one albumin molecule in a billion for troponin would count about four false molecules for every real one.

01 — Forty grams against five nanograms

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The second case in this guide sits lower still. Phosphorylated tau 217, the protein fragment measured, together with beta-amyloid 1-42, by the first blood test cleared to aid the diagnosis of Alzheimer's disease (Chapter 21), circulates at fractions of a picogram per milliliter. A picogram per milliliter equals a nanogram per liter, so 0.5 pg/mL of tau is a tenth of the troponin concentration by mass. Counted as full-length tau, an assumption because the forms measured are fragments, it is closer to a thirteenth by number: about 260,000 molecules in the 40 µL that one cleared test uses.

Measuring without seeing

No balance weighs a quarter of a picogram, and no microscope picks one protein out of a liquid crowded with billions of similar ones. Every immunoassay therefore does three things in sequence. It catches the target selectively, using antibodies that bind its shape and little else. It makes each caught molecule produce a signal large enough to measure, by attaching a label that gives color, light or current. And it separates that signal from everything else that could produce one: label that never bound, label that bound to the wrong thing, light from the sample itself, and the electronic noise of the detector.

Each step can fail on its own. An antibody that binds too slowly catches only part of the target in nine minutes. A label that gives too little signal per molecule leaves a few caught molecules lost in the detector's noise. And a separation that leaves some unbound label behind creates a signal with no troponin at all. The guide takes the steps in order: catching in Part I, signal in Part II, and the machinery of separation and measurement in Part III.

What "the floor" means

A test's floor is not one number but three, defined by how they are measured. The limit of blank is the highest result expected from samples that contain none of the target, usually set so that only 5 percent of blank results exceed it. The limit of detection is the lowest concentration that reliably gives a result above the limit of blank. The limit of quantitation is the lowest concentration measured with an agreed precision, often a scatter of no more than 20 percent. Chapter 5 shows where each comes from, and Chapter 23 how many measurements it takes to establish them.

For the US version of the thread's assay, cleared in 2017 on two analyzers, the FDA's decision summary gives a limit of blank of 2.5 to 3 ng/L, a limit of detection of 3 to 5 ng/L depending on the analyzer, and a limit of quantitation of 6 ng/L. Its 99th-percentile upper reference limit, the value that only one healthy person in a hundred exceeds, was 19 ng/L overall, 14 ng/L for women and 22 ng/L for men, from 1,301 healthy adults. The rule-out cut-off that the European guidelines use for this assay sits lower still, below 5 ng/L.

02 — Where the thread's assay can see

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The plate shows why the order matters. A test whose limit of detection sat above the 99th percentile could only say "high" or "not high". A test that measures well below the 99th percentile can see a value change between two samples taken an hour apart, even when both are normal, and that change is what the one-hour rule-out in Chapter 20 uses. The floor is therefore not a matter of pride in a datasheet. It decides which clinical questions a test can answer at all.

+ The questionWhat is an antibody, and how can one of them pick a single protein out of thousands?

A Y-shaped protein with two tips

An immunoglobulin G molecule, the kind of antibody used in nearly every immunoassay in this guide, weighs about 146,000 grams per mole and is built from four protein chains: two identical heavy chains of about 50,000 grams per mole each and two identical light chains of about 25,000. The chains fold into a Y. The two arms, each formed by one light chain and the upper half of one heavy chain, are the fragments that bind antigen, and the stem joined to them by a flexible hinge is the part the immune system uses to signal what the antibody has caught. Measured by how it moves through water, the whole molecule behaves like a sphere about 13 nanometers across.

The binding happens at the tip of each arm. There, six loops of the protein chain, three from the heavy chain and three from the light, come together to form a small surface whose shape and chemistry vary from one antibody to the next. The rest of the molecule is nearly the same in every antibody of its class. An immune system can make enormous numbers of different tips because the genes for those loops are assembled from pieces and then mutated as the immune response matures, so that cells making tighter binders are favored.

03 — Two binding sites on one Y-shaped protein

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For an immunoassay the important consequence is that one antibody carries two identical binding sites, and that the binding site is small compared with the antibody itself. The surface of the target that the antibody touches, its epitope, is only a patch of the target, so a protein the size of troponin offers room for several different antibodies to bind at once, each at its own epitope. Chapter 4 shows why that room is what makes the most sensitive format possible.

Recognition is shape and chemistry over a small patch

A survey of 53 crystal structures of antibodies bound to protein targets, published in 2008, found that a typical epitope contains about 20 amino acids of the target and covers about 790 square ångströms, an area about 3 nanometers on a side. Three in four epitopes fell between 15 and 25 amino acids and between 600 and 1,000 square ångströms. Most epitopes on proteins are not a continuous stretch of the chain but residues from different parts of it, brought together by the way the protein folds. And only a fifth to a third of the residues in the antibody's loops actually touch the target.

Binding over such a patch is the sum of many weak contacts: hydrogen bonds, attractions between opposite charges, and the close packing of surfaces that fit. Each contact alone would break apart in an instant at body temperature. Together, when the shapes match, they hold the two molecules for minutes to hours, and a mismatch of a few atoms removes enough contacts to weaken the hold by a large factor. That is the basis of specificity: not a lock that admits one key, but a fit whose strength falls off quickly as the shape departs from the one the antibody was selected against.

The strength of the fit is measured by the dissociation constant, the concentration of target at which half of the antibody's sites are occupied at equilibrium. Smaller is tighter. The founding patent for two-antibody assays with monoclonal antibodies, filed in 1980 by the company Hybritech, required each antibody to bind with an affinity equivalent to a dissociation constant of 10 nanomolar or less, and a 2010 analysis of single-molecule assays assumed antibodies a thousand times tighter, at 10 picomolar. Chapter 3 shows what such numbers mean for how much target an assay can catch.

One clone, one epitope

An animal immunized with a protein makes many different antibodies against it, each from a different line of antibody-producing cells and each binding its own epitope. Serum collected from the animal holds this mixture, a polyclonal antibody. It binds strongly because it attacks many sites, but no two animals make the same mixture, and a supply ends when the animal's serum runs out.

Working at the Medical Research Council's Laboratory of Molecular Biology in Cambridge, Georges Köhler and César Milstein fused antibody-producing cells from an immunized mouse with tumor cells that grow without limit. Each fused cell line, a hybridoma, produced one antibody indefinitely. Their paper reached Nature on 14 May 1975 and was published on 7 August, and the work shared the Nobel Prize in Physiology or Medicine in 1984. A monoclonal antibody binds one epitope, and the same cell line can supply the same antibody for decades. That consistency, more than any gain in strength, is what turned immunoassays into products whose results could be compared from one reagent lot to the next.

Choosing what the antibody sees

Troponin shows how much rests on the choice of epitope. The troponin complex that controls contraction has three proteins, T, I and C, and both heart and skeletal muscle make forms of T and I. The heart's forms differ from the skeletal ones, notably at one end of the chain; human cardiac troponin T has 298 amino acids and cardiac troponin I has 210. An assay for heart damage must bind the cardiac forms and ignore the skeletal ones, which are released by injured muscle elsewhere in the body.

The first generation of the troponin T assay did not fully manage that. Its second generation, described in 1997, replaced one antibody that cross-reacted with skeletal troponin T with a different antibody specific for the cardiac form. The paper reported no cross-reactivity with purified skeletal troponin T at 1,000 micrograms per liter, two hundred thousand times the concentration of interest in Chapter 1, and no false results in 43 marathon runners.

04 — One antibody pair, several circulating forms

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The cardiac forms also do not circulate as one molecule. In the first hours after a heart attack, much of the released troponin travels as the three-protein complex; later, more of it circulates as smaller complexes and fragments cut by enzymes, according to a 2021 review whose authors work at an antibody supplier. Troponin I is mostly bound to troponin C in the blood, and an international task force on cardiac biomarkers has advised that troponin assays should recognize every circulating form equally. In practice each pair of antibodies sees a subset, defined by where its two epitopes sit on the chain and whether those regions survive cutting.

The thread's assay uses two monoclonal antibodies against cardiac troponin T, one made in mouse and one a mouse and human hybrid, each at 2.5 milligrams per liter in its reagent according to the FDA's decision summary. Those amounts make the antibody far more plentiful than the target.

+ The questionIf an antibody grips its target so tightly, why does the time an assay allows change how much of the target it catches?

Binding is a balance

Placed in a small tube with an excess of antibody, each troponin molecule of Chapter 1 is close to thousands of binding sites, the fit is good, and the bond, once made, lasts for minutes or hours. It seems the target should be caught almost at once and kept for good. Neither is true, and the reason governs every incubation time, every wash and every protocol in the analyzers of Part III.

Two molecules bind only when diffusion brings them together in the right orientation, and a bound pair comes apart whenever the jostling of the surrounding water happens to break enough of its weak contacts at once. Both events happen all the time. The rate of binding is the association rate constant: for proteins meeting proteins, antibodies included, it is typically about a million per molar per second, a hundred to a thousand times slower than the limit that pure diffusion would allow, because most collisions arrive at the wrong angle. The rate of coming apart is the dissociation rate constant: for good antibodies, between about one in a thousand and one in a hundred thousand per second.

The ratio of the two is the dissociation constant of Chapter 2, so an antibody that binds at a million per molar per second and lets go at one in ten thousand per second has a dissociation constant of 0.1 nanomolar. The dissociation rate also gives the lifetime of a single bond. Half of a set of bound pairs comes apart in about 0.7 divided by the dissociation rate constant: about 12 minutes at one in a thousand per second, about two hours at one in ten thousand and about 19 hours at one in a hundred thousand. Binding is never permanent; it is a balance that shifts with concentration and time.

How much is caught at equilibrium

When the target is scarce and the antibody plentiful, the fraction of target bound at equilibrium depends only on the antibody concentration measured against the dissociation constant. It equals the antibody concentration divided by the sum of the antibody concentration and the dissociation constant. Roger Ekins, who spent much of his career on the limits of immunoassays, put numbers to it: antibody at a hundredth of the dissociation constant binds about 1 percent of the target, at a tenth about 9 percent, at the dissociation constant itself about half, and at twenty times the dissociation constant more than 90 percent.

Sandwich assays, the format of Chapter 4, are designed to sit at the top of that range: they use far more antibody than the dissociation constant, so that almost all the target can be caught. The thread's assay carries its antibodies at about 17 nanomolar in the reagent (Chapter 2), and several-fold less once sample, reagents and beads are mixed. For an antibody with a dissociation constant of 0.1 nanomolar, even a few nanomolar is many times the constant, and more than 99 percent of the troponin would be bound at equilibrium. The question is how long equilibrium takes to arrive.

How long equilibrium takes

With the antibody in excess, the approach to equilibrium follows a simple rule. The observed rate is the association rate constant times the antibody concentration, plus the dissociation rate constant, and half of the remaining distance to equilibrium is covered in 0.7 divided by that rate. After five such half-times the binding is about 97 percent complete, a rule of thumb from a 2020 guide to measuring binding. The more antibody, the faster the approach, which is a second reason for the large excess.

05 — More antibody, faster capture

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That calculation assumes that every antibody and every troponin molecule are free in a well-mixed liquid. In many immunoassays they are not: one antibody is fixed to a surface, the wall of a plastic well or the surface of a bead, and the target has to travel to it. Then a second clock starts to matter.

Distance is time

A protein the size of troponin T diffuses through water at about 0.0000007 square centimeters per second, a figure scaled from the measured value for albumin. The distance a molecule wanders grows only with the square root of time: about 0.7 millimeter in an hour, and four times that distance takes sixteen times as long. In a standard plastic well holding 100 microliters, the liquid stands about 3.2 millimeters deep. A troponin molecule near the top of an unstirred well needs most of a day to reach antibodies coating the bottom, so in an hour's incubation the antibody on the walls samples only a thin layer of the liquid next to it.

06 — Distance is time

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Laboratory assays in plates answer with long incubations, shaking and warmth. Analyzers answer with geometry. The thread's assay lets its two antibodies find the troponin while all three are dissolved, which is fast because the antibody concentration is high. Beads coated with streptavidin, a bacterial protein that grabs biotin, a small vitamin attached to one of the antibodies, almost irreversibly, then collect the complexes; depending on the analyzer they are added after the first incubation or are present from the start. The beads, a few micrometers across, are spread through the whole reaction volume, so no molecule is ever more than a short distance from one. A 2008 analysis of surface sensors made the general point: transport of the target to the surface "can play as critical a role as the chemical reaction itself".

The Roche analyzers of the thread show the trade in their protocols. Each test takes 9, 18 or 27 minutes, and the troponin T assay comes in a 9-minute version for emergencies and an 18-minute version for routine work. In a table of figures supplied by the makers and compiled by the International Federation of Clinical Chemistry, the 18-minute version on one analyzer family had a limit of detection of 2.05 ng/L and the 9-minute version 2.85 ng/L, while on the newer cobas e 801 both were listed at 3 ng/L. Shorter incubations can cost floor; the size of the cost depends on the whole system.

+ The questionWhy are large proteins measured between two antibodies, while small molecules are measured by competing for one?

A binder nobody wanted to call an antibody

At the Radioisotope Service of the Veterans Administration Hospital in the Bronx, New York, Solomon Berson and Rosalyn Yalow injected insulin labeled with radioactive iodine into patients and followed how fast it left the blood. In people who had been treated with insulin for diabetes, the labeled hormone disappeared much more slowly than in people who had never received it. Electrophoresis of the plasma, which separates proteins by how they move in an electric field, showed why: the labeled insulin was riding on a globulin, a class of blood protein, that bound it. The binder appeared only in patients who had been injected with insulin, and it was present in every patient treated for three months or more.

The two researchers concluded, in the body of their paper, that the binder met a textbook definition of an antibody, and they called it an "insulin-transporting antibody". The immunologists of the time did not accept that insulin could provoke antibodies at all. Science rejected the paper, and the Journal of Clinical Investigation rejected it at first and accepted it, on 17 October 1955, only after the title was changed to speak of an "insulin binding globulin". Yalow reproduced the rejection letter in her Nobel lecture of 8 December 1977. Berson had died in 1972, and Yalow received half of that year's Nobel Prize in Physiology or Medicine, for the development of radioimmunoassays of peptide hormones.

Competition: measuring what the sample displaces

The binding that the editors doubted became a measuring method. In a letter to Nature published on 21 November 1959, and in a full paper in July 1960, Yalow and Berson used serum from guinea pigs immunized with beef insulin to measure insulin in human plasma. Their mixtures held a fixed, small amount of antibody, a trace of insulin labeled with radioactive iodine at about 0.05 to 0.15 nanogram per milliliter, and the patient's plasma diluted tenfold. Over four days at 4 °C, the unlabeled insulin from the plasma and the labeled insulin competed for the same limited binding sites. The researchers then separated bound from free insulin on strips of paper, by a combination of chromatography and electrophoresis, and counted the radioactivity in each.

The more insulin the plasma contained, the less of the labeled insulin found a site, and the lower the ratio of bound to free label. Comparing that ratio with the ratios given by known amounts of insulin gave the patient's concentration. This is a competitive immunoassay: the label is a labeled copy of the target, the antibody is scarce on purpose, and the signal falls as the concentration rises.

07 — Insulin from plasma pushes labeled insulin off the antibody

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Competition has a structural weakness. When the sample holds no target at all, the bound signal is at its largest, since the labeled copies meet no competition for the limited sites. A small amount of target shows up as a small decrease in a large signal, and a small decrease is hard to tell from the ordinary scatter of a large count. A competitive assay can be made more sensitive by using less antibody and less label, but only up to the point where the count becomes too small to measure precisely.

The sandwich: two antibodies, two epitopes

The alternative puts the label on the antibody instead of on a copy of the target. In 1968 L. E. M. Miles and C. N. Hales at the University of Cambridge described immunoassays with labeled antibodies, and the form that dominates today followed: a sandwich, or two-site, assay. One antibody, the capture antibody, is fixed to a solid surface or carries a tag that a surface will grab. A second antibody, carrying the label, binds a different epitope on the same target. Only a target molecule bound by both holds the label to the surface, so the signal rises with concentration and, ideally, starts from zero.

Monoclonal antibodies made the sandwich practical. A patent filed by Hybritech in August 1980 and granted on 8 March 1983 claimed two-site assays built from two monoclonal antibodies against different sites, each binding with an affinity of at least about 10⁸ liters per mole. Polyclonal reagents could be used for sandwiches too, but they had to be purified laboriously to make a capture and a detection antibody that did not compete for the same sites.

A sandwich needs two epitopes far enough apart for two antibodies, each about 13 nanometers across, to bind at once. Proteins offer that room. Small molecules, such as drugs, steroids and thyroid hormones with masses below about 1,000 grams per mole, do not, which is why they are still measured by competition. A 1998 patent from the immunoassay maker Chiron Diagnostics put the difference in numbers: the most sensitive sandwich assays then in use detected several million molecules of thyroid-stimulating hormone per test cuvette, against several billion molecules for the most sensitive competitive assays for digoxin and the thyroid hormone T3, a gap of about a thousandfold.

08 — Competition starts high; a sandwich starts near zero

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The thread's format, and how it began

Troponin T, a protein of 298 amino acids, has been measured by sandwich from the start. In 1992 Hugo Katus and colleagues, together with staff of the manufacturer Boehringer Mannheim, described that company's troponin T immunoassay. It used two monoclonal antibodies: a capture antibody carrying biotin, held by streptavidin coated on the inside of a plastic tube, and a detection antibody carrying an enzyme. It ran in 90 minutes at room temperature and measured from 0.1 to 15 micrograms per liter.

The 2017 assay of the opening keeps the same architecture. Its capture antibody still carries biotin, its beads are coated with streptavidin, and its second antibody still carries the label, now a ruthenium compound read by the method of Chapter 9. It runs in 9 minutes and quantifies from 6 ng/L. The lower end of the 1992 range, 0.1 micrograms per liter, is 100 ng/L, so in 25 years the bottom of the reported range fell about seventeenfold while the time fell tenfold, with the same two-antibody format throughout.

One property of the one-step sandwich, in which both antibodies meet the sample together, needs a name here because later chapters return to it. At very high target concentrations, free target molecules can occupy both the capture and the labeled antibodies separately, so fewer complexes carry both, and the signal falls instead of rising. This high-dose hook can make an enormous concentration read as a modest one. Chapter 13 shows how analyzers detect it, and Chapter 19 what it does to a patient's result.

+ The questionWhen a sample holds none of the target protein, why does the instrument still read a signal, and why is that reading the real floor?

The blank that is not blank

A sandwich assay kit for prostate-specific antigen, made by the Institute of Isotopes in Budapest with a radioactive label on its detection antibody, prints an example calibration in its maker's instructions, last updated in 2020. Each tube receives 25 microliters of sample and labeled antibody giving 392,944 counts of radioactivity a minute. After the incubation and two washes, the tube with a standard containing no antigen at all still holds 292 counts. The tube with 0.1 nanogram per milliliter holds 630. The difference made by the antigen, 338 counts, is about the same size as the signal from nothing.

That zero standard is the part of the measurement that every immunoassay fights. It is not a fault of that kit. A label attached to an antibody does not know whether the antibody is holding its target; it reports its position, and some labeled antibodies end up on the surface without any target to hold them there. The 292 counts, which include the counter's own background, are 0.074 percent of the label added: a small fraction, but a fraction of a very large number. And at the bottom of the range, the specific signal is no larger than this residue.

Separating bound from free: the wash

Every sandwich and competitive assay described so far has a step in which bound label is separated from free label. Yalow and Berson did it on paper strips; most assays since have done it with a solid phase, a surface to which the capture antibody is fixed: the inside of a plastic tube or well, or the surface of beads. Once the target has bound to the capture antibody and the labeled antibody has bound to the target, the liquid is drawn off and replaced with a clean buffer, once or several times. That rinse is the wash. What the surface holds stays; what floats free leaves with the liquid.

Washing is a dilution, not a filter. Each rinse removes most of the free label that remains, and the residue after several rinses depends on how much liquid clings to the surface between them, how long each takes and how well the liquid mixes. Beads add one more trick: a magnet holds them against the wall of the vessel, or against an electrode, while the liquid around them is exchanged. Lateral flow strips (Chapter 15) wash by flow, with the sample stream itself carrying unbound label past the capture line. A few designs, described in Chapter 8, avoid washing altogether by making the label give a different signal when it sits next to its partner.

09 — What a wash removes, and what it cannot

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Why label sticks where it should not

A wash removes most free label, never all of it, and none of the label that something holds. In a blank, two things leave label behind. Labeled antibody adsorbs directly onto the plastic or the bead, because proteins stick to most surfaces. And a small amount of label stays in the liquid film that clings after the last rinse. In a patient's sample a third can join them: a molecule such as a human antibody that recognizes the animal antibodies used as reagents can hold labeled antibody to the capture antibody with no target at all, a sample-specific error that Chapter 19 treats.

Assay makers counter the first with blockers: inert proteins such as bovine serum albumin or milk proteins, at a few percent, coat any surface the capture antibody left bare, and mild detergents such as Tween 20, at a tenth of a percent or so, discourage loose adsorption. The second is countered by adding animal immunoglobulin that soaks up the interfering human antibodies. A 1998 patent from Chiron Diagnostics stated the general principle: in a labeled-antibody assay, sensitivity improves by reducing the nonspecific adsorption of the labeled antibody "to the solid phase and the reaction vessel".

The consequence for design is the central claim of this guide's answer. Making each label brighter multiplies the signal from target-bound label and the signal from wrongly bound label by the same factor. The ratio between them does not change. A brighter label lowers the floor only while some other source of noise, such as light reaching the detector from elsewhere or the detector's own electrical noise, is larger than the zero signal. Once the zero signal dominates, only less wrongly bound label, or a steadier count of it, lowers the floor further.

The limit of blank, and the floor it sets

The zero signal is not one number but a spread. Measured many times, blank samples give a range of results, because the counts scatter, the wash leaves slightly different residues and the instrument drifts. A result just above the average blank therefore does not prove that any target was present. The Clinical and Laboratory Standards Institute publishes a guideline on detection limits, EP17, that is widely used in laboratory medicine and recognized by the FDA. It defines the limit of blank as the result that blanks exceed only 5 percent of the time: in its simplest form, the mean of the blanks plus 1.645 times their standard deviation, the usual measure of scatter. The limit of detection is then the concentration whose results exceed the limit of blank 95 percent of the time, found by adding 1.645 times the scatter of low-level samples.

10 — From blank scatter to a limit of detection

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For the thread's assay, the FDA's 2017 decision summary reports how the limit of blank was set: 60 results from blank samples for each reagent lot, run over at least three days on two analyzers of each type, gave a limit of blank of 2.5 ng/L on one analyzer and 3 ng/L on the other. The limit of detection was 3 and 5 ng/L. The limit of quantitation, 6 ng/L, was set where the scatter fell to 20 percent.

The isotope kit shows how much the definition matters. Its instructions give an older-style "sensitivity" of 0.02 ng/mL, the mean zero standard plus two standard deviations from 20 repeats, and also an EP17 limit of blank of 0.035, a limit of detection of 0.09 and a limit of quantitation of 0.12 ng/mL. All four describe the same tubes.

+ Part II · Making a bond visible

Four ways to read a label.

In 1971 two groups, one at Stockholm University and one in the Netherlands, independently published ways to replace the radioactive label of the first immunoassays with an enzyme, a catalyst that keeps working for as long as it has something to work on. Their idea opened a sequence of labels, each giving more signal per bound molecule against less unwanted light. This part follows four of them in order: an enzyme read by the color it makes, molecules that give off light in a chemical reaction, labels that glow long after a flash of exciting light, and a label switched on by a voltage at an electrode, the method of the thread's assay. By its end, the floor of Chapter 5 has a precise shape: a label's light against everything else that reaches the detector.

+ The questionHow does an enzyme turn a few bound antibodies into a color a photometer can read, and where does color run out?

An enzyme as an amplifier

A single molecule of horseradish peroxidase, the enzyme most often attached to antibodies in color-based immunoassays, can convert a few hundred to a few thousand molecules of a colorless substrate into colored product every second. Measurements put the rate at about 240 per second near neutral pH and room temperature, while figures more than ten times higher are often quoted for the acidic, warmer conditions in which the enzyme works fastest. Over a half-hour incubation, even the lower rate gives each enzyme molecule some 400,000 colored molecules to its name.

That multiplication is why enzymes replaced radioactivity. A radioactive iodine atom on an antibody gives at most one decay, and an iodine-125 label decays slowly, so in a counting time of a minute only a small fraction of the labels on a tube report at all. Radioactive labels also needed shielding, licenses and disposal, and they lost activity on the shelf. An enzyme label, by contrast, is stable, safe, and keeps adding signal for as long as substrate remains. The signal of each bound antibody is no longer one event but a pile of product that grows with time.

The pile has a price. The enzyme does not know whether it is attached to an antibody that holds a target or to one stuck on the plastic, and both make product at the same rate. Enzyme amplification raises the signal from wrongly bound label exactly as much as the signal from target, which is the argument of Chapter 5 in its first practical form.

The first ELISA

The enzyme-linked immunosorbent assay, ELISA, was first worked out by Eva Engvall, then a graduate student of Peter Perlmann at Stockholm University. In a retrospective published in 2010, she recalled obtaining the first calibration curve early in 1970. The assay was a competitive one, adapted from a standard radioimmunoassay, with antibodies coupled to particles of cellulose; bound and free were separated by repeated centrifugation and washing. The enzyme was alkaline phosphatase, chosen because it had a soluble and sensitive substrate whose product could be measured. The target was rabbit immunoglobulin G. "We were continually asked why anyone would want to measure rabbit IgG!" she wrote.

The paper appeared in 1971 in the journal Immunochemistry. In the Netherlands, B. K. van Weemen and A. H. W. M. Schuurs published an enzyme immunoassay the same year, and a 2005 history describes the two methods as developed independently and simultaneously. The form most laboratories now picture under the name, a sandwich assay in the wells of a plastic plate, came later, as plates, monoclonal antibodies and plate readers arrived. The principle did not change: catch, wash, add substrate, and measure the color.

11 — Catch, wash, add substrate, measure the color

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Reading color: absorbance

A colored solution absorbs part of the light shone through it. A photometer, or a plate reader that holds many wells at once, measures how much light of one wavelength passes through the well and how much passed through a blank, and reports the absorbance, a logarithmic measure of the loss. An absorbance of 1 means a tenth of the light got through, 2 means a hundredth, and 3 a thousandth. Absorbance is proportional to the concentration of the colored molecule, to the length of liquid the light crosses, and to how strongly that molecule absorbs at that wavelength, its molar absorptivity.

The common peroxidase substrate, tetramethylbenzidine, turns blue as the enzyme works, and yellow when the reaction is stopped with acid. Its yellow product absorbs at 450 nanometers with a molar absorptivity of about 59,000 per molar per centimeter, high for a small molecule. The product of alkaline phosphatase, para-nitrophenol, absorbs at about 400 nanometers with 18,380 per molar per centimeter, a value measured carefully enough in 1980 to serve as a reference material. Plate readers made for this work resolve a thousandth of an absorbance unit and are specified, by their makers, to repeat readings within about half a percent plus 0.005 units.

Laboratory sandwich ELISAs reach low concentrations by putting more sample in, waiting longer and catching more of the target. One commercial kit for the inflammatory protein interleukin 6, according to its maker, reached a sensitivity of 0.7 picogram per milliliter with 100 microliters of sample in an assay that took four and a half hours. Analyzers, which must answer in minutes, cannot buy sensitivity that way.

Where color runs out

Color fails at both ends. At the low end, a trace of product changes the light leaving the well by a small fraction of a large beam. The reader has to measure that beam twice, through the well and through a blank, and subtract; any flicker of the lamp, any scratch on the plastic and any bubble enters both measurements and survives the subtraction as noise. The enzyme stuck on the plastic makes its own color in the meantime. At the high end, absorbance stops following concentration. Readers report up to 4 absorbance units, but their makers specify linearity only to about 2 or 3, because very little light gets through a dark well and stray light inside the reader starts to dominate.

12 — Color between two walls

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Between a noise floor near 0.005 and a ceiling near 2 lies a factor of about 400, less than three factors of ten. A sample whose concentration falls outside that window must be diluted and run again, or read at a different time. The light-emitting labels of the next three chapters attack both ends at once, by measuring light where there should be darkness instead of a dip in a bright beam.

+ The questionWhy does light from a chemical reaction reach lower concentrations than color does?

A measurement with no lamp

An absorbance reader cannot work in the dark: it needs a lamp, and it reports the target as a small shadow in the lamp's light. If instead the label itself makes the light, and the detector sits in a light-tight box, the measurement changes character. With no label present, the detector sees nothing but its own small electrical noise. Every photon that arrives was made by a reaction in the tube, and the instrument counts upward from near zero instead of subtracting one large reading from another.

That is chemiluminescence: a chemical reaction whose product is formed in an excited state and releases its energy as light. Fireflies and some deep-sea animals do it with enzymes; immunoassays do it with small organic molecules. The gain over color is not that each label gives more signal, though it often does, but that the signal is measured against darkness. A shadow of 0.005 absorbance units is a change of about 1 percent in a bright beam; a light of a few hundred photons a second, against a detector that registers a few dozen in the dark, is unmistakable.

The label still has to be attached to an antibody, still has to be separated from free label by washing, and still counts when it sticks where it should not. What changes is how small a quantity of label the instrument can see, and over how wide a range. Two families of chemistry dominate analyzers today: labels that flash once, and enzymes that keep glowing.

Flash: one label, one burst

In 1983 Ian Weeks and colleagues described acridinium esters as labels for immunoassays. Attached to a monoclonal antibody at up to about 2.8 labels per antibody, the ester could be detected at about 0.8 attomole, the paper reported, which is roughly 480,000 label molecules, in a two-site assay that incubated for 30 minutes and read each sample in 5 seconds.

The light comes when the ester meets hydrogen peroxide in alkali. In the scheme used on one maker's analyzers, the reagents are added in two steps: an acidic peroxide solution and then sodium hydroxide, with a detergent that speeds and strengthens the emission. A 2014 study by chemists at Siemens Healthcare Diagnostics, which uses these labels, found that with the detergent at least 95 percent of the light from four esters arrived within 1.2 to 5.3 seconds, against up to a minute without it. Each acridinium molecule reacts once and gives at most one photon, so the measurement is a race: the trigger is injected in front of the detector, and the whole flash is counted in a few seconds.

Abbott's ARCHITECT and Alinity analyzers use acridinium-labeled antibodies on paramagnetic microparticles, a method the company calls chemiluminescent microparticle immunoassay, and Siemens Healthineers' Atellica and ADVIA Centaur analyzers use acridinium esters too, according to FDA decision summaries and the makers' own pages.

Glow: enzymes that make light

The other family keeps the enzyme of Chapter 6 and changes its substrate. In 1983 researchers in Birmingham reported that firefly luciferin multiplied the light from luminol oxidized by horseradish peroxidase, and in 1985 the same group found phenol compounds that raised it more than a thousandfold and kept it glowing for several minutes. Alkaline phosphatase gained light-producing substrates in the late 1980s, molecules called 1,2-dioxetanes: the enzyme removes a phosphate group, and the unstable remainder falls apart and emits light. A 1991 review gave the detection limit for alkaline phosphatase with such a substrate as 0.001 attomole, about 600 enzyme molecules, with emission lasting more than 30 minutes.

Here the enzyme's turnover returns as an amplifier of light. Each enzyme keeps converting substrate molecules for minutes, each of which can give a photon, so a few hundred enzymes can be seen. Fujirebio's Lumipulse analyzers use alkaline phosphatase with the dioxetane substrate AMPPD, whose light peaks at 477 nanometers, according to the FDA's 2025 decision summary for the second case of this guide. QuidelOrtho's VITROS analyzers measure peroxidase with luminol.

13 — One burst, or a glow that keeps building

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Glow has costs of its own. The light rises over minutes, so the analyzer must wait before reading, and that wait sets a minimum time per test. Beckman Coulter's older substrate for its Access analyzers needed about 6.3 minutes to generate its signal, according to a 2019 poster by the company and its substrate subsidiary. Its DxI 9000 analyzer uses a newer acridan-based substrate that gives its light in about a minute, and by the company's account the time to the first troponin result fell from 17 to 11 minutes.

What light changes, and what it does not

Measuring light against darkness widens the range as well as lowering the floor. A detector that counts single photons can register a few dozen per second in the dark and millions per second before it saturates (Chapter 13), five or more factors of ten, against less than three for absorbance. A sample can then be measured from near the floor to far above the clinical decision point without dilution, which saves reruns and reagent.

14 — A shadow in a bright beam, or light in the dark

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What light does not change is the label that stays without a target. An acridinium ester stuck to the wall of a cuvette flashes as brightly as one on a sandwich, and an enzyme on the plastic glows as long as one on a bead. Chemiluminescence removed the lamp, the scratches and the bubbles from the noise of Chapter 6, so the floor that remains is much closer to the one set by wrongly bound label. The next two chapters remove more of the remaining unwanted light, one by waiting and one by moving the light source to an electrode.

+ The questionFluorescence works well enough to count single cells in a flow cytometer; why did immunoassays have to wait before looking at it?

Light in, light out, and light everywhere

A europium ion held in an organic chelate, whose ligand absorbs ultraviolet light near 340 nanometers and passes the energy to the ion, gives back red light near 615 nanometers, and it keeps giving it back for close to a millisecond after the exciting flash has ended. That last property, unusual among labels, is the subject of this chapter, because it solved a problem that ordinary fluorescence could not.

Fluorescence is the obvious way to read a label. A dye absorbs light of one color and returns light of a longer wavelength, and filters separate the two, as the flow cytometry guide in this library explains in detail. But an immunoassay reads its label in a well or a cuvette full of serum, plastic and buffer, not in a narrow stream past a focused laser. The exciting light scatters off the plastic and the liquid. Serum contains proteins and other molecules that fluoresce on their own. And some of the exciting light leaks through any filter. All of that unwanted light arrives together with the label's light, from the moment the lamp fires.

What separates them is time. The unwanted fluorescence and the scattered light die away in about 10 nanoseconds, according to the documentation of one time-resolved fluorescence system. Ordinary fluorescent dyes fade just as fast, so their light and the background arrive together and cannot be told apart by timing. A label whose glow lasts tens of thousands of times longer can be read after the background has gone.

A label that glows for a millisecond

The lanthanides, a row of metals at the bottom of the periodic table, have electrons whose transitions are slow by atomic standards. Europium, samarium and terbium, wrapped in a molecule that absorbs light and passes the energy on, emit sharp lines with decay times of tens of microseconds to about a millisecond. In 1983 E. Soini and H. Kojola described a time-resolved fluorometer for such labels, built around an ordinary xenon flash lamp, whose one-second measurements matched the sensitivity of the radioactive methods then in use.

DELFIA, a time-resolved method now sold by Revvity, shows the timing in its user guide: an excitation flash, a wait of 400 microseconds, a counting window of 400 microseconds, and a thousand such cycles a second. Before reading, an acidic enhancement solution pulls the europium out of the label and into a new, brightly fluorescent complex, which the maker says multiplies the fluorescence by one to ten million times.

15 — Wait, then count

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The gain is the same in kind as chemiluminescence's: the detector reads its label against near-darkness. But the darkness here is made by timing, with a lamp in the instrument, and so the lamp's own afterglow, the plate's phosphorescence and the dark counts of the detector set the optical background; in an assay, labeled antibody stuck without target, and stray europium, usually set the floor. The DELFIA guide gives typical plate backgrounds of 100 to 1,000 counts, depending on the plate.

Reading without washing: energy transfer

Long-lived labels allowed a further step, one of the few ways around the wash that Chapter 5 mentioned: can bound and free label be told apart without washing? In 1993 a chemist at the French company CIS bio international described europium held in a cage-like molecule called a cryptate, used as an energy donor on one antibody. A second antibody carries an acceptor molecule. When both antibodies bind the same target molecule, donor and acceptor sit a few nanometers apart, and the europium hands its energy to the acceptor, which then emits at its own wavelength with a delay borrowed from the europium. Free antibodies, scattered through the liquid, are too far apart for the transfer.

Thermo Fisher Scientific's B·R·A·H·M·S KRYPTOR analyzers use this method under the name TRACE. According to the company's white paper, a laser pulse at 337 nanometers excites the europium, the instrument waits 50 microseconds, and it measures light at 665 nanometers, from the acceptor, and at 620 nanometers, from the europium itself. The ratio of the two corrects for a cloudy or colored sample, and the paper states that no washing or separation step is needed. The 1993 paper measured prolactin down to 0.3 microgram per liter in human serum.

16 — Light from the acceptor only when two antibodies meet

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Outside the central laboratory

Long-lived labels suit small instruments, because the timing that rejects background does not depend on a dark, precisely built optical chamber as much as on a fast switch between flash and count. Radiometer's AQT90 FLEX, cleared by the FDA in 2017 for two cardiac tests, reads a europium chelate by time-resolved fluorescence with single-photon counting, in a test cup that holds the antibodies, according to the FDA's decision summary; the maker lists troponin among its tests and gives results in 11 to 21 minutes. Fluorescence can also read an enzyme instead of a lanthanide. bioMérieux's VIDAS analyzers use what the company calls an enzyme-linked fluorescent assay: an enzyme on the detection antibody turns a substrate into a fluorescent product, so the amplification of Chapter 6 is read as light rather than color. The company lists a high-sensitivity troponin I test on these analyzers that takes 20 minutes and covers 1.5 to 40,000 pg/mL, a span of more than four factors of ten.

Fluorescent beads carry a different idea: Luminex's bead arrays, described in 1997 by authors from the company, color-code tiny beads so that dozens of assays run in one sample and are read in a flow cytometer, a method the flow cytometry guide in this library covers.

+ The questionWhat does starting the light with a voltage gain over starting it with a reagent?

A label the electrode re-arms

On 12 June 1996 the FDA cleared an assay for the pregnancy hormone hCG submitted by Boehringer Mannheim, one of the first tests for an analyzer the company called Elecsys. Its label was a metal complex that gives off light only while an electric voltage drives a reaction at an electrode, a method called electrochemiluminescence. The technology came from IGEN, a company in Maryland that had licensed it in 1992 to the business that Roche later acquired, and twenty years on it reads the troponin T assay of this guide's opening.

The label is a ruthenium atom held by three ring-shaped organic molecules, a complex that weighs about a thousand grams per mole, small enough to attach several to one antibody. In the measuring solution it sits with a large excess of a second chemical, tripropylamine, called the coreactant. When the electrode is raised to a positive voltage, it takes electrons from the tripropylamine, which breaks into short-lived, highly reactive fragments. In the sequence described by a 2020 study, one kind of fragment hands the ruthenium complex an extra electron and another then takes an electron away, which leaves the complex in an excited state. It relaxes by emitting a photon of orange-red light near 620 nanometers and returns to exactly the form it started in.

That return is the point of the method. An acridinium ester reacts once and is spent; a ruthenium label, after emitting, is ready to go round again, as long as the electrode keeps the voltage on and fresh coreactant reaches it. The 2020 study of the mechanism, carried out by university chemists with researchers from Roche Diagnostics and Hitachi High-Tech, set out the reaction steps and applied 1.4 volts to drive them. Roche's own patent on the measuring method describes the label reaction as preferably circular, so that one label emits "a plurality of photons".

The measuring cell

The light is made in a small flow cell, not in the reaction vessel. In the arrangement described in a patent by Roche Diagnostics and Hitachi High-Technologies, the reaction mixture, holding streptavidin-coated magnetic beads typically 2 to 3 micrometers across, is drawn into the cell over a working electrode. A permanent magnet under the electrode pulls the beads, with the sandwiches they carry, onto the electrode surface. A buffer containing the coreactant then flows through, washing away unbound label. The voltage is applied, a photomultiplier tube looks through the electrode assembly and records the light for about two seconds, and the magnet is moved away so that the beads can be flushed out and the cell cleaned for the next test.

17 — Beads held on an electrode, read through it

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The thread's assay follows this sequence. According to the FDA's 2017 decision summary, a biotin-labeled antibody and a ruthenium-labeled antibody form a sandwich around troponin T, streptavidin-coated microparticles collect the sandwich, the microparticles are captured magnetically on the electrode, and a voltage stimulates the emission that a photomultiplier measures. The cobas e 801 analyzer that now runs the assay holds its measuring cells at 28 °C, separate from the 37 °C of its incubator, according to its maker's specifications.

Why the thin layer matters

The reactive fragments that excite the ruthenium last only a fraction of a millisecond. The 2020 study estimated a half-life of about 200 microseconds and concluded that they are not expected to travel more than about 3 micrometers from the electrode before they decay. Light is made only in that thin layer.

18 — A label the electrode re-arms

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Three consequences follow. The method adds a second separation on top of the wash: label that is not on a bead held against the electrode contributes little light, whatever the wash left behind. There is no exciting light, so there is no scatter and no autofluorescence to subtract, as in chemiluminescence. And because each label can emit many photons during the read, the signal per label is larger than a single flash would give. A 1991 paper by IGEN's scientists reported that electrochemiluminescent labels could be measured over six factors of ten, with a detection limit of 200 femtomolar for the label.

The same 2020 study measured how much light depends on geometry: beads of 0.3 micrometer gave about eight times more light than beads of 2.8 micrometers, consistent with more of their labels sitting close to the electrode, and a new coreactant raised the signal of commercial cobas e 801 assays by up to 47 percent. Neither change is a statement about any current product; both show that the electrode's few micrometers are a design variable.

The thread, measured

The steps are consistent with the 2017 assay's numbers, though its fall from the fourth generation's 10 ng/L came within the same detection method, from changes to the assay itself. Those numbers are a 9-minute protocol, a limit of blank of 2.5 ng/L and a measuring range up to 10,000 ng/L, more than three factors of ten above its limit of quantitation without dilution. Other companies use electrochemiluminescence differently. Meso Scale Discovery builds carbon electrodes into the bottoms of plastic plates, so that a research laboratory can read several spots, each with its own antibody, in one well; only labels near the electrode surface are detected, according to the company.

+ Part III · The analyzer as a system

Holding the floor steady, test after test.

One cobas e 801 module, the analyzer that now runs the thread's assay in many hospital laboratories, can report up to 300 immunoassay results an hour, according to its maker, every one of them from a reaction that must be timed, warmed, washed and read exactly like the calibrators it is compared with. The chemistry of Parts I and II sets how low a floor can be. This part is about the machine that has to reach that floor on every test: how a tube becomes a result, how liquids are moved without carrying one sample into the next, why minutes and tenths of a degree matter, how light becomes a number, and how the number is tied to a scale that another laboratory can share.

+ The questionWhat has to happen, and in what order, between a tube of blood and a reported number?

A tube's path through the machine

A blood tube arriving at an analyzer carries a barcode, and the barcode is the first input the machine reads. From it the analyzer learns, through the laboratory's information system, which tests have been ordered on that sample. A sampling arm then fits a fresh disposable tip, finds the surface of the liquid, draws up a few microliters of plasma while sensors watch for clots, bubbles and foam, and dispenses it into a single-use reaction cup. On a cobas e 801 the sample volume per test can be set between 4 and 60 microliters, and every pipetting uses a new tip, according to the maker's specifications.

Reagents come next. Each test's antibodies, beads and buffers sit in packs in a refrigerated store on board, at 5 to 10 °C on the e 801, which holds up to 48 of them and identifies each by a radio-frequency tag. A reagent arm adds the right volumes to the cup, a mixer stirs it, and the cup moves into an incubator held at 37 °C, plus or minus 0.3. At the end of the incubation the mixture goes to the measuring cell of Chapter 9, where the beads are captured, washed and read. The analyzer's software turns the light into a concentration with the calibration curve, checks the result against flags and limits, and sends it to the laboratory system.

19 — One tube's path through the analyzer

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Each of those steps belongs to a subsystem that an engineer can name: sample handling, liquid handling, reagent storage, incubation, separation, detection, and the control software that schedules them. None of them is clever on its own. The design problem is that all of them must act on each test at exactly the right moment, the same way for every test and for every calibrator, while dozens of other tests are at other stages in the same machine.

Many tests in flight

An 18-minute assay cannot be run one test at a time if the analyzer is to report hundreds of results an hour. The answer is a pipeline. On the e 801, a new cup starts every 12 seconds, so 300 tests can begin each hour. Each spends its incubation in one of 94 positions on the incubator, then leaves for the measuring cell as the next cup arrives.

20 — A new test every 12 seconds

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The same arithmetic shows where the limits lie. On the maker's own numbers, a run made only of 27-minute assays would need 135 positions at full speed, more than the incubator holds, so such a mix must slow down. For its smaller cobas e 402, the maker states that throughput "may differ based on the mix of test orders per sample". Consumables impose another limit: the e 801 holds 15 magazines of 105 tips and 105 cups, 1,575 tests, which at 300 an hour is the 5 hours 15 minutes of unattended running that its specifications list.

The conversion chain

An analyzer never measures troponin. It measures something several steps removed and converts it back. Troponin molecules in the sample become sandwiches on beads, and the sandwiches put labels within a few micrometers of an electrode. The labels emit photons, a fraction of the photons release electrons in the detector, and the electrons become a count or a current. The count becomes a signal value for that test, the calibration curve turns the signal into a concentration, and software compares the concentration with limits and flags before it becomes a reported number.

21 — From troponin to a reported number

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Every arrow in that chain can change the result without any change in the troponin. A pipetting error changes how many molecules enter the cup; a warm incubator changes how many become sandwiches in nine minutes; a fouled electrode changes how many photons each label gives; a drifting detector changes the count; an out-of-date calibration changes the conversion. The rest of this part takes the arrows in turn, and the quality controls of Chapter 14 exist because the chain cannot be watched link by link on every test.

Other makers, the same skeleton

Other analyzers make different choices on the same skeleton. Abbott's ARCHITECT i2000SR, rated by its maker at up to 200 tests an hour, draws samples with a fixed probe that is washed between samples and specified to carry over no more than 0.1 part per million; its troponin assay has two incubations with a wash after each, and a flash label. Siemens Healthineers' Atellica IM 1600, rated at up to 440 tests an hour, uses disposable tips, an acridinium label read by a photomultiplier, sample volumes of 10 to 100 microliters and assay times from 10 to 54 minutes, according to its 2017 specification sheet. The subsystems are the same in each machine; what differs is where each maker spends the time, the space and the cost.

+ The questionHow does a machine pipette a few microliters, wash particles a few micrometers across, and leave nothing behind for the next sample?

A drop that should not travel

On 9 August 2021 Beckman Coulter began a recall of its Access high-sensitivity troponin I reagent, more than 290,000 kits, according to the FDA's recall record. The firm attributed the problem to carry-over: after a sample with an extremely high troponin result, traces could pass "through the probe into the particle well" of the reagent pack and contaminate later tests. Its instructions to laboratories were concrete. After any result above 270,000 picograms per milliliter, they were to discard the open reagent packs, load a new one, run a low-level control on every pipettor, and repeat any positive results that might have been affected.

Carry-over is the transfer of material from one sample, or one reagent, into the next. A troponin assay is especially exposed to it because its range is so wide. The thread's assay is tested free of the high-dose hook up to 100,000 ng/L (Chapter 13), and troponin I assays meet samples above 270,000 ng/L, as the recall's trigger shows, while the decision limits sit near 14 to 19 ng/L. A sample ten thousand times above the decision limit needs to leave behind less than one part in ten thousand of itself, and in practice far less, to keep the next result honest.

One common design measures carry-over with a fixed sequence: a high sample run twice, then a low sample three times. If the first low result is higher than the third, the difference, divided by the high concentration, estimates the fraction carried. The test has to be designed in, because routine quality control cannot see the problem. The guideline on statistical quality control from the Clinical and Laboratory Standards Institute excludes from its scope random errors that affect individual samples, such as carry-over, because control samples run between patients will not reveal them.

22 — What a part per million does

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Tips, probes and pressure

The first defense is the pipetting hardware. A disposable tip used once and thrown away cannot carry sample on the probe to the next sample, though carry-over can still arise through reagent probes, wash stations or a shared measuring cell. The cobas e 801 uses "one disposable AssayTip per pipetting", in its maker's words, and Siemens Healthineers and QuidelOrtho both say that their disposable tips eliminate sample carry-over, a claim the makers state rather than a measured value. A fixed probe, washed inside and out between samples, saves the cost and waste of tips and can be cleaned to a specified carry-over, as Abbott's figure shows; the Beckman recall shows what happens when a path is left that the cleaning does not cover.

The second defense watches the liquid as it moves. A probe that touches a clot, a bubble or foam draws the wrong volume, and the result is wrong with no visible sign. A 2000 patent from Bayer, whose diagnostics business later passed to Siemens, describes one way to catch it: record the vacuum inside the probe while it aspirates. A normal draw of about 100 microliters shows a smooth rise and decay within about half a second; a clot blocks the tip and drives the pressure higher and the draw longer; foam lets the pressure collapse early; a second rise before the decay means the tip lost contact with the liquid. Analyzers also sense the liquid surface before aspirating, and the e 801 adds a camera for foam.

23 — Listening to the probe as it draws

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Small volumes make every error larger in relation. A 4-microliter draw that loses a tenth of a microliter to a droplet on the tip is 2.5 percent short, and the troponin result is 2.5 percent low with it. Pipetting precision at such volumes does not appear in the makers' public specifications, but the total precision of the assay, measured in the studies of Chapter 23, includes it.

Magnets and beads

Most analyzers separate bound from free label with magnetic microparticles. The thread's reagent contains streptavidin-coated particles at 0.72 milligram per milliliter, according to the FDA's decision summary, and patents describe the Elecsys beads as typically 2 to 3 micrometers across, a size also used in research products such as one supplier's 2.8-micrometer streptavidin beads, which bind about 500 to 750 picomoles of biotin per milligram. Beads that small stay suspended long enough to collect the sandwiches throughout the reaction volume, as Chapter 3 required, yet a permanent magnet can still pull them out of suspension.

The magnet then defines the wash. The beads are held against a wall or an electrode, the liquid around them is drawn off, and fresh buffer is added. How much unbound label survives depends on the volume of liquid left clinging to the pellet of beads, the number of exchanges, and whether the beads are released and resuspended between them. Those figures do not appear in public sources, but each exchange works as a dilution: two exchanges that each leave 1 percent behind leave one part in ten thousand.

Magnets attract more than beads. In October 2019 Roche issued a field safety notice for one lot of its troponin T reagent on the cobas e 801, after laboratories reported non-reproducible high results. The final version of the notice, in January 2020, gave the cause as contamination with magnetic particles, not beads, introduced during filling of the reagent for that analyzer only. Until the lot was replaced, laboratories were told to repeat any result of 14 ng/L or more, not to shake the reagent packs, and to discard each pack after its first 200 tests.

+ The questionWhy do the analyzer's clock and thermostat shape the result as much as the antibody does?

A tolerance of three tenths of a degree

The incubator of a cobas e 801 is specified at 37 °C, plus or minus 0.3 °C, and the module's measuring cell at 28 °C, plus or minus 0.5, according to the maker. Those numbers are requirements on the chemistry as much as on the heaters. Every rate in an immunoassay rises with temperature: molecules diffuse faster in warmer liquid, antibodies bind and release faster, and enzymes turn over substrate faster. If the reaction is stopped and read before it is complete, which in a 9- or 18-minute protocol it usually is, a warmer reaction gives more signal for the same amount of target.

How much more depends on the assay. An application note from Revvity, the maker of DELFIA reagents, gives one measured case: in a two-step assay for thyroid-stimulating hormone, a second incubation of two minutes reached 67 percent of its maximum signal at 35 °C, against 25 percent at 25 °C. The note derives from such data a rate that roughly triples for every 10 °C, which corresponds to an activation energy, the energy barrier the reaction must climb, of about 84 kilojoules per mole. That is one assay on one platform, but the order of magnitude is common: chemistry slows sharply as it cools.

24 — What three tenths of a degree is worth

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The measuring cell has its own temperature for a related reason. Its light comes from reactions at the electrode surface, which are also faster when warmer, and in one patent example it is read for about two seconds, so a difference in the cell's temperature from one test to the next passes straight into the signal per label. Holding the cell at a fixed temperature, separate from the incubator's, keeps that signal the same from test to test.

The same minutes for every cup

Timing matters for the same reason as temperature. A reaction that has not reached equilibrium gives a signal that depends on how long it ran, so every cup must run exactly as long as the calibrators did. That is why analyzers run on a fixed cycle, a new cup every 12 seconds on the e 801, and why the protocols come in fixed steps of 9, 18 or 27 minutes: the scheduler can then guarantee that each cup spends its incubation in the same number of cycles, whatever else is happening in the machine.

The fixed protocol also hides a useful check. Chapter 3 showed that the fraction caught after a given time depends on how fast the target binds. A target that binds unusually slowly, because it is bound up with something else in the blood, can give different results in a 9-minute and an 18-minute version of the same assay, and Chapter 19 describes laboratories that used exactly that difference to find an interference.

Shorter protocols, higher floors

When a protocol is shortened, binding has less time to approach equilibrium, so fewer sandwiches form for the same amount of troponin, and the signal from target shrinks while the background does not. The floor can rise. The International Federation of Clinical Chemistry publishes a table of troponin assay characteristics supplied by their makers. Its 2019 version lists, for the high-sensitivity troponin T assay on one family of Roche analyzers, a limit of detection of 2.05 ng/L for the 18-minute protocol, below the 5 ng/L the European label gives, a reminder that a limit depends on how it was measured, and 2.85 ng/L for the 9-minute one. On the newer cobas e 801 both protocols were listed at 3 ng/L.

The US clearance of 2017 shows the same trade made a different way. On the cobas e 411 the 9-minute assay runs in two steps, with the beads added in a second incubation, and its limit of detection was 5 ng/L; on the cobas e 601 it ran as one step, with the beads present from the start, and its limit was 3 ng/L, according to the FDA's decision summary. The reagent was the same, but the two analyzers differ both in how they arrange the minutes and in their measuring hardware, and the IFCC table also lists a higher limit for the e 411 in the 18-minute protocol, so the arrangement of steps is at most part of the difference.

25 — The same reagent, arranged in time

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Reagents on board and on the shelf

Time acts on the reagents too. Antibodies slowly lose activity, labels decay or degrade, and the beads can clump, so every reagent pack has a shelf life in the refrigerator and a shorter life once opened on the analyzer. The e 801 keeps its packs at 5 to 10 °C on board and gives stabilities of up to four months depending on the assay; Abbott gives 14 to 30 days on board for its ARCHITECT i reagents, and Siemens Healthineers 4 to 90 days for the Atellica IM, according to the makers.

A calibration ages with its reagent. The FDA's decision summary for the thread's assay states that a calibration remains valid for 7 days while the same reagent pack stays on the analyzer, and for 12 weeks with packs from the same lot. After that the analyzer requires a new calibration, because the reagent's response may have drifted. The Clinical and Laboratory Standards Institute's guideline EP25, in its second edition of 2023, describes how makers establish reagent shelf-life and in-use stability, by real-time and accelerated studies.

+ The questionHow does one detector report a few hundred photons and a few million with the same accuracy?

A detector that multiplies electrons

A photomultiplier tube is a glass vacuum tube a few centimeters long with a thin metal-alkali film, the photocathode, on the inside of its window. A photon striking the film can knock out one electron. Inside the tube, a chain of electrodes called dynodes, each at a higher voltage than the last, accelerates that electron into the next dynode, where it knocks out several more, and so on down the chain, so that one electron at the cathode becomes a pulse of a million to ten million electrons at the end. Hamamatsu, which makes such tubes, gives that range of gain in its handbook.

Not every photon starts a pulse. The fraction that releases an electron, the quantum efficiency, depends on the cathode material and the wavelength. Hamamatsu quotes up to 43 percent at 350 nanometers for its best blue-sensitive cathodes, and a further loss occurs because only 60 to 90 percent of released electrons reach the first dynode. The maker's figures for one photon-counting module, at 380,000 counts a second per picowatt of 400-nanometer light, work out to about 19 percent of arriving photons counted. At 600 nanometers the same module counts only about 3 percent, which matters for the orange-red light of electrochemiluminescence near 620 nanometers: an analyzer reading that light is better served by a cathode chosen for the red.

26 — One photon, a million electrons

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The tube also produces pulses when no light falls on it, from electrons released by heat in the cathode. This dark count is small, about 50 a second typical for that module, but it is never zero, and it is the detector's own floor. A light-tight chamber and a label that makes its own light are what let a blank reading approach that floor.

Counting statistics

Counting light is counting random events. Photons from a steady source arrive at random moments, so a count made over a fixed time scatters from one reading to the next even when nothing changes. The scatter follows a simple rule from probability: if the average count is N, the standard deviation is the square root of N. A reading of 100 counts carries a scatter of about 10, or 10 percent; 10,000 counts, 100, or 1 percent; and a million counts, 1,000, or 0.1 percent. The more light a measurement collects, the more precisely it knows the light.

27 — Counts scatter by their square root

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Counting statistics apply at every level of the chain. A digital assay that counts beads (Chapter 17) obeys the same square-root rule in its bead counts, and a low-level sample in any assay has a scatter set partly by how few labels, and how few photons, it produces. Improving the light collection of an analyzer, or reading for longer, lowers the scatter of the blank, and so the limit of blank, when photon counting contributes much of that scatter; when label bound without target dominates, as Chapter 5 described, it helps little.

From dozens to millions

At the other end of the scale the detector runs out of speed. Each pulse takes time to register, and if pulses arrive faster than the electronics can separate them, some are lost. The same Hamamatsu module stays linear to about 5 million counts a second, at which point it loses about 10 percent. From 50 dark counts to 5 million is five factors of ten, a far wider span than the less than three factors of absorbance in Chapter 6.

Analyzers report the result of this counting as a signal value. Abbott's documents call it relative light units and state that there is a direct relationship between the troponin in a sample and the light units detected. Above the counting range, an analyzer can measure the tube's current instead of counting pulses, reduce the light reaching it, or dilute the sample. The thread's assay quantifies from 6 to 10,000 ng/L without dilution, and its US clearance allows a tenfold dilution, which extends reporting to 100,000 ng/L. The cobas e 801 can dilute automatically by factors up to 27,000, according to its maker.

The hook, seen from the detector

A detector reports how much light it saw, not which side of a curve the light came from. In a one-step sandwich assay, as Chapter 4 described, a very high concentration of target fills the capture and the labeled antibodies separately, so fewer complexes carry both and the light falls again after a peak. A single reading cannot tell a moderate sample on the rising side of the curve from an enormous one past the peak.

28 — One light value, two concentrations

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Makers therefore test how far the signal keeps rising and state a limit. The FDA's decision summary for the thread's one-step assay reports no hook effect up to 100,000 ng/L, ten times the top of its measuring range. Abbott's high-sensitivity troponin I assay, which captures the target and washes before adding the labeled antibody, reported none up to 500,000 ng/L, a hundred times its top of range. A two-step design avoids the classic hook because excess free target is washed away before the label arrives; its signal levels off at the capacity of the capture antibody instead of falling. Roche's insulin assay, by its method sheet, shows no hook up to 20,000 microunits per milliliter, twenty times its range.

+ The questionTwo well-made troponin assays give different numbers for the same blood. Which one is right?

Ten times apart on the same samples

In an international pilot study published in 2015 by a working group of the International Federation of Clinical Chemistry, sixteen troponin I assays from six manufacturers measured the same 79 patient samples. Plotted against one another, the lowest-reading and highest-reading assays differed by about a factor of ten, with regression slopes between assays ranging from 0.33 to 2.87. None of the assays was broken. Each did what its maker designed it to do, with its own antibodies, its own calibrators and its own definition of a nanogram per liter of troponin I.

That is the puzzle of this chapter. A concentration reported by an immunoassay is not counted molecule by molecule; it is read off a curve that converts the analyzer's signal into a number, and the curve is only as true as the materials used to draw it. Chapter 2 showed that two antibody pairs can see different subsets of the troponin in blood. This chapter shows how each assay is tied to a scale, and why for troponin I there is still no single scale to tie them all to.

A curve made by the maker, adjusted in the laboratory

A calibration is the set of measurements that relates an assay's signal to concentration. Calibrators are samples with assigned concentrations, and an immunoassay's curve through them is not a straight line: plotted against the logarithm of concentration, it is flat at the bottom, rises through the middle and flattens again near the top, a shape that a four-parameter logistic function, a standard S-shaped curve, has been found to fit well. A 1997 patent on immunoassay calibration describes the usual practice as five or six standards each measured in duplicate.

Roche splits the work in two. For each reagent lot the manufacturer measures six master calibrators on many instruments and runs and encodes the resulting master curve with the lot. In the laboratory, the analyzer measures a two-level calibrator set, for the thread's assay at about 18 and about 4,200 ng/L, and uses the two results to adjust the master curve to that instrument and that reagent pack. Abbott's earlier ARCHITECT troponin I assay, by contrast, was calibrated on the analyzer with six calibrators, one of them a zero made from human serum and five containing recombinant cardiac troponin complex, according to its FDA decision summary.

29 — A master curve, adjusted on site

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Calibrations do not last. The thread's assay holds a calibration for 12 weeks on the same reagent lot and for 7 days with the same pack on the analyzer, according to its FDA decision summary, and Roche's method sheets also call for recalibration with each new lot and whenever quality control fails.

Lots, drift and quality control

Every new lot of reagent is a slightly different product: antibodies from a new production run, calibrators from a new batch, beads from a new coating. The maker assigns each lot its own curve, but the assignment can be wrong. In July 2020 Siemens Healthineers began a recall of three lots of its Dimension Vista high-sensitivity troponin I reagent for a negative bias, results reading lower than they should, and the remedy issued was a correction factor specific to each affected lot, according to the FDA's recall record. Laboratories check new lots themselves with patient samples, by protocols such as the Clinical and Laboratory Standards Institute's guideline EP26.

Between lots, laboratories watch for drift with quality control samples of known concentration, run at intervals and plotted on charts. The rules most laboratories use were set out by James Westgard and colleagues in 1981: a run is rejected, for example, when one control falls more than three standard deviations from its target, or two in a row fall more than two on the same side. Each rule is a compromise between catching real shifts and rejecting good runs: treating a single result beyond two standard deviations as a failure would reject about 9 percent of good runs with two controls, according to Westgard's own guidance.

Who decides what a nanogram is

For a measurement to mean the same in every laboratory, each assay's calibrators must be linked, through an unbroken chain of comparisons, to a reference that everyone accepts: ideally a reference measurement procedure and a certified reference material, which in turn are tied to the International System of Units. The international standard for that chain in laboratory medicine is ISO 17511, whose 2020 edition was confirmed in 2026.

30 — An unbroken chain, or a missing top

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Troponin I has no such chain end to end. In 2006 a committee of the American Association for Clinical Chemistry found a between-assay variability of 82 to 97 percent among 15 assays before harmonization. Adjusting them with shared serum pools cut that to 9 to 23 percent, but the purified troponin materials tested behaved like patient samples in only 45 and 39 percent of the assays, too few to serve as a common calibrator. The 2015 pilot with which this chapter opened reduced the scatter between assays from 40 to 22 percent at low concentrations by recalibrating them to common samples, and concluded that troponin I results did not yet have traceability to a higher-order reference. The US National Institute of Standards and Technology issued a new certified troponin complex, SRM 2921a, on 10 March 2026, intended for validating methods; its certification report notes that assay standardization is still needed.

Troponin T has a different situation, because one company makes the assay. Its results agree between laboratories by design, and the FDA's decision summary states that the 2017 assay is traceable to Roche's own earlier troponin T assay. Even that agreement has limits across generations. Roche's sixth-generation troponin T assay, CE-marked by September 2025 by the company's account, comes with 99th-percentile limits of 27 ng/L overall, 18 for women and 32 for men, from its own reference study, against 19, 14 and 22 ng/L for the fifth-generation assay cleared in the US, so its results cannot be read against the older limits.

The practical consequence runs through every later chapter. In the International Federation's 2019 table of high-sensitivity assays, the makers' 99th-percentile limits for troponin I ran from 8.67 to 58.2 ng/L, and the European guideline's one-hour algorithm gives a separate set of cut-offs for each assay. A troponin result has meaning only against the limits of the assay that produced it.

31 — Each assay its own limits

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+ Part IV · On a strip

The same chemistry without a machine.

The Science Museum Group in Britain keeps a Clearblue One Step pregnancy test made in Bedford, England, in 1988: a plastic stick with a pen-shaped sampler to hold in a stream of urine and a window where, three minutes later, a thin blue line meant the hormone hCG was present. The brand's present owner describes it as the world's first lateral flow test, a claim the museum's record does not make. Everything an analyzer does with pumps, magnets, heaters and a photomultiplier, that stick did with a strip of porous material, dried reagents and the eye. The two chapters of this part take the strip apart and then ask what changes when a machine, or a phone, reads the line instead of a person.

+ The questionHow does a strip of porous material do, in fifteen minutes and with no power, what an analyzer does with pumps and magnets?

The parts of a strip

A lateral flow test is an immunoassay run inside a strip of porous material, in which the liquid sample moves by capillary action, the same pull that draws water up a paper towel, and carries the reagents with it. A 2009 review of the format lists the parts. A sample pad receives the liquid. A conjugate pad holds the labeled antibody, dried. A membrane, usually nitrocellulose, carries a printed test line of capture antibody and, beyond it, a control line. An absorbent pad at the far end draws the liquid through, and a backing and a plastic housing hold it all together.

The Unilever patent behind the one-step pregnancy test, with a first priority date of 27 April 1987, describes the trick that makes the format work. The labeled antibody is dry on its pad, but it becomes "freely mobile within the porous carrier when in the moist state": the arriving sample dissolves it and carries it along. If the sample contains hCG, the labeled antibody binds it on the way, and the test line, where a second antibody is "permanently immobilised", catches the complex as it passes. Labeled antibody that bound nothing flows on to the control line, which catches the labeled antibody itself and shows that the liquid has run the whole strip.

32 — A sandwich that runs itself

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Each part maps onto a subsystem of the analyzer in Chapter 10. The sample pad is sample handling; the conjugate pad is reagent addition; the flow along the membrane is incubation and transport together; the test line is the solid phase; the stream that carries unbound label past the line and into the absorbent pad is the wash; and the eye, or a reader, is the detector. The strip has no clock and no thermostat. Its timing is set by the material.

A label that can be seen

The labels are particles, not molecules, so that a line holding enough of them becomes visible. The 2009 review gives sizes from about 15 to 800 nanometers, with colloidal gold the most common and colored latex, carbon and others in use. Gold particles of 20 to 40 nanometers absorb most strongly at 524 to 530 nanometers, in the green, so the light they leave looks red; the effect, called surface plasmon resonance, comes from the free electrons of the metal oscillating together. A patent filed in 1979 by J. H. W. Leuvering, and assigned to Akzona, the US arm of the Dutch group Akzo, described such gold sols as immunoassay labels; one of its examples, a sandwich assay run in tubes, detected hCG at about 5 milli-international units per milliliter.

Capillary flow sets the clock

Membrane makers describe their products by how long water takes to climb a set distance. Sartorius gives 65 to 115 seconds per 40 millimeters for one of its faster nitrocellulose grades and 95 to 155 for a slower one; Cytiva names its grades by the same measure, from about 80 to about 170 seconds per 4 centimeters. Flow is fastest at the start and slows as the wetted length grows, because the pull of the advancing front has more and more liquid behind it to drag. In the ideal case, described by Lucas and Washburn early in the twentieth century, the distance traveled grows with the square root of time.

33 — Seconds over the line

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Short contact is the strip's central limit. Millipore's guide for membrane users states that the time reactants spend close enough to bind is set by the capillary flow rate. It gives a rule of thumb: the effective concentration of analyte falls with the square of the flow rate, so halving the flow time, from 180 to 90 seconds per 4 centimeters, costs about four times in sensitivity. Designers buy back binding time with slower membranes, with test lines placed farther from the start where the flow is slower, and with larger sample volumes, and they pay for it in minutes and in liquid.

How little a line can show

The limits that result can be read from regulators' records. The FDA's decision summary for one professional pregnancy test cassette, cleared through a 510(k), the US route for a test similar to one already sold, sets its cut-off at 10 milli-international units of hCG per milliliter in serum and 20 in urine. At 8 milli-units, 78 percent of serum samples still read positive, as expected of a cut-off that splits a gradual change into two answers. The makers tested for a hook up to 2,000,000 milli-units per milliliter, a hundred thousand times the urine cut-off, and found none, because a strip that runs sample and labeled antibody together can still saturate at extreme concentrations, and the makers have to test how high.

Troponin shows where strips stop. Roche's cobas h 232, a point-of-care reader for its whole-blood cardiac strips, measures troponin T from 100 to 2,000 ng/L in 12 minutes, according to its maker. The low end of that range is about seven times the 14 ng/L 99th-percentile limit of the laboratory assay. That strip, with seconds of binding time at its line, has not reached the floor of the laboratory's sensitive assays; the instrument-read cartridges of Chapter 16 go lower.

+ The questionWhat does a reader add to a test a person can read by eye, and what changes when the person does the reading?

The same strips, read twice

From November 2020 the city of Liverpool, with about 498,000 residents, offered rapid antigen tests for the virus that causes COVID-19 to the people living there, using the Innova lateral flow test. As part of the pilot's evaluation, people without symptoms at 48 testing sites took two swabs minutes apart under supervision, one for the strip and one for a laboratory PCR test. The interim evaluation, published by the University of Liverpool on 23 December 2020, reported 5,869 paired results. Of the 70 people whose PCR test was positive, the strips, read on site, had found 28: a sensitivity, the share of infected people a test finds, of 40 percent, after void results were set aside. Among those with the highest viral loads, the strips found about two thirds.

The evaluators then had the photographs of the same strips re-read by reviewers who did not know the PCR results. On re-reading, the sensitivity rose to 53.4 percent overall, and among the higher viral loads from 66.7 to 83.3 percent, with specificity unchanged at 99.9 percent. Most of the discordant readings had come in the first week of the pilot, which the report linked to the testing staff's inexperience; it also cautioned that the numbers were small. The strips had not changed. What changed was who looked at them, and how.

A separate national evaluation, at the government laboratory at Porton Down and the University of Oxford, found the same pattern from a different angle. Its preliminary report, as quoted in a briefing for journalists on 11 November 2020, gave the strip's sensitivity as about 79 percent when laboratory scientists performed the test, 73 percent with trained health workers and 58 percent with members of the public who had trained themselves from the instructions and swabbed and tested themselves. The groups were not randomized, so the figures are not a like-for-like comparison.

34 — The same strips, read twice

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What an instrument reader measures

An instrument reader replaces the eye with a light source and a detector. The BD Veritor system for influenza, cleared by the FDA on 23 March 2012, uses a battery-powered bench reader that measures the light reflected from each line of the strip. Its decision summary records that the reader subtracts the signal at a negative-control line, a measure of the strip's nonspecific color, from the signal at each test line, and scores the difference against a cut-off; the strip "cannot be interpreted visually", and the subtraction is one that "the human eye is unable to accurately perform".

That subtraction is the reader's real contribution. The eye judges a faint line against the color of the membrane around it, under whatever light is in the room, and different people set their own threshold for "a line". A reader measures both the line and the background in fixed geometry, with its own light, and applies the same threshold every time. It can also save the reading, flag a strip that did not run, and send the result to a record.

35 — What an instrument reader subtracts

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Readers also allow labels that the eye cannot see. Quidel's Sofia analyzer, cleared by the FDA on 24 October 2011, reads beads dyed with a europium chelate, excited at 365 nanometers and glowing at 618, by scanning the whole strip, according to its decision summary. A 2012 comparison of europium nanoparticles read by time-resolved fluorescence with colloidal gold in two model lateral flow assays found that the fluorescent label detected about 7 times less target in one and about 300 times less in the other. A 2017 study by academic developers of a thermal reader, which detects the heat that gold particles release when lit by a laser, reported detecting about ten times less target than reading by eye.

Phones as readers

A phone carries a camera, a light and a computer, and has become a reader in its own right. A 2023 study of one smartphone reader, built around a 3D-printed holder and a real-time app, found that at a low antigen concentration volunteers judged only 20 percent of test strips positive while the app classified all of them correctly; the study used a single phone model. In August 2021 BD and Scanwell Health received emergency use authorization in the US for a home COVID-19 test read by a phone's camera, which the company described as the first of its kind.

India used the phone differently. When the Indian Council of Medical Research issued its advisory on home testing with rapid antigen tests, on 19 May 2021, naming one approved kit, it required users to photograph the strip with the phone used to register in a mandatory app, which recorded the result and sent it to the national testing portal. The same council had validated 95 rapid antigen kits by April 2021 and set its bar for use at the point of care at a sensitivity of at least 50 percent and a specificity of at least 95 percent.

Troponin at the bedside

The readers that now bring high-sensitivity troponin to the bedside are not strips read by eye but cartridges read by instruments. The PATHFAST high-sensitivity troponin I test was cleared by the FDA on 20 March 2024, and its distributor called it the first such test cleared for use at the point of care in the US. Abbott's handheld i-STAT high-sensitivity troponin I cartridge followed on 3 January 2025: an assay with electrochemical detection on about 22 microliters of whole blood in about 15 minutes, with 99th-percentile limits of 13 ng/L for women and 28 for men, according to its FDA decision summary. Siemens Healthineers' Atellica VTLi, which gives a result from a fingerstick in eight minutes, has carried a CE mark since 2021 and was not found among US clearances as of October 2026.

+ Part V · One molecule at a time

The floor, counted.

The disc at the heart of one digital immunoassay analyzer holds 216,000 wells, each with a volume of about 40 femtoliters, a twenty-five-millionth of a microliter, and each just large enough to hold one bead. In wells that small, a single enzyme molecule soon fills its well with enough fluorescent product to see, and the instrument can count the wells that light instead of measuring a glow. The two chapters of this part follow that idea from its first immunoassay paper to its limits, and they complete the answer to the central question: what sets the floor once single molecules can be seen.

+ The questionIf a few hundred labels are too few to see as a glow, can each one be seen on its own?

One enzyme in fifty femtoliters

On 23 May 2010 the journal Nature Biotechnology published a paper describing an immunoassay that counted single protein molecules in blood. Fourteen of its fifteen authors worked at Quanterix, a company in Cambridge, Massachusetts; the fifteenth, David Walt of Tufts University, was an adviser to the company, and all held shares or options in it. The paper called the method a single-molecule enzyme-linked immunosorbent assay, or digital ELISA.

The chemistry is a sandwich on beads, as in Chapter 9. Capture antibodies on beads 2.7 micrometers across catch the target; a detection antibody carrying biotin binds it; and streptavidin attached to an enzyme, beta-galactosidase, binds the biotin. When target is scarce, most beads catch no molecule at all, and the few that do catch one. The beads are then loaded into an array of tiny wells, about 50 femtoliters each in the 2010 paper, so that each well holds at most one bead. The wells are sealed after a substrate is added, and any well holding an enzyme fills with a fluorescent product.

The trick is concentration. One molecule of product in a tenth of a milliliter of liquid, the volume of an ordinary assay, is undetectable; the same molecule in 50 femtoliters is at about 33 picomolar, and the enzyme goes on making more. Each well is either bright or dark, and the instrument photographs the array and counts the bright ones. According to the abstract, the method detected as few as 10 to 20 enzyme-labeled complexes in 100 microliters of sample, and it measured prostate-specific antigen in the serum of men who had had their prostates removed at 14 femtograms per milliliter, or 0.4 femtomolar, where the prefix femto- means a thousandth of pico-.

The ratio of molecules to beads is what makes the count digital. A company poster reporting the same work describes one experiment with about 400,000 beads and a consumable of more than 50,000 wells. If that many beads share the paper's lowest level, 10 to 20 labeled complexes in 100 microliters, there is one complex for every 20,000 to 40,000 beads, so a bead that lights almost certainly carries exactly one molecule, and the number of lit beads is the number of molecules seen. The same arithmetic shows the price: a few lit wells have to be found among tens of thousands of dark ones, so the array and the camera that reads it matter as much as the label.

36 — One enzyme lights a 40-femtoliter well

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Counting the lit beads

A count of bright wells becomes a concentration through a rule from probability. When molecules are spread at random over many beads, the number on any one bead follows what statisticians call a Poisson distribution, and the share of beads carrying none is fixed by the average number per bead. The company's papers call that average the AEB, for average enzymes per bead, and use it as the common unit: the instrument counts the share of beads that are lit and converts it into AEB.

As concentration rises, more beads carry two or more enzymes and the count stops tracking the concentration simply. The analyzer described in 2016 switches to measuring the average brightness of the beads, an analog mode, once more than 70 percent of them are lit, the same limit as an average of about 1.2 enzymes per bead given in a 2011 paper by the company. The digital range spans about two and a half factors of ten, and with the analog mode the total exceeds four.

37 — Counting lit beads

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From a paper to an analyzer

The method reached laboratories as an automated analyzer, the Simoa HD-1, described in 2016 by authors from Quanterix and from Stratec, the company that built the instrument. Its disc holds 216,000 wells of 40 femtoliters, and a single-analyte test loads 400,000 to 500,000 beads, of which the instrument images about 25,000 to 40,000. It runs 66 samples an hour, with the first result after about an hour. The paper defines the limit of detection as 2.5 standard deviations above the background, reports limits such as 0.020 picogram per milliliter for prostate-specific antigen and 0.0020 picogram per milliliter for interleukin 6, and states an average sensitivity more than 1,200 times that of ELISA across 16 proteins.

Those limits, for other proteins, sit a hundred to a thousand times below the floor of the troponin assays in Part III. The current analyzer, the HD-X, is labeled by its maker for research use only. Quanterix offers blood tests for neurofilament light and pTau217, proteins linked to damage and disease in the brain, as laboratory-developed tests, and in early 2026 submitted a 510(k) for an algorithmic Alzheimer's test combining several markers; no FDA clearance of it had been found as of October 2026. Chapter 18 asks why, with single molecules countable, the clinical market for the same markers went largely to conventional analyzers.

+ The questionOnce single labels can be counted, what limits a test?

Four cleared tests, none of them digital

By October 2026 this guide had found four FDA clearances of blood tests that aid in assessing the amyloid plaques of Alzheimer's disease, and none of the tests ran on a digital analyzer. Three were immunoassays on conventional analyzers of the kinds described in Part III, two of them measuring tau protein that carries a phosphate group at amino acid 217, called pTau217; the fourth used mass spectrometry. Quanterix, whose method Chapter 17 described, had submitted its own test for clearance, and no decision on it had been found.

The puzzle is that pTau217 circulates at very low concentrations. In a small 2024 study on one analyzer, people with amyloid plaques had on average about 0.66 picogram per milliliter in their plasma and people without them about 0.11. A picogram per milliliter is the same as a nanogram per liter, so these concentrations sit well below the troponin limits of Part III. Yet the conventional assays reach them. Fujirebio's pTau217 assay on its Lumipulse analyzer, a chemiluminescent enzyme immunoassay of the kind in Chapter 7, has a limit of detection of 0.041 pg/mL, and Roche's electrochemiluminescent pTau217 assay 0.056, according to their FDA decision summaries. Roche's cut-offs sit at 0.223 and 0.378 pg/mL, four to seven times above its limit of detection.

The digital analyzer of Chapter 17 reported limits of about 0.002 to 0.02 pg/mL for other proteins, lower again, though limits for different proteins and antibodies are not directly comparable. Whatever the commercial reasons for the outcome, the arithmetic shows that the conventional assays had already reached the concentrations at which patients differ. That changes the question this part set out to answer. Counting single labels removed the detector as the limit. What remains is the label that lights without a target, the share of the sample the assay actually looks at, the antibody's grip, and the number of molecules in the drop.

The blank, one bead at a time

In a digital assay the blank is not a faint glow but a count. The 2016 paper on the HD-1 analyzer reported that backgrounds were generally between 0.005 and 0.01 enzymes per bead, which means that 0.5 to 1 percent of the beads light in a sample with no target: of 25,000 beads read, about 125 to 250. The paper does not say which molecules light them. By definition they are beads carrying label that no target holds, the background of Chapter 5 seen one molecule at a time.

The example shows why a more sensitive camera would not help: every lit bead in the blank is already seen. What sets the floor is how many beads light without a target and how much that number scatters from one blank to the next. A 2012 theoretical paper by the company's scientists notes that lower concentrations of label reduce the background. By the balance of Chapter 3, less label also binds fewer of the captured molecules, so the trade between signal and background that Chapter 5 described returns at the scale of single molecules.

Molecules in the drop

Counting also exposes a limit that a glow hides: the number of molecules there are to count. On the HD-1, only about 5 percent of the beads in a test are sealed into wells and read, according to a 2020 paper from the laboratory of David Walt, a co-author of the 2010 method. At 10 attomolar, a hundredth of a femtomolar, a 100-microliter sample holds about 600 molecules of the target. Even if every one were caught and labeled, a reading of one bead in twenty would see about 30 of them, with a counting scatter near 18 percent.

The same group sealed beads into droplets instead of wells and read up to 60 percent of 100,000 beads. Limits of detection fell to 30 attomolar for interferon gamma, against 350 on the commercial analyzer, and to 20 attomolar for interleukin 2, against 550: about 1,200 molecules in the whole sample at the lowest. The method was a research technique, not a product. Its authors also noted that sensitivity is limited by the antibodies' dissociation constants. As Chapter 3 showed, the share of target an antibody holds depends on how its concentration compares with that constant, and a complex that lets go during the washes takes its label with it. The company's 2012 theory paper put the efficient range for digital ELISA at antibodies with dissociation constants from about 10 picomolar to 1 nanomolar.

38 — Where the molecules go

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Sample volume is the last lever. A larger sample holds proportionally more molecules, and the tests in the 2016 paper used from 25 microliters of sample for most proteins up to 144 microliters for one viral protein. The limit is practical: more sample means more time to capture from it, more beads to read, and more of everything else in blood that can stick.

Two antibodies that must meet

Another way to lower the background is to demand more before a signal counts. In a sandwich, a detection antibody stuck to the surface reads as if a target held it in place. In 2002 a group at Uppsala University in Sweden described a proximity ligation assay, in which two binders, each carrying a short strand of DNA, must attach to the same molecule before their strands lie close enough to be joined into a new piece of DNA. The joined piece is then copied and measured, and a binder on its own leaves nothing to copy. The paper, which used short DNA molecules called aptamers as its binders, detected a growth factor at 40 zeptomoles, a zeptomole being a thousandth of an attomole, or about 24,000 molecules, "without washes or separations".

A 96-plex version of the proximity extension assay, described in 2014 by authors mostly from the company Olink, used pairs of antibodies carrying matched DNA strands and ran 92 assays together in 1 microliter of sample; the omics guide in this library explains how the resulting DNA barcodes are read. Its authors argued that conventional multiplex sandwich assays stop at about ten targets, because the chances of a wrong antibody pairing multiply with every target added. In their format a signal needs both antibodies on the same molecule and DNA strands that match each other.

39 — Two binders must meet

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What sets the floor

Chapter 4 traced the troponin T floor from 100 ng/L in 1992 to a quantitation limit of 6 ng/L in the 2017 assay. Abbott's high-sensitivity troponin I assay detected 1.7 ng/L in its 2019 US clearance, and a single-molecule counting assay described in 2007 by scientists at the company Singulex detected 0.2 ng/L. Across those years the limit changed hands. The early floors in immunoassays were set by the label and the way it was read: a radioactive count, a color in a photometer. Light from chemistry and from electrodes, read in the dark, cut the reader's own noise until the detector stopped being the limit, and the analyzer's washing, timing and calibration kept the floor steady from run to run.

With labels counted one by one, the floor is set by what the antibodies do and by what the sample offers: the labels left stuck without a target, the share of target caught and kept through the washes, and the number of molecules in the volume tested. So the answer to the central question runs in sequence. The label and its reader set the floor first, the instrument held it, and today the antibodies and the drop set it.

40 — Who set the floor

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A lower floor is worth having only where the biology calls for it. The thread's troponin T assay, at the limit of detection of 5 ng/L then on its label, measured troponin in 43 percent of healthy men and 7 percent of healthy women in an international reference population. Abbott's troponin I assay, with a limit of detection then given as 1.9 ng/L, measured troponin in 80 percent of the same men and 67 percent of the women. Chapter 20 shows why measuring healthy people matters for a decision made within an hour. For pTau217 the conventional analyzers already reach the concentrations that separate patients, and below that point a lower floor adds little to the decision the test supports unless it also improves precision at the cut-offs, where about a fifth of patients in the cleared tests fall between answers.

+ Part VI · Reading the number

When a result is right, and when it is not.

On 28 November 2017 the FDA issued a safety communication warning that biotin, a B vitamin sold in high doses in supplements for hair, skin and nails, can make laboratory tests read falsely high or falsely low. The agency reported that one patient taking high levels of biotin had died after falsely low troponin test results. Biotin harms neither a reagent nor an analyzer; it works through the chemistry many tests are built on. The three chapters of this part read results: what makes a well-run assay wrong for one patient, how the thread's troponin numbers become a decision within an hour, and why the first blood test cleared to aid the diagnosis of Alzheimer's disease reports a ratio, with a zone that gives no answer.

+ The questionWhat can make a well-run assay give one patient a wrong number?

Seven samples in 21,000

Seven samples in 21,000 is the share of hospital samples in which one study found a patient's own antibodies interfering with immunoassays that contained blocking reagents, according to a 2004 review of interference in immunoassays. The same review reports that human antibodies against mouse proteins have been found in 30 to 40 percent of patient samples. Both numbers can be true at once, and much of the gap between them is the work that the reagent does.

Human antibodies that bind the antibodies of other animals are called heterophile antibodies, or, when they are directed at mouse proteins, human anti-mouse antibodies, HAMA for short. A 1999 review notes that they can arise from treatment with animal antibodies and from other causes, can reach high levels and can persist for years. Many immunoassay antibodies are made in mice, as Chapter 2 described, so such an antibody can grip the capture antibody with one arm and the labeled antibody with the other. The analyzer then reads a sandwich with no target in it, and the result is falsely high. An antibody that binds only one reagent antibody can instead stop it catching the target, which the review offers as the likely route to a falsely low result. Rheumatoid factor, an antibody that binds the stem of other antibodies, can bridge in the same way, and the review notes that it can cause false positives in troponin assays.

41 — Four ways a sandwich can mislead

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Makers defend against all of these in the reagent. They add non-immune animal serum or mouse antibodies that soak up the patient's antibodies before they reach the sandwich; they use antibody fragments that lack the stem; and they use antibodies built partly from human sequence, like the labeled antibody of the thread's assay. These defenses are not complete. The review summarizes a study of ten donors chosen because their blood was known to interfere, in which 66 laboratories in seven countries measured 74 analytes: about 6 percent of the results were false positives, and the kits' blockers failed to prevent about two thirds of the interferences. Because the donors were selected for interference, the 6 percent is a measure of how hard the problem is, not of how often it occurs.

Biotin, the reagent's own glue

Biotin interferes for a different reason: many assays use it themselves. Chapter 4 described the thread's assay, in which the capture antibody carries biotin and the magnetic beads carry streptavidin, a protein that grips biotin very tightly. Free biotin in the patient's blood occupies the streptavidin on the beads, so the capture antibody, with the troponin and the label it holds, has nowhere to attach and is washed away. In a sandwich assay the result reads falsely low. In a competitive assay, where less label means more target, the same loss reads falsely high.

A 2017 study in JAMA measured the effect directly. Six healthy adults took 10 milligrams of biotin a day for a week, and their blood, drawn about two hours after the last dose, was run on 37 immunoassays on four systems. Interference appeared in 9 of the 23 assays that used biotin and in none of the 14 that did not. Five of eight competitive assays read falsely high and four of fifteen sandwich assays falsely low; one assay for thyroid-stimulating hormone fell by about 94 percent.

The FDA's warnings of 2017 and 2019 described supplements of up to 20 milligrams, then pills of up to 100 milligrams, and doses of up to 300 milligrams a day recommended by physicians for some conditions, giving blood levels of up to 1,200 ng/mL. Its 2020 guidance asked makers to test biotin up to 3,500 ng/mL, three times the highest concentration expected in patients. The thread's assay had been cleared in 2017 after testing biotin only up to 20 ng/mL, and its instructions said not to draw blood from patients taking more than 5 milligrams a day until at least 8 hours after the last dose.

Roche changed the reagent rather than the instructions. It added an antibody that captures free biotin and lengthened the linker that holds biotin on the capture antibody, and the FDA cleared the reformulated assay on 21 September 2021 with no interference up to 1,500 ng/mL, according to its decision summary. Abbott states that its Alinity assays are free from biotin interference, a maker's claim. The FDA's list of troponin assays whose biotin risk had not been addressed, as it stood in June 2022, still named the 2017 version of the thread's assay, because the list goes by clearance number.

42 — A vitamin dose against a reagent's tolerance

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Too much target, or target in disguise

The high-dose hook of Chapters 4 and 13 turns into a clinical error when an enormous concentration reads as a modest one. The 2004 review cites a patient whose prolactin read 1,500 milliunits per liter undiluted and 950,000 on dilution, about 630 times higher. In a 2017 case report, a woman with an advanced molar pregnancy, an abnormal growth of placental tissue, had two negative urine pregnancy tests, while her serum hCG, measured after proper dilution, was above 900,000 milli-units per milliliter. The defense is dilution: a laboratory that suspects an extreme value reruns the sample diluted, and an analyzer can do so automatically.

Some targets circulate bound to the patient's own antibodies. Prolactin bound mostly to immunoglobulin G, called macroprolactin, is generally considered biologically inactive, yet assays detect it. The review reports it in 24 percent of sera with raised prolactin, and differences of 2.3- to 7.8-fold between assays. Troponin can do the same. In a 2022 case report, a patient's troponin I read 872 ng/L on one analyzer, against an upper limit of 54, and peaked at 3,165. Roche's high-sensitivity troponin T assay gave 12 ng/L, and a second troponin I assay 11 ng/L, against its own limit of 9. Precipitating the antibodies with polyethylene glycol left only 31 percent of the troponin I, below the 40 percent threshold the authors cited for suspecting a complex. The patient had already been treated for inflammation of the heart in error, and the troponin I was still 130 ng/L a year later.

Small molecules bring a different problem. A competitive assay for a drug or hormone can bind related molecules of similar shape, so it measures them too. The review cites immunoassays for the transplant drug cyclosporin that read up to 174 percent higher than a chromatography method, because they also counted the drug's breakdown products.

What the analyzer and the laboratory can do

The reagent carries most of the defense: blockers, fragments, partly human antibodies, a biotin scavenger, a two-step format that washes away excess target, and a hook-free limit far above the measuring range. The analyzer adds rules: it flags results above the measuring range and reruns them diluted.

The laboratory holds the last checks. Comparing results that should agree can expose a complex: in a 2021 case series, four patients with troponin T bound to antibodies were identified when the 9-minute and 18-minute versions of the same assay disagreed, with a ratio below 0.80. The 2004 review lists the other checks: dilute the sample and see whether the result falls in proportion, add a blocking reagent, precipitate the antibodies, or measure on a method that uses antibodies from another species or no biotin. A result that changes by more than the test's own scatter under one of those checks was not a measurement of the target alone.

+ The questionHow does a number measured at a few nanograms per liter let an emergency department send a patient home within an hour?

An hour in a guideline

At the end of August 2015 the European Society of Cardiology released new guidelines for patients with a suspected heart attack whose electrocardiogram, the tracing of the heart's electrical activity, does not show one particular pattern, called ST-segment elevation. The guidelines recommended a decision from two blood samples taken an hour apart, as an alternative to waiting three hours, wherever a high-sensitivity troponin assay with a validated algorithm was available. They added that the cut-off levels in that one-hour algorithm are specific to each assay. The 2020 edition made the one-hour algorithm the best option and two hours the second best, and the 2023 guideline on acute coronary syndromes kept both.

An hour is not long for a protein to accumulate. Troponin leaks from dying heart muscle over hours, and a patient who arrives soon after the pain began may not yet have much of it in the blood. The opening scene showed the problem: in the Basel study, the standard assay separated heart attacks from other causes much less well in patients who had arrived within three hours. The guideline's answer uses two properties of the sensitive assays together, a limit set in healthy people and a change measured between two draws.

The 99th percentile

The international definition of a heart attack, in its fourth version of 2018, calls troponin above the 99th-percentile upper reference limit a sign of injury to heart muscle. That limit is the value below which 99 percent of a healthy reference population falls. An acute heart attack requires a rise or fall in troponin, with at least one value above that limit, together with clinical evidence that the heart muscle was starved of blood. The definition also requires the high-sensitivity assays to measure with a coefficient of variation of 10 percent or less at the 99th percentile, and it recommends separate limits for women and men.

For the thread's assay, the FDA's decision summary gives limits of 19 ng/L overall from 1,301 healthy people, 14 ng/L from 656 women and 22 ng/L from 645 men. A percentile at the extreme of a distribution rests on few people: 1 percent of 656 is between six and seven, so the women's limit is set by the handful of highest values among them. Abbott's high-sensitivity troponin I assay, cleared in 2019, reported its overall limit of 28 ng/L from 1,531 people with a 90 percent confidence interval of 22 to 33, a range that shows how loosely even a large reference study pins the number down. The European label of the thread's troponin T chemistry gave 14 ng/L overall, from a different reference population, and Chapter 14 showed that the next generation of the assay has different limits again.

43 — A limit set by the top 1 percent

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The FDA's review of the thread's assay shows why one number is not enough early on. Using the 718 Basel patients of the opening, it found that a single sample on arrival, judged against 19 ng/L, identified 93.5 percent of the heart attacks; a sample at three hours identified 98.3 percent. Judged against the 99th percentile alone, a single draw at the door misses one heart attack in fifteen and cannot send patients home.

From one number to a change

The one-hour algorithm adds the change between two draws. For the thread's assay, the 2020 guideline's table rules a heart attack out when the first result is below 12 ng/L and the second, an hour later, differs from it by less than 3 ng/L; a single very low first result, below 5 ng/L, can also rule out under conditions the guideline sets. It rules one in when the first result is 52 ng/L or more, or the change in one hour is 5 ng/L or more. Everyone else is observed and tested again. The same table gives a separate row of numbers for each assay: for Abbott's troponin I assay, for instance, below 5 ng/L with a change under 2 (or below 4 alone) to rule out, and 64 or more, or a change of 6 or more, to rule in.

A prospective study published in 2015, led by the Basel researchers of the opening at six centers in Switzerland, Spain and Italy, tested the algorithm with the thread's assay on 1,320 patients, 229 of whom had a heart attack. The study was funded in part by several assay makers, and Roche donated the assay. The algorithm ruled out 786 patients, 59.5 percent, with a sensitivity of 99.6 percent and a negative predictive value of 99.9 percent; ruled in 216, of whom 78.2 percent had a heart attack; and left 318 for observation.

44 — Ruling out in one hour

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A negative predictive value of 99.9 percent depends on how many of the patients tested have the disease, here 17 percent. The PCR guide in this library works through that dependence of predictive values on prevalence, and it applies unchanged: the same algorithm in a population with more heart attacks would leave more of them among the patients ruled out.

What "high-sensitivity" means

The word has a definition. A task force of the International Federation of Clinical Chemistry, writing in 2017 and restating criteria it first proposed in 2012, set two criteria: a coefficient of variation of 10 percent or less at the 99th percentile, and results above the limit of detection in at least half of healthy people, reported in whole nanograms per liter. Its authors disclosed research funding or consulting for several troponin makers. In the healthy population the task force used, the thread's assay, with a limit of detection of 5 ng/L, measured troponin in 7 percent of women and 43 percent of men. It met the precision criterion but not the second one, and the task force noted its lower-than-recommended rates.

When the FDA cleared the assay in January 2017, it asked the maker, according to a trade report, to call it "next generation" rather than high-sensitivity, as the opening noted. Assays that measure most healthy people do more than lower a limit. A value that can be measured in a healthy person can be compared with the same person's value an hour later, which is what the change in the algorithm needs at the bottom of the range.

45 — Measurable in healthy people

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+ The questionWhy does the first cleared Alzheimer's blood test report a ratio of two proteins, with a middle zone that gives no answer?

Two numbers and a remainder

Of the positive results, 91.7 percent were confirmed by a brain scan or a spinal fluid test; of the negative results, 97.3 percent. The FDA gave those figures on 16 May 2025, when it cleared what it called the first in vitro diagnostic device that tests blood to aid in diagnosing Alzheimer's disease. Fewer than 20 percent of the 499 patients in the study received neither answer. The device was Fujirebio's Lumipulse G pTau217/β-Amyloid 1-42 Plasma Ratio, cleared through a 510(k) about eight months after the application arrived, with the company's spinal fluid test of 2022 as its predicate.

The test does not diagnose the disease. It looks for amyloid pathology: deposits in the brain of a protein fragment called beta-amyloid, which in clinical practice is assessed by a scan with positron emission tomography (PET) or by a test of the fluid around the spinal cord. The cleared indications limit it to adults aged 50 and older, seen in a specialized care setting with signs and symptoms of cognitive decline, and state that it is not intended as a screening or stand-alone diagnostic test. Its results must be interpreted with other clinical information about the patient.

Two assays, one number

The test is two immunoassays run on the same analyzer. Each is a two-step sandwich with an alkaline phosphatase label and a dioxetane substrate that glows at 477 nanometers, the chemistry of Chapter 7, run on Fujirebio's LUMIPULSE G1200 using 40 microliters of plasma, according to the decision summary. One measures tau protein carrying a phosphate group at amino acid 217, pTau217, from 0.047 to 10 picograms per milliliter, with a limit of detection of 0.041. The other measures beta-amyloid 1-42, the 42-amino-acid form of the fragment, from 0.8 to 500 pg/mL. The pTau217 range sits far below even the troponin floor of Chapter 1.

The analyzer reports the two concentrations, and the ratio is then calculated by hand: pTau217 divided by beta-amyloid 1-42, rounded to five decimal places. The FDA's decision summary says that it "must be manually calculated because the instrument does not report the final result". The maker adds that both values must come from the same sample on the same analyzer, and that results from other makers' assays must not be mixed in. Neither plasma assay has an international reference of its own. The summary states that pTau217 is traceable only to an in-house standard, and that no international reference material or method exists for plasma beta-amyloid 1-42, whose calibrators are tied to the maker's spinal fluid assay, which is the troponin I problem of Chapter 14 in a new field.

The decision summary does not say why the test divides one protein by the other, and the predicate, itself a ratio of two amyloid fragments in spinal fluid, was granted without an explanation of that choice in its own review. Arithmetic says what a ratio does and costs. Any factor that scales both concentrations in one tube by the same proportion cancels when one is divided by the other. But each measurement brings its own scatter, and the ratio carries both. The summary reports total within-laboratory coefficients of variation of 2.2 to 10.3 percent for pTau217, 2.5 to 3.8 percent for beta-amyloid 1-42 and 3.1 to 10.2 percent for the ratio, so at low concentrations the scarcer protein dominates the ratio's scatter. Between reagent lots, the ratio's coefficient of variation was 7.3 to 16.3 percent.

46 — Two assays, one number

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Two cut-offs and a middle zone

The test sorts each ratio into three results. A ratio of 0.00738 or more is positive, one of 0.00370 or less is negative, and anything between is indeterminate. The cut-offs were set on 208 stored plasma samples from three research cohorts. According to the decision summary, an indeterminate result "is consistent with patients who are uncertain to have amyloid pathology", and those patients "should be considered for further testing", such as a PET scan or a spinal fluid test.

The clinical study tested 499 people aged 52 to 93 against a PET scan read by eye, for 152 of them, or the spinal fluid test, for 347. Just over half, 51.1 percent, had amyloid pathology. The decision summary's table shows what the two cut-offs did.

47 — What the middle zone holds

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Two cut-offs are a choice to answer fewer patients more reliably. A single cut-off would have given every patient a yes or no, and since the middle group split evenly, roughly half of those patients would have been sorted wrongly. With two, a fifth of patients go on to the scan or spinal test they would have needed without the blood test, and the other four fifths get an answer that matches the reference most of the time. The algorithm of Chapter 20 makes the same trade, with an observation zone in place of an indeterminate one. The predictive values also depend on how common amyloid pathology is among those tested, which in this specialist population was about half.

What the clearance established, and what came after

The clearance established that the ratio, run as specified on the named analyzer, agrees with amyloid PET or the spinal fluid test, in symptomatic adults in specialist care, closely enough for the FDA to find it substantially equivalent to its spinal fluid predicate. It did not establish use in people without symptoms, for screening, or in primary care, and it did not make a positive result a diagnosis. Samples matter too: the summary warns that specimens "degrade rapidly at room temperature", a reminder that the analyzer's floor is only as good as the tube that reaches it.

Three more blood tests for amyloid pathology were cleared within fifteen months. Roche's Elecsys pTau181 assay, cleared on 8 October 2025, uses a single cut-off to rule out amyloid pathology in primary care: in 312 people, of whom 13.1 percent were amyloid-positive, a negative result was right 97.9 percent of the time and a positive one only 22.4 percent of the time. On 19 August 2026 the FDA cleared C2N's PrecivityAD2, which measures by mass spectrometry, and Roche's Elecsys pTau217 assay, which uses a single protein and two cut-offs. In Roche's study of 958 people, 19.9 percent of results fell between its cut-offs, about the same share as for the ratio, according to its decision summary. In the EU, Fujirebio announced a CE mark under the IVDR for its single-protein pTau217 plasma assay on 11 May 2026. No approval of such a test by India's regulator was found as of October 2026.

48 — Blood tests for amyloid, 2022 to 2026

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+ Part VII · The field in 2026

Who builds the analyzers.

A cobas pro integrated solutions line joins clinical chemistry and immunoassay modules into one machine, and in its largest configuration it is rated at up to 6,400 tests an hour, according to figures Roche supplied to the trade magazine CAP TODAY in July 2026. Roche first sold one in the US in 2019 and reported more than 1,000 installed there and more than 5,000 elsewhere. The business those lines belong to, which Roche calls Core Lab and defines as immunoassays, clinical chemistry and its CustomBiotech unit, sold 7.6 billion Swiss francs' worth in 2025, 55 percent of the company's diagnostics sales. This part, a single chapter dated October 2026, sorts the makers of such machines, and of the analyzers and strips around them, by the way each reads its label.

+ The questionWho makes the analyzers and strips in use in 2026, and which way of reading a label does each rely on?

Four ways of reading, many makers

Two hundred tests an hour is a common rating for one immunoassay module in 2026, and the makers reach it with the different labels of Part II. The figures below are the makers' own, from their product pages, unless a trade survey is named, and each is a best case for one test mix, as Chapter 10 warned.

Light switched on at an electrode, the method of Chapter 9, belongs to Roche among the central-laboratory makers in this chapter. Its cobas e 801 module is rated at up to 300 immunoassay tests an hour and its smaller e 402 at up to 120, both with reaction times of 9, 18 or 27 minutes.

A flash from an acridinium label, the first family of Chapter 7, runs on Abbott's Alinity i, rated at up to 200 tests an hour per module, which the FDA's review of one Alinity assay describes as a chemiluminescent microparticle immunoassay with acridinium-labeled antibodies. Abbott states that its Alinity assays are free from biotin interference. Siemens Healthineers uses acridinium esters on its Atellica IM 1300 and 1600, rated at 220 and 440 tests an hour, and on the ADVIA Centaur XPT, rated at 240, with a first result in 18 minutes.

Glow from an enzyme, the second family of Chapter 7, is the widest group. Beckman Coulter's Access analyzers use alkaline phosphatase with a light-producing substrate; its DxI 9000, announced in 2023, moved to a faster substrate that the company's 2019 poster described as producing its light within a minute instead of about six. Fujirebio's LUMIPULSE G1200, rated at 120 tests an hour, uses alkaline phosphatase and a dioxetane substrate, according to the FDA's decision summary for its Alzheimer's ratio. Mindray's CL-2000i states the same principle and is rated at 240 tests an hour. QuidelOrtho's VITROS systems measure a peroxidase label with luminol, which the maker calls enhanced chemiluminescence.

Snibe's MAGLUMI X8 is rated at up to 600 tests an hour per module and 2,400 with four; its maker's page describes chemiluminescence without naming the label.

Fluorescence, the method of Chapter 8, survives in smaller systems. bioMérieux's VIDAS KUBE runs single-dose tests read by fluorescence at up to 36 an hour per module, stackable to six modules. Thermo Fisher Scientific's KRYPTOR analyzers use the energy-transfer method TRACE, with no washing. Counting single molecules, the subject of Chapter 17, is sold by Quanterix on analyzers labeled for research use only; in July 2025 the company completed its purchase of Akoya Biosciences.

49 — Who reads which label

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At the bedside

Chapter 16 described the high-sensitivity troponin tests that reached the bedside: Abbott's handheld i-STAT, cleared in January 2025, the PATHFAST system of PHC Corporation of Tokyo, cleared in March 2024, and Siemens Healthineers' Atellica VTLi, CE-marked in 2021. Other point-of-care analyzers carry broader menus. Radiometer's AQT90 FLEX runs up to 30 tests an hour from whole blood or plasma, with results in 11 to 21 minutes, for troponin I and T, natriuretic peptide, D-dimer, procalcitonin, C-reactive protein and hCG, among others. In December 2023 Roche agreed to buy the parts of LumiraDx tied to that company's point-of-care platform, which runs immunoassays and clinical chemistry on one instrument, for 295 million US dollars, in a sale out of administration in the UK.

India

India's makers, judged from their own pages, sell mostly rapid tests, ELISA kits and licensed chemiluminescence. The rapid antigen test that India's medical research council recommended for point-of-care use in June 2020, SD Biosensor's Standard Q, was made at a unit in Manesar, Gurugram. Agappe Diagnostics of Kochi signed a supply and license agreement with Fujirebio in March 2023: Fujirebio supplies the Mispa i60 and i121 analyzers and reagent raw materials, and Agappe makes the cartridge reagents in India and sells them under its own name. Agappe's 2024 brochure also lists acridinium-ester reagents and two chemiluminescence analyzers rated at 200 and 180 tests an hour, without saying where the analyzers are made.

J. Mitra & Co. of New Delhi sells rapid tests, ELISA kits and two bench-top chemiluminescence analyzers, again without a stated place of manufacture, and an analyzer developed with a technology center at the Indian Institute of Technology Madras. Transasia Bio-Medicals sells ELISA kits and an automated ELISA processor that reads absorbance. Mindray sells its chemiluminescence analyzers in India from an office in Gurugram. No maker's page opened for this guide described a central-laboratory immunoassay analyzer designed in India.

A field in motion

The field changes by the quarter. The makers' own counts, reported to the same 2026 trade survey, put about 3,625 Alinity ci-series systems in the US and about 16,600 elsewhere, and more than 950 of Diasorin's LIAISON XL analyzers in the US and more than 6,500 elsewhere; these are self-reported and unaudited. Siemens Healthineers said in 2026 that it was preparing to separate its diagnostics business, after a quarter in which that business's reported revenue fell 12 percent, 7 percent at constant currency, according to a trade report. Fujirebio has developed reagents for Sysmex's HISCL analyzers since 2020, and the two companies announced a basic agreement on wider collaboration around the start of 2024.

The products move too. Roche's sixth-generation troponin T assay was CE-marked by September 2025, by the company's account, with new reference limits (Chapter 14). QuidelOrtho's VITROS high-sensitivity troponin I assay was cleared by the FDA on 29 October 2025 with a claimed limit of detection of 0.59 ng/L. Four blood tests for amyloid pathology were cleared between May 2025 and August 2026 (Chapter 21), three of them immunoassays on conventional analyzers.

50 — A field in motion

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+ Part VIII · Quality and regulation, as of October 2026

What a test system must show.

About 8 percent of the in vitro diagnostics on the EU market needed a notified body's assessment under the 1998 directive; under the regulation that replaced it, around 80 percent do. The European Commission gave those figures in October 2021, when only six notified bodies had been designated to do the work; by February 2026 there were 19. The four chapters of this part set out what a reagent and its analyzer must show before they are sold, as of October 2026. Each puts the lasting mechanism first, the evidence and the reason for it, and the clauses and their dated status last, because the clauses will change and the reasons will not.

+ The questionHow many measurements does it take to show a test's floor and precision, and why that many?

A definition that is also an experiment

The limit of blank is the highest result expected, 95 times in 100, from a sample that contains none of the target. Chapter 5 worked the arithmetic once. This chapter asks what that sentence commits a maker to measure, because every word in it hides a choice: which samples count as blank, on how many analyzers and reagent lots, over how many days, and how many results are enough to locate the 95th percentile of a spread.

The Clinical and Laboratory Standards Institute, a US standards body for laboratory medicine, answers in a series of documents whose names begin with EP. EP17, in its second edition of 2012, covers the limits of blank, detection and quantitation, and requires that they fall in that order: the limit of detection above the limit of blank, and the limit of quantitation at or above the limit of detection. The thread's assay followed it. According to the FDA's decision summary, its limit of blank rests on 60 results from blank samples for each reagent lot on each type of analyzer, and its limit of detection on another 60 per lot from low-level samples. Its limit of quantitation, the lowest concentration measured with no more than 20 percent coefficient of variation, rests on 84 results per sample per lot. A laboratory that only wants to check a maker's claim needs far fewer: the guideline's implementation guide sets at least 20 blank results in all, from at least two samples measured at least twice on each of at least three days.

Why precision takes eighty results

A measurement's scatter has more than one source. Results within one run differ because pipetting, mixing and counting are never identical; that part is called repeatability. Runs on the same day differ a little more, because each run starts afresh with its own controls and handling, and days differ again, with new reagent packs, operators or calibrations. The total scatter a laboratory sees over weeks, the within-laboratory precision, combines all three, and a standard deviation from one run alone understates it.

51 — Where the scatter comes from

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To separate the sources, the design has to repeat each one. CLSI's EP05, in its third edition of 2014, measures each sample in two replicates, in two runs a day, on 20 days: 80 results per sample, from which an analysis of variance splits the scatter into its within-run, between-run and between-day parts. The thread's assay used the previous edition's version, 21 days of two runs of two, or 84 results per lot. The Alzheimer's ratio of Chapter 21 used 20 days of two runs of two on five panels, and then two further studies: three reagent lots on five days each, and three sites on five days each, both with two runs of three replicates.

A laboratory again checks rather than establishes. CLSI's EP15, also of 2014, verifies a maker's precision claim and estimates bias with at least 25 measurements over at least five days, and its product page states that such verification cannot establish precision. The difference matters in practice: the maker's 80 results support a claim, and the laboratory's 25 test whether its own analyzer lives up to it.

Range, interference and carry-over

Linearity is the claim that results stay proportional to concentration across the measuring range. EP06, in a second edition of 2020, dropped the curve-fitting of its predecessor and recommends a weighted straight-line fit, with the deviation at each level judged against what would matter clinically. The thread's assay was cleared with a measuring range of 6 to 10,000 ng/L and no hook up to 100,000 ng/L, the limit Chapter 13 explained.

Interference testing asks how much a result moves when a possible interferent is added to a real sample. EP07, in its third edition of 2018, sets the method, and a companion, EP37, lists the concentrations at which common substances should be tested. Chapter 19 showed the price of choosing too low a level: the thread's assay had been tested for biotin only up to 20 ng/mL, and the FDA's 2020 guidance asks for 3,500.

Carry-over is the part of one sample that reaches the next, the subject of Chapter 11. It is tested by running high and low samples in alternation and is reported as a fraction. The cost of a small fraction showed in 2021, when Beckman Coulter recalled its high-sensitivity troponin I assay because, by the firm's account, a sample above 270,000 ng/L could contaminate the reagent pack and so the results that followed. Taking 14 ng/L, the thread's limit for women, as an illustration, a carry-over of about 52 parts per million from such a sample would by itself turn a blank into a result at a 99th-percentile limit.

Comparison, reference limits and the documents regulators recognize

A new assay is compared with an existing one on patient samples, by the method of EP09, third edition, 2018, which estimates the bias between them across the range. Reference intervals follow EP28, whose minimum of 120 healthy people per group gives a nonparametric interval between the 2.5th and 97.5th percentiles. A 99th percentile needs many more: with 120 people it would be the single highest value, which is why the thread's assay drew on 1,301.

Regulators adopt these documents instead of writing their own. The FDA's database of recognized consensus standards listed, as of October 2026, EP05-A3, EP15-A3, EP17-A2, the second edition of EP06, the third of EP07, the second edition of EP37 (recognized in December 2025), the third edition of EP09 in part, and EP28-A3c, among others. It also listed an older CLSI document on interference by the patient's own antibodies in immunoassays. Recognition lets a maker cite the standard in a submission instead of justifying its method from first principles. In the EU, the IVDR divides the evidence for an IVD into scientific validity, analytical performance and clinical performance, and the Medical Device Coordination Group's 2022 guidance gives a list of analytical characteristics names carry-over and comparison between instruments beside the limits, linearity and precision of this chapter. Clinical performance studies follow ISO 20916 of 2019, confirmed in 2025, and how often a positive or negative result is right is worked through in the PCR guide in this library.

52 — Each claim and its document

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+ The questionWhy is an assay cleared together with the analyzer it runs on, and what happens when either changes?

A recall with nothing wrong in the bottle

On 2 January 2018 Siemens Healthineers began a recall of its troponin I assay for the Dimension Vista analyzer. The reagent itself had not changed. The instructions for use did not state the concentration of biotin below which the assay was free of interference, and the FDA's record gives the root cause as an error in labeling. The recall followed an earlier one, in March 2017, over biotin values printed in the wrong units in the same instructions.

A label is part of the product because a result means only what the label says it means. The same is true of the calibrators that set the curve, the controls that check it, the analyzer that runs the protocol and the software that turns light into a number. Earlier chapters showed each of them shaping the result: calibration in Chapter 14, timing and temperature in Chapter 12, the detector in Chapter 13. Regulators therefore judge the performance of the combination, and this chapter follows what that means when one part changes.

Cleared as a system

The FDA's guidance on replacement reagents states the principle in one sentence: clearance of a test system "is based on assay performance characteristics demonstrated with an instrument (or instruments) specified by the assay manufacturer". The same guidance notes that most automated clinical instruments by themselves are class I and exempt from premarket review. The risk sits in the claim, and the claim belongs to the assay run on named analyzers.

The thread's assay shows the arrangement. Its 2017 clearance covered the reagent, its calibrator set, its control material and a calibration-check material, and its decision summary names the analyzers, the cobas e 411 and the cobas e 601. Troponin assays are class II in the US, classified under a regulation that, read literally, describes tests for creatine kinase, the older cardiac marker; troponin sits there through its own product code. The exemption for analyzers has limits: the regulation withdraws it for devices used at the point of care and for those that assess the risk of cardiovascular disease.

The EU and India sort the parts differently but reach the same place. Under the EU's guidance on classifying IVDs, revised in September 2026, a troponin test for acute patients is class C and needs a notified body, while instruments intended specifically for diagnostic procedures are class A, which a maker can declare itself. India's list of IVD classes, updated in October 2023, puts a chemiluminescent immunoassay analyzer in class A and a clinical chemistry analyzer for near-patient testing in class C. In the US and the EU the reagent carries the higher class and the analyzer the lower; India's class for a troponin reagent was not found for this guide. In each place the performance claim ties the two together.

53 — What was cleared in 2017

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Open systems, families and migration

An assay that runs only on its maker's analyzers is a closed system; one whose maker validates it on other instruments is open, at least to that maker. The FDA's Replacement Reagent and Instrument Family Policy, final since 17 August 2022, lets a maker move a cleared assay to another cleared instrument in the same family without a new 510(k), provided it performs a risk assessment, verifies and validates against the protocols and acceptance criteria of the original clearance, and documents the work. The policy applies only to open systems, and not to class III devices, blood-bank tests, home tests or tests waived under CLIA. An instrument family shares architecture, design, tolerances, regulation and product code.

Moving an assay to a new kind of system is a bigger step. The FDA's 2013 guidance on assay migration lets a sponsor bring a cleared or approved assay to a new combination of instrument and software with focused analytical studies instead of new clinical trials. The studies cover detection limits compared with the old system, precision over days, runs and calibrations, reproducibility at three or more sites, linearity, and comparison on panels of samples. The intended use must stay the same. A migration is a way of showing that nothing important changed; it cannot add a claim.

When a part changes

Every change asks the same question: could it move a result that a patient's care depends on? The FDA's 2017 guidance on deciding when to submit a new 510(k) for a changed device has a section and a flowchart for IVDs, and a companion document for software. The biotin story of Chapter 19 ran through that process. Adding a scavenger antibody and lengthening a linker changed the reagent's formulation, and Roche obtained a new clearance for the reformulated assay in September 2021 rather than updating the old one.

Reagent lots change routinely, each with its own assigned curve, as Chapter 14 described. When a lot reads with a bias, the remedy is a correction, as in the Siemens lot-specific factors of that chapter, and the laboratory's own check is a lot-to-lot comparison on patient samples by the method of CLSI's EP26.

The EU's guidance for devices still in transition from the old directive, issued in May 2022, sorts changes into significant and not. Changing an essential ingredient such as a capture antibody, a new algorithm, a major change of operating system, or a move from visual to automated reading is significant; replacing an ELISA instrument, adjusting incubation time or temperature, fixing bugs or adding a manufacturing site is not. Where a maker is in doubt, it says, the maker should ask its notified body. The Commission's simplification proposal of December 2025, still not adopted in October 2026, would sort changes to certified devices into those needing no notification, prior notification or a notified body's approval, and would allow plans that agree certain changes in advance.

India's rules require prior approval for major changes after licensing, deemed granted if the licensing authority does not respond within 60 days, and notice of minor changes within 30 days. An addendum of 13 March 2026 to the regulator's questions and answers on IVDs states that each new version of approved software needs a post-approval change application.

54 — What a change triggers

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+ The questionAn analyzer never touches a patient. What can it harm, and which standards cover that?

A medical device with laboratory hazards

An immunoassay analyzer is a medical device, yet the standard that governs its electrical safety is written for laboratory equipment. Its particular part, IEC 61010-2-101 of 2018, sets requirements for in vitro diagnostic equipment, including equipment for self-testing, and it is used with the general laboratory standard, IEC 61010-1. The standard for medical electrical equipment, IEC 60601-1, is a different family. The puzzle dissolves once the question is asked the other way round: what could this machine harm?

It can harm the people who run it. An analyzer holds mains electricity, moving pipetting arms, heated incubators, magnets, chemicals such as the acidic peroxide and sodium hydroxide that trigger a flash label (Chapter 7), and blood that may carry infection; those are the hazards of laboratory equipment. It can disturb other equipment, or be disturbed by it, through electromagnetic interference. And it can harm a patient it never meets, through a wrong number. The FDA's overview of IVD regulation ties the safety of an IVD to its false-negative and false-positive results, and that third route is the one most particular to a diagnostic analyzer.

The wrong result as the hazard

Risk management for medical devices follows ISO 14971, whose 2019 edition was confirmed in 2025. Applied to an analyzer, it traces each fault forward to a wrong result, then to a situation in which the result is acted on, then to harm. The PCR guide in this library works through that chain for a thermal cycler. For an immunoassay analyzer the faults are often the ones earlier chapters met, and real cases show the chain end to end.

Carry-over is one. In 2021 Beckman Coulter recalled its high-sensitivity troponin I assay because, by the firm's account in the FDA's recall record, a sample above 270,000 ng/L could carry over through the probe into the particle well of the reagent pack; the record gives the root cause as undetermined. The fault as described is mechanical, the wrong result a falsely raised troponin in a later patient, and the harm a needless cardiac workup or a missed alternative diagnosis. The remedy was a procedure: after any such result, discard the open packs, load a new one, run low-level quality control on every pipettor and repeat the affected positive results. Biotin is another. In assays such as the thread's original version, the fault was a design tolerance as low as 20 ng/mL, the wrong result a falsely low troponin, and the harm of the kind the FDA reported in 2017, a death. One risk was controlled first by a warning in the instructions and later by a change to the reagent, the route Chapter 19 described.

55 — From a fault to a harm

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Safety and compatibility, as of October 2026

The general laboratory safety standard, IEC 61010-1, stands at its third edition of 2010 with an amendment of 2016. A second amendment, aligning it with the standard for audio, video and information technology equipment, was at the committee draft vote in April 2025, according to a test house; its publication could not be confirmed as of October 2026. The FDA recognizes the consolidated IEC 61010-1 with US national differences. Its database did not list the IVD part, IEC 61010-2-101, which a maker can still apply as the state of the art. The part for automatic laboratory equipment in general, IEC 61010-2-081, excludes IVD equipment and sends it to Part 2-101.

Electromagnetic compatibility has moved more recently. IEC 61326-2-6, the particular part for IVD equipment, reached its fourth edition in June 2025, and the FDA recognized that edition in full on 28 July 2025, having recognized the 2020 edition only in part. A maker designing an analyzer in 2026 therefore tests against an edition little more than a year old.

56 — Hazard, standard and status

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Software, usability and labels

The software that schedules an analyzer's steps and computes its results is developed under IEC 62304, still current in October 2026 as its 2006 edition amended in 2015 and recognized by the FDA. A second edition, widened to health software in general, remained a draft: the German standards association VDE described it in July 2026 as a working draft, expected to replace the three software safety classes with two levels of process rigor. One software firm reported that the new edition was published in August 2026, but the IEC's own catalogue listed no newer version, so this guide treats it as unpublished.

Usability engineering, IEC 62366-1 as amended in 2020, addresses the errors users make, and the FDA recognizes it. The information a maker must supply with its products is set out in the five parts of ISO 18113, all in second editions of October 2022, with Part 3 for instruments used by professionals and Part 5 for self-testing instruments; the FDA database does not list them. The symbols on labels follow ISO 15223-1 of 2021, whose 2025 amendment the FDA recognized in December 2025. Under all of these sits the quality management system: ISO 13485 of 2016, confirmed again in October 2025 and incorporated by reference into the FDA's Quality Management System Regulation, in force since 2 February 2026.

The regulators thus agree on what an analyzer must show, and differ on which documents carry formal weight. The FDA recognizes most of the IEC set, the EU leaves most of it outside its harmonized list, and both treat it as the established way to demonstrate safety, compatibility and sound software.

+ The questionWhat does each regulator, and the program that UN buyers of rapid tests rely on, ask of a reagent and its analyzer, as of October 2026?

The European Union, from 26 May 2022

On 26 May 2022 the EU's In Vitro Diagnostic Medical Devices Regulation, the IVDR, began to apply, five years after its publication. Its central mechanism is the one every route in this chapter shares: the risk a wrong result poses decides who must check the evidence before a test is sold. The IVDR sorts devices into four classes, A to D, by rules in an annex, and from class B upward, together with sterile class A devices, a notified body, a conformity assessment body designated under the regulation, must take part in assessing the maker's quality system and technical documentation, more fully for the higher classes.

The EU's guidance on classification, revised in September 2026, shows where the products of this guide fall. A troponin test for patients presenting acutely is class C, as a cardiac marker. Tests for self-testing are class C, except a short list, including pregnancy tests, that is class B. Instruments intended specifically for in vitro diagnostic procedures are class A, which the maker declares itself, and class B is the default for anything the rules do not place elsewhere. The analyzer and its troponin reagent therefore reach the market by different doors, as Chapter 24 showed.

The capacity to check has lagged behind the need. A Commission survey counted 19 notified bodies designated under the IVDR at the end of February 2026, which had received 3,418 applications and issued 2,318 certificates. A regulation of 2024 extended the transition for devices that need a notified body for the first time, on conditions and with deadlines that depend on the class.

57 — The IVDR transition, by class

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The rules may change again. In December 2025 the European Commission proposed to amend the IVDR and its companion regulation for other medical devices: replacing the five-year expiry of certificates with periodic reviews based on risk, reducing notified-body involvement for lower-risk devices, and allowing changes to be agreed in advance. As of October 2026 the proposal had not been adopted; the Council had issued a progress report in May, and the European Parliament's committee rapporteur had published a draft report in July.

The United States

The FDA also sorts devices into classes, I to III, and the class decides the route. A device similar to one already on the market reaches it through a 510(k), a demonstration of substantial equivalence to that predicate; a low- or moderate-risk device with no predicate goes through a De Novo request; and a class III device needs premarket approval. The thread's troponin assay, class II, was cleared through a 510(k). The first spinal fluid test for amyloid had no predicate and was granted through De Novo in May 2022, and it then served as the predicate for the blood test of Chapter 21.

A second decision follows clearance. Under the Clinical Laboratory Improvement Amendments, the law that governs US laboratories, the FDA assigns each test a complexity: waived, moderate or high. Seven criteria, each scored from 1 to 3, separate moderate from high complexity: a test whose scores sum above 12 is high complexity, at or below 12 moderate, and waived status is decided separately; until it is categorized, a test is treated as high complexity. The category decides which laboratories may run the test, so a maker designs for it as much as for the clearance.

Tests developed and run within a single laboratory have a separate history. A rule of May 2024 would have made explicit that such laboratory-developed tests are devices under the law. A federal court in Texas vacated it on 31 March 2025, and the FDA took the added words back out of its regulation in a rule published that September. As of October 2026 the Alzheimer's blood tests that Quanterix sells as laboratory-developed tests remained outside FDA clearance, while no decision on its 510(k) for one of them, submitted in early 2026, had been found.

India

India regulates IVDs under the Medical Devices Rules, 2017, which took effect at the start of 2018 and use the same four classes, A to D. The Central Licensing Authority, within the Central Drugs Standard Control Organisation (CDSCO), licenses all imports, the manufacture of class C and D devices, the clinical performance evaluation and approval of new IVDs, and test licenses; state licensing authorities license the manufacture of class A and B devices and their sale. An importer applies on Form MD-14 and receives a license on Form MD-15. Analyzers, instruments and software came under licensing in two phases, a year apart: the lower classes on 1 October 2022 and classes C and D on 1 October 2023, and the regulator's 2023 list puts a chemiluminescent immunoassay analyzer in class A.

India's distinctive requirement is local evidence. A new IVD in class B, C or D needs a clinical performance evaluation in India whatever its approvals in the EU, the US or elsewhere, and for imported kits the evaluation uses three batches made from three lots of key raw materials, at central or registered testing laboratories. The regulator updated its lists of those laboratories three times between June 2025 and June 2026, and in September 2025 it and the medical research council published standard evaluation protocols and a portal for kit validation. The class of a troponin reagent in its lists was not found for this guide.

WHO prequalification

The fourth route is not a regulator. The World Health Organization, which says of itself that it "is not a regulatory authority", assesses products for prequalification, a benchmark that UN and other international procurement agencies use when they buy, and that national regulators can rely on. Its program for IVDs became operational in 2010, growing out of a scheme for evaluating test kits begun in 1988. An assessment has three parts, a review of the dossier, an inspection of the manufacturing site and a review of the labeling, with an abridged dossier allowed where a recognized regulator has already assessed the product stringently. WHO has also made the laboratory evaluation of performance a separate procedure that comes first, done by designated evaluation laboratories.

Eligibility is limited to priority diseases, mainly infections such as HIV, hepatitis B and C, malaria, syphilis and tuberculosis, and a few point-of-care tests. There is no category for cardiac markers or for Alzheimer's disease, so neither of this guide's two cases has a route through it. In October 2026 the list held 125 prequalified IVDs. The entries from makers whose names indicate Indian companies were all rapid tests on strips, for HIV, hepatitis C, syphilis and malaria: the technology of Chapter 15 reaches the world's procurement through this door.

58 — Four routes, one evidence base

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Back to Basel

The blood drawn from 718 patients in the Basel-coordinated study between 2006 and 2008 asked nothing of a regulator. It was frozen, measured blind in batches, and judged against a diagnosis made from the records, which is the design of a clinical performance study. The paper appeared in August 2009, the assay reached laboratories in Britain and Ireland that year, the European guideline built a one-hour decision on such assays in 2015, and in January 2017 the FDA cleared the assay for the US, using the same 718 patients as part of its evidence. Nearly seven and a half years separated the paper from the clearance.

Every chapter of this guide is in that interval. The antibodies and the label set how few molecules the test could see; the analyzer held that floor from run to run; the calibration tied it to a scale; the studies showed its blank, its scatter and its interferents; and the regulators decided, each in its own way, what evidence was enough. A few molecules in a drop became a number a physician could act on in an hour only when all of those held at once.

The chapters reduce to thirteen working rules for people who design, verify, buy, regulate or write about immunoassays and the analyzers that run them. Each rule points to the chapters that explain why it holds.

1. Write the target as a count of molecules before choosing a method. A concentration of a few nanograms per liter is a few million molecules in a drop, and the count that reaches the detector is smaller still after capture, washing and sampling (Chapters 1, 17 and 18).
2. Name the epitopes and the circulating forms, not only the protein. Two antibody pairs against the same troponin can count different subsets of it, and assays with different epitopes and calibrators read the same blood differently (Chapters 2 and 14).
3. Budget the minutes of binding against antibody concentration and distance. Excess antibody buys speed, beads spread through the sample shorten diffusion, and a strip gives each part of the sample only seconds over its line (Chapters 3, 12 and 15).
4. Choose the format from the size of the molecule, and state the hook-free limit. Large proteins go between two antibodies and small molecules compete for one; a one-step sandwich can hook at extreme concentrations, so the limit belongs in the specification (Chapters 4, 13 and 19).
5. Treat the blank as the floor, and report every limit with the design that produced it. Label held without target sets the limit of blank, and limits of blank, detection and quantitation mean little without their replicates, lots, analyzers and days (Chapters 5, 18 and 23).
6. Judge a label by the background it leaves, not by its brightness alone. Color, chemiluminescence, delayed fluorescence and electrochemiluminescence each won by measuring against less unwanted light, and an amplifying label amplifies wrongly bound label too (Chapters 6 to 9).
7. Specify the analyzer as a chain of subsystems, and test the joins. Sampling, reagent handling, incubation, washing, detection and software each move the number, and carry-over and temperature drift can escape routine controls (Chapters 10 to 12).
8. Specify the detector from both ends of the range. Dark count and light collection set the detector's share of the blank, counting statistics set the scatter of small signals, and saturation and dilution set the top (Chapter 13).
9. Tie every result to its calibration chain and to the limits of its own assay. A master curve adjusted on site, a lot change or a new generation can move results, and troponin cut-offs cannot be carried from one assay to another (Chapters 14 and 20).
10. Validate the reader with the test, whether it is a person or a machine. The same strips gave different sensitivities when read by different people, and an instrument reader applies its threshold equally well to a strip that ran badly (Chapters 15 and 16).
11. Ship the interference checks with the assay, and trigger them on disagreement with the patient. Heterophile antibodies, biotin, macrocomplexes and the hook produce plausible numbers, which a mismatch with the patient, or between methods, exposes (Chapter 19).
12. Count the middle zone and the prevalence in every performance claim. An observation zone or an indeterminate result moves work instead of removing it, and predictive values change with the share of patients who have the condition (Chapters 20 and 21).
13. Plan the evidence once for the whole system, then file it by each route. Regulators judge the reagent, calibrators, analyzer, software and label together, sort them by risk in different ways, and change their rules on calendars a maker does not control (Chapters 23 to 26).

Terms are defined as they are used in this guide.

510(k)

A US premarket submission showing that a device is substantially equivalent to a similar device already on the market, its predicate; the guide's troponin T assay and the first Alzheimer's blood test were cleared this way.

99th-percentile upper reference limit

The troponin value below which 99 percent of a healthy reference population falls; above it, troponin is a sign of injury to heart muscle. For the guide's troponin T assay in the US it is 19 ng/L overall, 14 ng/L for women and 22 ng/L for men.

Absorbance

A logarithmic measure of the light a colored solution removes from a beam, read by a photometer or plate reader: 1 means a tenth of the light got through, 2 a hundredth and 3 a thousandth. Readers stay linear only to about 2 or 3.

Acridinium ester

A chemiluminescent label that reacts once with hydrogen peroxide in alkali and gives its light in a flash lasting a few seconds; Abbott and Siemens Healthineers analyzers use it.

AEB

Average enzymes per bead: the unit in which a digital ELISA reports its result, worked out from the share of beads that light.

Amyloid pathology

Deposits in the brain of the protein fragment beta-amyloid, assessed in clinical practice by a PET scan or a spinal fluid test. The cleared Alzheimer's blood tests aid in assessing it; they do not diagnose the disease.

Analyte

The substance an assay measures; this guide mostly calls it the target.

Antibody

A protein the immune system makes to bind one molecular shape. The kind used in immunoassays, immunoglobulin G, is a Y-shaped protein of about 146,000 grams per mole with an identical binding site at the tip of each arm.

Assay

A procedure that measures how much of a particular substance a sample contains.

Assay migration

Moving a cleared or approved assay to a new combination of instrument and software, supported by focused analytical studies instead of new clinical trials; the intended use must stay the same, and no claim can be added.

Association and dissociation rate constants

The rate at which two molecules bind, typically about a million per molar per second for proteins, and the rate at which a bound pair comes apart, between about 1 in 1,000 and 1 in 100,000 per second for good antibodies. Their ratio is the dissociation constant.

Beta-amyloid

A protein fragment that forms deposits in the brain; its 42-amino-acid form, beta-amyloid 1-42, is measured in plasma beside pTau217 in the first cleared Alzheimer's blood test.

Biotin

A small B vitamin attached to an antibody so that streptavidin will grab it. Free biotin from high-dose supplements occupies the streptavidin, so assays built on it read falsely low in a sandwich and falsely high in a competitive format.

Blocker

An inert protein, such as bovine serum albumin or milk protein, that coats any surface the capture antibody left bare; added animal serum or mouse antibody likewise soaks up a patient's interfering antibodies.

Calibration curve

The curve, drawn through calibrators, that converts an analyzer's signal into a concentration. For an immunoassay it is S-shaped, flat near zero and near the top, and in a competitive assay it slopes downward.

Calibrator

A sample with an assigned concentration, measured to draw or adjust the calibration curve; calibration is repeated with each new reagent lot, at set intervals and whenever quality control fails.

Capillary flow

The pull that draws liquid through a porous material, as water climbs a paper towel. On a lateral flow strip it moves the sample and sets the timing, slowing as the wetted length grows.

Carry-over

The transfer of material from one sample, or one reagent, into the next, reported as a fraction of the high sample, such as parts per million; routine quality control does not reveal it.

CDSCO

The Central Drugs Standard Control Organisation, India's regulator; its Central Licensing Authority licenses all imported IVDs, the manufacture of class C and D devices and the approval of new IVDs.

CE mark

The marking under which a device is sold in the EU; under the IVDR most IVDs need a notified body's assessment first, while the maker of a class A instrument declares conformity itself.

Chemiluminescence

Light from a chemical reaction whose product forms in an excited state. Measured against darkness, with no lamp, it reaches lower concentrations and a wider range than color.

CLIA categorization

The FDA's assignment of a test to waived, moderate or high complexity under the Clinical Laboratory Improvement Amendments, which decides which US laboratories may run it. Seven criteria are each scored 1 to 3, and a sum above 12 means high complexity.

Clinical performance study

A study that judges a test's results against a reference diagnosis of each patient, as the Basel troponin study did; in the EU it follows ISO 20916, and India requires a local clinical performance evaluation for new class B to D IVDs.

Closed and open system

An assay that runs only on its maker's analyzers is a closed system; one whose maker validates it on other instruments is open, at least to that maker. The FDA's 2022 instrument family policy applies only to open systems.

CLSI

The Clinical and Laboratory Standards Institute, a US standards body for laboratory medicine; its EP documents, such as EP05 for precision and EP17 for detection limits, set study designs that the FDA recognizes.

Coefficient of variation

The standard deviation of repeated results divided by their mean, as a percentage; a high-sensitivity troponin assay must reach 10 percent or less at the 99th percentile.

Colloidal gold

Gold particles a few tens of nanometers across, the most common label on lateral flow strips; particles of 20 to 40 nanometers absorb most strongly in the green, at 524 to 530 nanometers, so a line of them looks red.

Competitive assay

An immunoassay in which a labeled copy of the target competes with the target in the sample for a scarce antibody, so the signal falls as the concentration rises. Small molecules, too small to hold two antibodies, are measured this way, as was insulin in the first radioimmunoassay.

Coreactant

The chemical, tripropylamine in the guide's troponin assay, that sits in large excess beside an electrochemiluminescent label; oxidized at the electrode, it breaks into short-lived fragments that excite the label.

Cross-reactivity

Binding of an antibody to a molecule other than its intended target, such as skeletal troponin T in a cardiac assay or a drug's breakdown products in a competitive assay.

Cut-off

A value that divides results into answers, such as positive and negative, or rule-out, observation and rule-in; troponin cut-offs belong to one assay and cannot be carried to another.

Dark count

Pulses a detector gives with no light falling on it, from electrons released by heat in the cathode; about 50 a second in one photon-counting module, it is the detector's own floor.

De Novo

The US route for a new low- or moderate-risk device with no predicate; the first spinal fluid test for amyloid was granted this way in May 2022.

Decision summary

The FDA's published summary of the review behind a clearance, the source of most of this guide's figures for cleared assays.

Digital ELISA

An immunoassay that seals beads into wells of tens of femtoliters, so that a single enzyme lights its well, and counts the lit wells instead of measuring a glow. Its analyzers are labeled for research use only.

Dioxetane

A light-producing substrate for alkaline phosphatase: the enzyme removes a phosphate group, and the unstable remainder falls apart and emits light, glowing for minutes.

Dissociation constant

The concentration of target at which half of an antibody's binding sites are occupied at equilibrium; smaller is tighter. With antibody at twenty times this constant, more than 90 percent of a scarce target is bound.

Electrochemiluminescence

Light from a label, a ruthenium complex in the guide's troponin assay, driven by a voltage at an electrode together with a coreactant. Each label returns to its starting form and can emit many photons, and only labels within about 3 micrometers of the electrode light.

ELISA

Enzyme-linked immunosorbent assay: an immunoassay read by the color an enzyme label makes from its substrate, first published in 1971 and now most often a sandwich in the wells of a plastic plate.

Energy transfer (TRACE)

A method in which a europium label on one antibody passes its energy to an acceptor on a second antibody only when both bind the same target molecule, so bound and free label are told apart without washing; Thermo Fisher Scientific's KRYPTOR analyzers use it under the name TRACE.

Enzyme label

An enzyme, usually horseradish peroxidase or alkaline phosphatase, attached to an antibody so that each bound antibody turns a substrate into many molecules of colored, fluorescent or light-giving product.

Epitope

The patch of a target's surface that an antibody touches, typically about 20 amino acids; a protein the size of troponin has room for several antibodies, each at its own epitope.

HAMA

Human anti-mouse antibodies: heterophile antibodies directed at mouse proteins, which matter because many reagent antibodies are made in mice.

Heterophile antibody

A patient's antibody that binds the antibodies of other animals. Bridging the capture and labeled antibodies, it gives a falsely high result; binding only one of them, it can give a falsely low one.

High-dose hook

The fall in signal at very high target concentrations in a one-step sandwich, when free target fills the capture and labeled antibodies separately, so an enormous concentration can read as a modest one. Makers state a hook-free limit, 100,000 ng/L for the guide's troponin T assay.

High-sensitivity troponin assay

A troponin assay with a coefficient of variation of 10 percent or less at the 99th percentile and results above its limit of detection in at least half of healthy people, as defined by a task force of the International Federation of Clinical Chemistry in 2017.

IEC 61010

The safety standard for laboratory equipment, which analyzers follow instead of the medical electrical standard IEC 60601-1: Part 1 covers laboratory equipment in general, and Part 2-101, of 2018, in vitro diagnostic equipment.

IEC 61326-2-6

The electromagnetic compatibility standard for IVD equipment, covering how an instrument disturbs other equipment or is disturbed by it; its fourth edition, of June 2025, was recognized in full by the FDA on 28 July 2025.

IEC 62304

The standard for developing medical device software, such as the software that schedules an analyzer's steps and computes its results; in October 2026 its 2006 edition, amended in 2015, was still current.

Immunoassay

An assay in which the part that recognizes the substance being measured is an antibody.

Indeterminate result

A result between two cut-offs, which gives neither a positive nor a negative answer and sends the patient to a confirmatory test; about a fifth of results in the first cleared Alzheimer's blood test.

Interference

A change in a result caused by something in the sample other than the target, such as biotin, heterophile antibodies, rheumatoid factor or molecules related to the target; the substance responsible is an interferent.

International Federation of Clinical Chemistry (IFCC)

The international body whose working groups and task forces compare troponin assays, publish a table of their characteristics supplied by the makers, and in 2017 defined a high-sensitivity troponin assay.

ISO 13485

The international standard for the quality management systems of medical device makers, in its 2016 edition; the FDA's Quality Management System Regulation (QMSR), in force since 2 February 2026, incorporates it by reference.

ISO 14971

The standard for risk management of medical devices, in a 2019 edition; applied to an analyzer, it traces each fault to a wrong result, to a situation in which that result is acted on, and to harm.

IVD

In vitro diagnostic device: a reagent, calibrator, analyzer or other product used to test samples, such as blood, outside the body.

IVDR

The EU's In Vitro Diagnostic Medical Devices Regulation, applied since 26 May 2022; it sorts IVDs into classes A to D and requires a notified body for around 80 percent of them, against about 8 percent under the 1998 directive it replaced.

Label

The part of an immunoassay that makes a bound antibody visible: an enzyme that makes color, a molecule that gives off light, or a particle that shows as a colored line.

Laboratory-developed test

A test developed and run within a single laboratory. A 2024 FDA rule that would have made such tests explicitly devices was vacated by a federal court in March 2025.

Lateral flow test

An immunoassay run inside a strip of porous material: the sample moves by capillary flow, dissolves a dried labeled antibody and carries it past a printed test line of capture antibody and then a control line.

Limit of blank

The highest result expected from samples that contain none of the target, set so that only 5 percent of blank results exceed it; in its simplest form, the mean of the blanks plus 1.645 standard deviations.

Limit of detection

The lowest concentration whose results exceed the limit of blank 95 percent of the time; 3 to 5 ng/L, depending on the analyzer, for the guide's troponin T assay in the US.

Limit of quantitation

The lowest concentration measured with an agreed precision, often a coefficient of variation of no more than 20 percent; 6 ng/L for the guide's troponin T assay in the US.

Macrocomplex

A target bound to the patient's own antibodies, such as macroprolactin, which is prolactin bound mostly to immunoglobulin G, or troponin bound to antibodies; it can read high on one assay and normal on another.

Magnetic microparticles

Beads a few micrometers across, typically coated with streptavidin, that collect sandwiches throughout the reaction volume and are held by a magnet while the liquid around them is exchanged.

Master curve

A calibration curve the maker measures for each reagent lot, from master calibrators on many instruments, and encodes with the lot; the analyzer adjusts it with a two-level calibrator set.

Measuring range

The span of concentrations an assay reports without dilution, across which results are claimed to stay proportional to concentration; 6 to 10,000 ng/L for the guide's troponin T assay in the US.

Medical Device Coordination Group (MDCG)

The EU group that issues guidance under the IVDR; its 2022 guidance divides the evidence for an IVD into scientific validity, analytical performance and clinical performance.

Medical Devices Rules, 2017

The rules under which India regulates IVDs, in effect since the start of 2018, with four classes, A to D, and licensing divided between the CDSCO and state authorities.

Mole

An amount of substance holding about 6.02 × 10²³ molecules; an attomole, a billionth of a billionth of a mole, is about 602,000 molecules. One mole per liter is one molar, so picomolar means trillionths of a mole per liter.

Monoclonal antibody

An antibody made by one line of cells, a hybridoma, that binds one epitope. Unlike a polyclonal antibody, the varying mixture in an immunized animal's serum, it can be supplied unchanged for decades, which made reagent lots consistent.

Multiplex assay

An assay that measures several targets in one sample. Each added target adds unintended antibody pairs that can cross-react, so a floor measured for one target alone does not carry over to a panel.

Notified body

An independent organization, designated by an EU member state, that assesses a maker's quality system and technical documentation before an IVD from class B upward can be sold; 19 were designated under the IVDR at the end of February 2026.

One-hour algorithm

The European Society of Cardiology's rule for ruling a heart attack in or out from two troponin samples an hour apart, using the first value and the change between them, with cut-offs specific to each assay; patients in neither group are observed.

PET

Positron emission tomography: a brain scan that, with a spinal fluid test, is how amyloid pathology is assessed in clinical practice.

Photomultiplier tube

A vacuum-tube detector in which a photon striking the photocathode releases an electron, and a chain of dynodes multiplies it into a pulse of a million to ten million electrons.

Poisson distribution

The pattern of counts of random events, which fixes, for example, the share of beads carrying no enzyme from the average number per bead. A count that follows it scatters by its own square root: 100 counts by 10 percent, 10,000 by 1 percent.

Positive and negative predictive value

The share of positive results that are right and the share of negative results that are right; both depend on prevalence among the people tested.

Premarket approval

The US route to market for a class III device, the highest of the FDA's three classes.

Prevalence

The share of the people tested who have the condition; 17 percent had a heart attack in the 2015 study of the one-hour algorithm.

Proximity ligation and extension assays

Methods in which two binders carrying DNA strands must attach to the same molecule before their strands yield a piece of DNA that is copied and measured; a binder on its own leaves nothing to copy, so background falls and many assays can share one sample.

pTau217

Tau protein carrying a phosphate group at amino acid 217, in forms that circulate as fragments, at fractions of a picogram per milliliter of plasma; measured to aid in assessing amyloid pathology.

Quality control

Samples of known concentration run at intervals and plotted on charts to catch drift; a run is rejected under rules such as one control more than three standard deviations from its target, or two in a row more than two on the same side.

Quantum efficiency

The fraction of photons striking a detector's cathode that release an electron; up to 43 percent at 350 nanometers for the best blue-sensitive cathodes, and much lower in the red.

Reader

An instrument that reads an assay's signal. A plate reader measures absorbance in many wells at once; a strip reader measures each line against the strip's own background with its own light and applies the same threshold every time.

Recall

A maker's correction or removal of a product already on the market, recorded by the FDA; the remedies in this guide include new instructions for laboratories and lot-specific correction factors.

Reference material

A material whose value is known well enough to anchor a calibration chain; it serves only if it behaves like patient samples in the assays that use it.

Rheumatoid factor

An antibody that binds the stem of other antibodies; it can bridge reagent antibodies like a heterophile antibody and cause false positives in troponin assays.

Sandwich assay

An immunoassay in which a capture antibody and a labeled antibody bind two different epitopes of the same target, so the signal rises with concentration from near zero; also called a two-site assay.

Sensitivity

Clinically, the share of people with the condition whose test is positive, such as 40 percent for the Liverpool strips read on site. Analytically, as in a more sensitive assay, how low a concentration a test can detect.

Solid phase

The surface that holds the capture antibody, such as the inside of a tube or well or the surface of beads, so that bound label stays while free label is washed away.

Specificity

Clinically, the share of people without the condition whose test is negative, 99.9 percent for the Liverpool strips. For an antibody, its selective binding to one shape, which weakens quickly as a molecule departs from that shape.

Standard deviation

The usual measure of the scatter of repeated results around their mean.

Streptavidin

A bacterial protein that grips biotin almost irreversibly; coated on beads, it collects the biotin-carrying capture antibody together with the sandwich it holds.

Time-resolved fluorescence

Reading a long-lived label such as europium, which glows for tens of microseconds to about a millisecond, after a delay long enough for scattered light and the sample's own fluorescence, which fade in about 10 nanoseconds, to disappear.

Traceability

The link, through an unbroken chain of comparisons, from an assay's calibrators to an accepted reference, ideally a reference measurement procedure and a certified reference material; troponin I still lacks such a chain end to end.

Troponin

A complex of three proteins, T, I and C, that controls muscle contraction; the cardiac forms of troponin T and I leak into the blood when heart cells die and are measured to rule a heart attack in or out.

Wash

Rinsing away everything that did not bind while the surface keeps what did, so that only label held by the target is left to measure; it works by repeated dilution, not as a filter.

WHO prequalification

The World Health Organization's assessment of an IVD as a benchmark for UN and other international buyers, limited to priority diseases such as HIV, hepatitis B and C, malaria, syphilis and tuberculosis; WHO is not a regulator, so registration stays national.

Within-laboratory precision

The total scatter of results a laboratory sees over weeks, combining repeatability within a run with scatter between runs and between days; CLSI's EP05 estimates it from 80 results per sample over 20 days.

Each source is listed under the chapter whose text first relies on it, with the opening scene and the front matter first. Company documents and papers by a company's own staff are marked, as they are in the text. Regulatory status, standards editions and product facts are as of October 2026.

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182. Ch. 20 — Clerico A., Lippi G. The state-of-the-art of "high-sensitivity" immunoassay for measuring cardiac troponin I and T. J Lab Precis Med 3, 53 (2018) (editorial; summarises the IFCC task force criteria first proposed by Apple F.S., Collinson P.O., Clin Chem 58, 54–61 (2012), which was not opened) (secondary)
183. Ch. 21 — US Food and Drug Administration. De Novo DEN200072, Lumipulse G β-Amyloid Ratio (1-42/1-40) (Fujirebio Diagnostics): database record and decision summary; granted 4 May 2022. accessdata.fda.gov/cdrh_docs/reviews/DEN200072.pdf
184. Ch. 21 — Fujirebio. Lumipulse G pTau 217/β-Amyloid 1-42 Plasma Ratio, product page (accessed 8 Oct 2026). fujirebio.com [manufacturer]
185. Ch. 21 — Palmqvist S., …, Tampi M.P. Clinical practice guideline for blood-based biomarkers in the diagnostic workup of Alzheimer’s disease within specialized care settings. Alzheimers Dement 21(7), e70535 (2025). doi:10.1002/alz.70535 (not opened; thresholds as reported in the Alzheimer’s Association AAIC release of 29 Jul 2025, its guideline page and a Barcelonaβeta summary) (secondary)
186. Ch. 21 — Fujirebio. Fujirebio announces CE marking of the fully automated Lumipulse G pTau 217 Plasma assay. Press release, Ghent and Tokyo, 11 May 2026. fujirebio.com/en/news-events/fujirebio-announces-ce-marking-of-the-fully-automated-lumipulser-g-ptau-217-plasma [manufacturer]
187. Ch. 22 — CAP TODAY. Chemistry and immunoassay analyzers, mid- and high-volume: product guide (July 2026; vendor-supplied data). captodayonline.com/2026/ProductGuides/CAPTODAY_ChemImmuno_MidHigh_ProductGuide_0726.pdf (secondary)
188. Ch. 22 — Roche Holding AG. Roche reports strong 2025 results with 7% sales growth. Media release (ad hoc announcement), 29 Jan 2026. roche.com/media/releases/med-cor-2026-01-29 [manufacturer]
189. Ch. 22 — US Food and Drug Administration. 510(k) K242870, Access hsTnI on the DxC 500i (Beckman Coulter): decision summary. accessdata.fda.gov/cdrh_docs/reviews/K242870.pdf
190. Ch. 22 — Beckman Coulter. Beckman Coulter unveils next generation immunoassay analyzer enabling elite laboratory performance with no daily maintenance. Press release, 15 May 2023. news.beckmancoulter.com [manufacturer]
191. Ch. 22 — Fujirebio. LUMIPULSE G1200, product page (accessed 8 Oct 2026). fujirebio.com/en/products-solutions/lumipulse-g1200 [manufacturer]
192. Ch. 22 — Mindray. CL-2000i chemiluminescence immunoassay analyzer, India product page (accessed 8 Oct 2026). mindray.com/in/products/laboratory-diagnostics/chemiluminescence-immunoassay/medium-test-volume/cl-2000i/ [manufacturer]
193. Ch. 22 — QuidelOrtho. VITROS ECiQ Immunodiagnostic System, product page (accessed 8 Oct 2026). quidelortho.com/global/en/products/vitros-systems/vitros-eciq-immunodiagnostic-system [manufacturer]
194. Ch. 22 — Snibe. MAGLUMI X8 and MAGLUMI M Series, product pages (accessed 8 Oct 2026). snibe.com/en/product/CLIA_analyzer/40.html; snibe.com/en/product/CLIA_analyzer/635.html [manufacturer]
195. Ch. 22 — bioMérieux. VIDAS KUBE, product page and brochure (accessed 8 Oct 2026). biomerieux.com/corp/en/our-offer/clinical-products/vidas-kube.html [manufacturer]
196. Ch. 22 — Quanterix. Quanterix completes acquisition of Akoya Biosciences. Press release (Business Wire, 7 Jul 2025). businesswire.com/news/home/20250707828058/en/ [manufacturer]
197. Ch. 22 — Quanterix. Quanterix releases financial results for the fourth quarter and full year 2025. Press release (via BioSpace, Mar 2026). biospace.com [manufacturer]
198. Ch. 22 — Roche. Media release, 29 Dec 2023, on the agreement to acquire the parts of LumiraDx tied to its point-of-care platform. roche.com/media/releases/med-cor-2023-12-29 [manufacturer]
199. Ch. 22 — Fujirebio. Fujirebio and Agappe enter into an agreement on business collaboration in the field of CLIA based immunoassay. News release, 19 Jan 2024. fujirebio.com/en/news-events [manufacturer]
200. Ch. 22 — Agappe Diagnostics. Corporate brochure 2024. agappe.com/media/Brochure/default/Corporate_Brochure_2024_latest.pdf [manufacturer]
201. Ch. 22 — J. Mitra & Co. Home page and CLIA analyzer page (accessed 8 Oct 2026). jmitra.co.in; jmitra.co.in/products/clia-analyzer/ [manufacturer]
202. Ch. 22 — Transasia Bio-Medicals. Lisa XL automated ELISA processor, product page (accessed 8 Oct 2026). transasia.co.in/instrument/immunology/lisa-xl/95 [manufacturer]
203. Ch. 22 — Roche. Investor update, 24 Aug 2026, on the FDA clearance of the Elecsys pTau217 plasma test. roche.com/investors/updates/inv-update-2026-08-24 [manufacturer]
204. Ch. 22 — 360Dx. Siemens Healthineers outlines diagnostics carve-out; business weighs down fiscal Q2 (2026). 360dx.com/business-news/siemens-healthineers-outlines-diagnostics-carve-out-business-weighs-down-fiscal-q2-0 (secondary)
205. Ch. 22 — Fujirebio. Fujirebio and Sysmex enter into basic agreement on business collaboration in the field of immunoassay. News release (undated, early 2024). fujirebio.com/en/news-events [manufacturer]
206. Ch. 23 — European Commission. Proposal for a Regulation amending Regulation (EU) 2017/746 as regards transitional provisions for certain in vitro diagnostic medical devices and the deferred application of requirements for in-house devices. COM(2021) 627 final, 14 Oct 2021, explanatory memorandum. eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:52021PC0627
207. Ch. 23 — European Commission. Notified Bodies survey on certifications and applications (MDR/IVDR), 20th survey, data to 28 Feb 2026 (presentation dated 2 Jul 2026). health.ec.europa.eu
208. Ch. 23 — CLSI. Evaluation of Detection Capability Implementation Guide (EP17-Ed2-IG), sample pages (undated). shop.clsi.org/media/fjedgxkz/ep17ed2ige_sample.pdf
209. Ch. 23 — CLSI. EP05-A3, Evaluation of Precision of Quantitative Measurement Procedures, 3rd ed. (1 Oct 2014; reaffirmed 2019). Catalogue page, clsi.org/shop/standards/ep05/
210. Ch. 23 — CLSI. EP15-A3, User Verification of Precision and Estimation of Bias, 3rd ed. (11 Sep 2014; reaffirmed 2019). Catalogue page, clsi.org/shop/standards/ep15/
211. Ch. 23 — CLSI. EP06, Evaluation of Linearity of Quantitative Measurement Procedures, 2nd ed. (24 Nov 2020). Catalogue page, clsi.org/shop/standards/ep06/
212. Ch. 23 — CLSI. EP07, Interference Testing in Clinical Chemistry, 3rd ed. (30 Apr 2018; reaffirmed 2022), and EP37, Supplemental Tables for Interference Testing in Clinical Chemistry (1st ed. 2018; updated 3 Jul 2025). Catalogue pages, clsi.org/shop/standards/ep07/; clsi.org/shop/subscription-products/ep37/
213. Ch. 23 — CLSI. EP09, Measurement Procedure Comparison and Bias Estimation Using Patient Samples, 3rd ed. (20 Jun 2018). Catalogue page, clsi.org/shop/standards/ep09/
214. Ch. 23 — CLSI. EP28-A3c, Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory (19 Oct 2010; reaffirmed 2020). Catalogue page, clsi.org/shop/standards/ep28/
215. Ch. 23 — Analyse-it Software Ltd. Choosing a reference interval method (web page, undated). analyse-it.com/learn/choosing-a-reference-interval-method (secondary)
216. Ch. 23 — LaValley C., Graff J.P. Reference intervals: the process to verify outside sources. UC Davis Health, Lab Best Practice blog (Apr 2022). health.ucdavis.edu/blog/lab-best-practice (secondary)
217. Ch. 23 — US Food and Drug Administration. Recognized Consensus Standards database: CLSI EP and I/LA documents (accessed 8 Oct 2026). accessdata.fda.gov/scripts/cdrh/cfdocs/cfStandards/search.cfm
218. Ch. 23 — Medical Device Coordination Group. MDCG 2022-2, Guidance on general principles of clinical evidence for in vitro diagnostic medical devices (Jan 2022). health.ec.europa.eu/system/files/2022-01/mdcg_2022-2_en.pdf
219. Ch. 23 — ISO. ISO 20916:2019, In vitro diagnostic medical devices: clinical performance studies using specimens from human subjects: good study practice (published 4 Jun 2019; confirmed 2025), catalogue page. iso.org/standard/69455.html
220. Ch. 24 — US Food and Drug Administration. Medical Device Recall Z-1671-2018, Dimension Vista CTNI Flex reagent cartridge (Siemens Healthcare Diagnostics; 510(k) K063756), Class 2; initiated 2 Jan 2018. accessdata.fda.gov/scripts/cdrh/cfdocs/cfRes/res.cfm?id=162500
221. Ch. 24 — US Food and Drug Administration. Replacement Reagent and Instrument Family Policy for In Vitro Diagnostic Devices. Guidance (17 Aug 2022). fda.gov/media/111186/download
222. Ch. 24 — US Code of Federal Regulations (eCFR). 21 CFR 862.1215 (creatine phosphokinase/creatine kinase or isoenzymes test system) and 21 CFR 862.9 (limitations of exemptions) (accessed 8 Oct 2026). ecfr.gov
223. Ch. 24 — Medical Device Coordination Group. MDCG 2020-16 rev.5, Guidance on classification rules for in vitro diagnostic medical devices under Regulation (EU) 2017/746 (9 Sep 2026). health.ec.europa.eu
224. Ch. 24 — Central Drugs Standard Control Organisation, India. Classification list of IVD medical devices (updated 25 Oct 2023; file IVD/Misc/196/2020). cdsco.gov.in/opencms/export/sites/CDSCO_WEB/Pdf-documents/medical-device/IVD_classifiction25oc23.pdf
225. Ch. 24 — US Food and Drug Administration. Assay Migration Studies for In Vitro Diagnostic Devices. Guidance (25 Apr 2013). fda.gov/media/73669/download
226. Ch. 24 — US Food and Drug Administration. Deciding When to Submit a 510(k) for a Change to an Existing Device. Guidance (25 Oct 2017), with its companion guidance on software changes. fda.gov/media/99812/download
227. Ch. 24 — Medical Device Coordination Group. MDCG 2022-6, Guidance on significant changes regarding the transitional provision under Article 110(3) of the IVDR (May 2022). health.ec.europa.eu/system/files/2022-05/mdcg_2022-6.pdf
228. Ch. 24 — European Commission. Proposal to amend Regulations (EU) 2017/745 and 2017/746 (simplification), COM(2025) 1023 final, 2025/0404(COD), 16 Dec 2025, as transmitted in Council document ST 16919/25 (18 Dec 2025). data.consilium.europa.eu/doc/document/ST-16919-2025-INIT/en/pdf
229. Ch. 24 — Austrian Parliament, EU database. Council progress report on the MDR/IVDR simplification proposal, RAT 9114/26 (27 May 2026). parlament.gv.at/gegenstand/XXVIII/EU/74474
230. Ch. 24 — Arnold & Porter. How will the EU Parliament shape the MDR/IVDR revision? (via Mondaq, 7 Aug 2026). mondaq.com (secondary)
231. Ch. 24 — Central Drugs Standard Control Organisation, India. IVD Medical Devices FAQ, Doc No. CDSCO/IVD/FAQ/04/2022, with addendum of 28 Mar 2025. cdsco.gov.in/opencms/export/sites/CDSCO_WEB/Pdf-documents/FAaddendum.pdf
232. Ch. 24 — Central Drugs Standard Control Organisation, India. Addendum No. 02 to Doc No. CDSCO/IVD/FAQ/04/2022 (13 Mar 2026). cdsco.gov.in
233. Ch. 25 — IEC. IEC 61010-2-101:2018, ed. 3.0 (5 Oct 2018), particular safety requirements for in vitro diagnostic medical equipment. IEC Webstore page, webstore.iec.ch/en/publication/60197
234. Ch. 25 — IEC. IEC 60601-1:2005+AMD1:2012+AMD2:2020 CSV, consolidated ed. 3.2 (20 Aug 2020). IEC Webstore page, webstore.iec.ch/en/publication/67497
235. Ch. 25 — US Food and Drug Administration. Overview of IVD regulation (web page, accessed 8 Oct 2026). fda.gov/medical-devices/ivd-regulatory-assistance/overview-ivd-regulation
236. Ch. 25 — ISO. ISO 14971:2019, application of risk management to medical devices, 3rd ed. (10 Dec 2019; confirmed 2025), catalogue page. iso.org/standard/72704.html
237. Ch. 25 — IEC. IEC 61010-1:2010/AMD1:2016 (published 16 Dec 2016). IEC Webstore page, webstore.iec.ch/en/publication/33314
238. Ch. 25 — SIQ Ljubljana. IEC 61010-1 Amendment 2: upcoming changes to safety requirements (3 Apr 2025). siq.si/en/news (secondary)
239. Ch. 25 — IEC. IEC 61010-2-081:2015, ed. 2.0 (23 Jan 2015; superseded by a 2019 edition). IEC Webstore page, webstore.iec.ch/en/publication/22287
240. Ch. 25 — IEC. IEC 62304:2006+AMD1:2015 CSV, ed. 1.1 (26 Jun 2015). IEC Webstore page, webstore.iec.ch/en/publication/22794
241. Ch. 25 — VDE. IEC 62304 Edition 2: Änderungen für Hersteller (22 Jul 2026). vde.com/iec-62304-edition-2-aenderungen (secondary)
242. Ch. 25 — Critical Software. IEC 62304 Edition 2 changes (newsroom item, Aug 2026). asd.criticalsoftware.com/en/newsroom/iec-62304-edition-2-changes-august-2026 (secondary)
243. Ch. 25 — ISO. ISO 18113-1, ISO 18113-3 and ISO 18113-5:2022, information supplied by the manufacturer for IVD medical devices, 2nd eds. (6 Oct 2022), catalogue pages. iso.org/standard/79866.html; iso.org/standard/79868.html; iso.org/standard/79870.html
244. Ch. 25 — ISO. ISO 15223-1:2021, symbols to be used with information to be supplied by the manufacturer, 4th ed. (6 Jul 2021), with Amendment 1:2025, catalogue page. iso.org/standard/77326.html
245. Ch. 25 — ISO. ISO 13485:2016, medical devices: quality management systems, 3rd ed. (25 Feb 2016; confirmed 31 Oct 2025), catalogue page. iso.org/standard/59752.html
246. Ch. 25 — US Code of Federal Regulations (eCFR). 21 CFR Part 820, Quality Management System Regulation (final rule 89 FR 7523, 2 Feb 2024; effective 2 Feb 2026); and US Food and Drug Administration, Quality Management System Regulation (QMSR) web page. ecfr.gov; fda.gov
247. Ch. 25 — Commission Implementing Decision (EU) 2021/1195 on harmonised standards for in vitro diagnostic medical devices, consolidated text to 9 Oct 2024. eur-lex.europa.eu/eli/dec_impl/2021/1195/2024-10-09/eng/pdf
248. Ch. 25 — European Commission. Harmonised standards: in vitro diagnostic medical devices (web page), with Implementing Decisions (EU) 2026/197 and 2026/1313 (landing pages and titles read). single-market-economy.ec.europa.eu
249. Ch. 26 — Regulation (EU) 2017/746 of the European Parliament and of the Council of 5 April 2017 on in vitro diagnostic medical devices. OJ L 117, 176–332 (5 May 2017). eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32017R0746 (read to Article 24)
250. Ch. 26 — Regulation (EU) 2024/1860 of 13 June 2024, amending the transitional provisions of Regulation (EU) 2017/746. OJ L (9 Jul 2024). eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32024R1860
251. Ch. 26 — US Food and Drug Administration. CLIA categorizations (web page, accessed 8 Oct 2026). fda.gov/medical-devices/ivd-regulatory-assistance/clia-categorizations
252. Ch. 26 — US Food and Drug Administration. Medical Devices; Laboratory Developed Tests; Implementation of Vacatur. Final rule. Federal Register 90, 45134 (19 Sep 2025), FR Doc. 2025-18217. govinfo.gov/content/pkg/FR-2025-09-19/pdf/2025-18217.pdf
253. Ch. 26 — Ministry of Health and Family Welfare, India. Medical Devices Rules, 2017, G.S.R. 78(E) (31 Jan 2017). main.mohfw.gov.in/sites/default/files/Medical%20Device%20Rules%2C%202017.pdf
254. Ch. 26 — Central Drugs Standard Control Organisation, India. IVD FAQ, Doc No. CDSCO/IVD/FAQ/03/2022. cdsco.gov.in/opencms/export/sites/CDSCO_WEB/Pdf-documents/IVD/FAQs/CDSCO-IVD-FAQ-03-2022-.pdf
255. Ch. 26 — Central Drugs Standard Control Organisation, India. In Vitro Diagnostics web page and notices, 2025–2026 (accessed 8 Oct 2026). cdsco.gov.in/opencms/opencms/en/Medical-Device-Diagnostics/InVitro-Diagnostics/
256. Ch. 26 — World Health Organization. Prequalification of health products, fact sheet (11 Jun 2026). who.int/news-room/fact-sheets/detail/prequalification-of-medicines-by-who
257. Ch. 26 — World Health Organization. IVD prequalification web pages: IVD prequalification assessment; IVD performance evaluation; IVDs eligible for WHO prequalification (tables updated 27 Sep 2026). extranet.who.int/prequal/vitro-diagnostics
258. Ch. 26 — World Health Organization. List of prequalified in vitro diagnostic products (accessed 8 Oct 2026). extranet.who.int/prequal/vitro-diagnostics/prequalified/in-vitro-diagnostics