Serology

Serology answers a different question from direct virus detection. Rather than finding the virus, it measures the antibody the host makes in response to it, and so reports on exposure, immune status, and, through the timing and class of that antibody, on whether infection is recent or past. The same laboratory discipline also performs antigen detection, which finds a viral protein directly and therefore marks current infection.

Serology remains essential for viruses that molecular methods detect poorly, for defining immunity, and for the large amount of diagnosis that turns on interpreting an antibody pattern rather than a nucleic acid result. Every such interpretation rests on one foundation: the kinetics of the antibody response. Read against that timeline, a result tells you not just whether antibody is present but what its presence means, and most serological error comes from reading a result without reference to it.

The antibody response and the diagnostic window

Virus-specific antibody is absent in someone never infected, and becomes detectable only some weeks after infection. Immunoglobulin M (IgM) appears first, typically declining within about one to two months although low levels can persist for a year or more, while immunoglobulin G (IgG) rises later and can persist for life.

This gives the basic grammar of serology on a single specimen:

• Neither IgM nor IgG: susceptible, no evidence of past infection.
• IgM, with or without IgG: current or recent infection.
• IgG without IgM: past infection, and often though not always immunity.

The interval between infection and detectable antibody is the diagnostic window, and it is the commonest reason a serological test is falsely negative: the sample was taken too early. Whether antibody is present at the onset of symptoms depends on the incubation period. In infections with a short incubation, such as many acute respiratory viruses, antibody is absent when symptoms begin, so acute serology is uninformative and a convalescent sample is needed. In infections with an incubation of a month or more, IgM and IgG are usually already present at onset, so a single specimen can be diagnostic.

Reinfection and reactivation complicate this. Both occur in someone who already has IgG, so an anamnestic (secondary) response may raise IgG with little or no IgM, and IgM cannot then be relied on to mark the event; in some herpesvirus reactivations IgM does reappear. For the chronic infections, human immunodeficiency virus (HIV) and the related human T-lymphotropic viruses, detectable antibody of any class effectively means current infection rather than past exposure, because these viruses are not cleared.

Antibody-detection assays

Several assay families detect antiviral antibody, and the choice among them turns on the virus, the clinical question, throughput and confirmatory needs. The older functional assays are now largely confined to reference and public-health laboratories, while solid-phase immunoassays do most routine work.

Assay	What it measures	Role	Limitation
Enzyme immunoassay (EIA/ELISA)	Antibody binding to fixed antigen	The routine workhorse, screening and immune status	Needs confirmation where specificity is critical
Chemiluminescence immunoassay (CLIA)	As EIA, light readout	High-throughput automated antibody and antigen testing	Instrument-dependent
Immunofluorescence (IFA)	Antibody binding to antigen on a slide	Flexible, multiplexable by microscopy	Subjective, needs skilled reading
Immunoblot / Western blot	Antibody to individual viral proteins	Confirmatory, high specificity	Laborious, can be indeterminate
Neutralisation	Antibody that blocks infectivity	Reference for protective antibody	Cumbersome, slow, live virus
Haemagglutination inhibition	Antibody blocking haemagglutination	Titre and immunity for agglutinating viruses	Largely superseded by EIA

The enzyme immunoassay (EIA), or enzyme-linked immunosorbent assay (ELISA), is the mainstay. In the common indirect format a viral antigen (whole viral lysate, purified protein, or a recombinant protein) is bound to a solid surface, patient serum is added so that specific antibody binds, and an enzyme-labelled anti-human immunoglobulin detector is then added and revealed by a colour change.

The antigen source shapes specificity: whole-lysate antigen is sensitive but prone to cross-reactivity, while recombinant antigen is cleaner but may miss some responses. Using isotype-specific detectors lets the same format measure IgM or IgA. The chemiluminescence immunoassay (CLIA) is the automated, high-throughput descendant, with a light rather than colour readout, and now runs much routine serology.

Immunofluorescence antibody (IFA) assays bind serum antibody to antigen on a slide and reveal it with a fluorescent anti-human immunoglobulin under the microscope; they are flexible but subjective and demand trained readers.

The immunoblot (Western blot) separates viral proteins by size, transfers them to a membrane, and identifies antibody to individual proteins, giving very high specificity by its characteristic banding pattern. Its main role is confirmatory, resolving a reactive screening EIA; a line immunoassay using recombinant antigens in defined bands is the modern equivalent, and an incomplete banding pattern is reported as indeterminate.

The functional assays measure what antibody does rather than that it binds. Neutralisation determines the highest serum dilution that blocks infection of cultured cells; it correlates with protection and is the standard against which other assays are judged, but it is slow, uses live virus, and is reserved for reference use and vaccine trials.

Haemagglutination inhibition (HAI) detects antibody by its ability to block virus-induced agglutination of red cells and is read as a titre, with a fourfold rise between acute and convalescent sera marking recent infection. Complement fixation is now largely historical, and rapid latex agglutination survives for quick answers such as varicella-zoster virus (VZV) immune status.

Immunoassay formats and their evolution

The immunoassay is versatile because it rearranges the same three elements: which reagent is fixed to the solid phase, what the patient sample supplies, and how the bound complex is revealed. These arrangements are architectures independent of the detection label, so the same format can be read by an enzyme, a chemiluminescent tag or electrochemiluminescence. The arrangements below cover almost all diagnostic use, and knowing them explains why one is chosen over another and which weakness each is designed to guard.

Format	Detects	Core arrangement	Signal	Chosen for
Indirect	Antibody	Antigen fixed to the plate binds patient antibody, revealed by labelled anti-human immunoglobulin	Rises with antibody	Routine IgG or IgM, by detector isotype; immune status
Sandwich (antigen-capture)	Antigen	Capture antibody on the plate binds antigen from the sample, revealed by a labelled detector antibody	Rises with antigen	Antigen detection such as HBsAg and p24
Double-antigen bridge	Antibody	Patient antibody is sandwiched between a plate-bound viral antigen and a labelled viral antigen	Rises with antibody	Modern total-antibody and combined antigen-antibody assays
Reverse-capture (antibody-capture)	Antibody (IgM)	Anti-human IgM on the plate captures the patient’s IgM, then viral antigen and a labelled antibody reveal it	Rises with specific IgM	IgM assays, avoiding rheumatoid-factor and IgG-competition error
Competitive	Antibody or antigen	Patient antibody competes with a labelled antibody for limited fixed antigen	Falls as sample antibody rises	High specificity; small or high-affinity targets

The indirect format is the simplest and most sensitive route to antibody, and switching the labelled detector between anti-IgG and anti-IgM reads either class. Its weakness is the IgM assay: a high level of specific IgG competes for the antigen and lowers IgM sensitivity, and rheumatoid factor can bind that IgG to produce a false-positive IgM. The reverse-capture format fixes both by grabbing the patient’s IgM first, which is why it is the preferred IgM design.

The sandwich (antigen-capture) format inverts the layout to detect a viral antigen, and the competitive format, in which sample antibody and a labelled antibody compete so that signal falls as the specific antibody rises, buys extra specificity.

The sandwich is no longer only an antigen method. The double-antigen bridge turns it around to detect antibody, sandwiching the patient’s antibody between a plate-bound viral antigen and a labelled viral antigen. Because a genuine result needs the antibody to bind specific viral antigen on both sides, the bridge is more specific than the indirect format, whose anti-human detector reports any bound immunoglobulin, and it now underlies many total-antibody and combined antigen-antibody assays.

The immunoassay’s logic has stayed constant while its parts have been rebuilt for sensitivity and throughput. The earliest solid-phase assays used a radioactive label (radioimmunoassay), soon displaced by the safer, cheaper enzyme label reading a colour change, the classic ELISA. Two later shifts mattered most.

First, the antigen improved: whole viral lysate, sensitive but cross-reactive, gave way to synthetic peptides and recombinant proteins, cleaner and more specific, and combined assays added antigen to antibody detection to shorten the window. This is the basis of the assay “generations”:

• First generation: whole viral lysate, detecting IgG only.
• Second generation: recombinant or synthetic-peptide antigens, more specific.
• Third generation: detecting IgM as well as IgG, so becoming positive earlier.
• Fourth generation: combined, adding p24 antigen (HIV) or core antigen (HCV) to antibody.

Second, the label improved: the enzyme-and-substrate colour reaction was joined by chemiluminescent labels (acridinium ester, which emits light with hydrogen peroxide) and time-resolved fluorescent lanthanide chelates (europium), both more sensitive and faster.

The term ELISA therefore names only the enzyme-labelled version, and today’s automated platforms are usually not enzymatic at all. The current high-throughput chemistry is the electrochemiluminescence immunoassay (ECLIA): a ruthenium label is carried on streptavidin-coated microparticles that are magnetically captured on an electrode, and an applied voltage makes the label emit light, read by a photomultiplier and expressed against a cutoff.

This delivers a wide dynamic range and very high sensitivity in a fully automated, random-access analyser, which is why electrochemiluminescent and chemiluminescent systems now run most routine serology. The progression, radioimmunoassay to enzyme colour to chemiluminescence to electrochemiluminescence, is one of steadily better labels on an unchanged immunoassay logic: the formats above still describe what is happening on the solid phase.

IgM, IgG and avidity

Distinguishing antibody classes turns a binding result into a statement about timing. A virus-specific IgM assay can diagnose recent infection from a single acute specimen, which is most reliable for viruses whose incubation is long enough that IgM is present when the patient presents, hepatitis A being the classic example.

The IgM-capture (antibody-capture) format is the preferred design: human IgM in the sample is captured first by an anti-IgM antibody on the solid phase, then viral antigen and a labelled detector are added. This configuration avoids two classic pitfalls of measuring IgM directly: it reduces the false positive from rheumatoid factor, the IgM anti-IgG that, together with specific IgG, can be misread as specific IgM, and it avoids the false negative from competing high-level specific IgG.

Even so, IgM results need clinical correlation, because IgM can persist for a year or reappear in reactivation, weakening its meaning as a marker of very recent infection.

IgG avidity exploits the maturation of the antibody response: antibody made early has low avidity for its antigen, and avidity rises with time, so low-avidity IgG points to recent primary infection and high-avidity IgG to past infection. Avidity testing is particularly valuable when a single specimen already contains both IgG and IgM and the question is whether infection is recent, above all in pregnancy, where dating a primary infection with cytomegalovirus (CMV) or rubella changes the assessment of fetal risk.

Antigen detection

Antigen detection finds a viral protein directly and so, like nucleic acid detection, reports current infection rather than the host’s history. It is done by several methods:

• Immunofluorescence or direct fluorescent-antibody staining of infected cells.
• Antigen-capture (sandwich) EIA, a capture antibody binding the antigen and a labelled second antibody detecting it.
• Lateral-flow immunochromatographic rapid tests for point-of-care use.
• Immunohistochemistry on fixed tissue.

Familiar examples are hepatitis B surface antigen (HBsAg), the HIV p24 antigen, and the rapid antigen tests for influenza, respiratory syncytial virus and SARS-CoV-2.

The recurring trade-off is sensitivity. Antigen detection is generally less sensitive than nucleic acid amplification, and rapid lateral-flow tests trade sensitivity for speed and accessibility, so a negative rapid antigen test in a symptomatic patient often needs confirmation by a more sensitive method. Where the antigen is abundant and specific, however, as with HBsAg, antigen detection is both diagnostic and central to management.

Interpreting serological patterns

Interpretation follows from the response kinetics and from whether one or two specimens are available.

Result on a single specimen	Usual interpretation
IgG negative, IgM negative	Susceptible (or too early, within the window)
IgM positive, IgG negative or positive	Current or recent infection
IgG positive, IgM negative	Past infection, often immune

A single specimen defines immune status (a positive IgG after vaccination or past exposure) or flags recent infection (IgM). Paired sera, an acute sample taken within a few days of onset and a convalescent sample about 10 to 14 days later and tested together, diagnose recent infection by seroconversion from negative to positive IgG, or by a fourfold or greater rise in IgG titre; no change indicates past exposure.

Because a single IgG is often uninformative for acute diagnosis, the paired approach remains important for the many viruses without a reliable IgM assay.

Confirmatory algorithms pair a sensitive screen with a specific confirmation: a reactive HBsAg EIA is confirmed by a neutralisation step, and a reactive HIV screening assay is worked up by a confirmatory or differentiation assay. Modern fourth-generation HIV assays detect p24 antigen alongside antibody and so shorten the window during which a recently infected, infectious person tests negative. For the chronic infections the presence of antibody signifies active infection (for hepatitis C, confirmed antibody corresponds to active infection in about ~85% of people, the remainder having cleared the virus).

Serology on cerebrospinal fluid (CSF) needs special care. Detecting virus-specific antibody in CSF is diagnostic of central nervous system infection only if it is made intrathecally, because antibody from blood crosses a damaged blood-brain barrier passively. Intrathecal synthesis is shown by comparing the CSF-to-serum ratio of specific antibody against that of total IgG or albumin, and for rare agents such as rabies the mere presence of specific CSF antibody is itself diagnostic.

Limitations and pitfalls

Serology’s limits follow from the same biology that gives it meaning:

• The diagnostic window makes early samples falsely negative.
• The response may be absent or delayed in neonates, the elderly, the immunocompromised and those with agammaglobulinaemia, so a negative result does not exclude infection in these groups.
• IgM persistence and its reappearance in reactivation blunt its specificity for recent infection.
• Passively acquired IgG, from intravenous immunoglobulin, transfusion, or maternal transfer across the placenta, can be mistaken for the patient’s own response, which is why congenital diagnosis relies on IgM (which does not cross the placenta) and on paired maternal and infant testing.

Three analytical pitfalls recur:

• Cross-reactivity between related viruses, and between antigenically similar epitopes, produces false positives, so a result that does not fit the clinical picture is checked with a more specific assay.
• Rheumatoid factor and other non-specific interferences distort IgM assays in particular, and are managed by the capture format or by serum pretreatment.
• The high-dose hook effect in one-step sandwich and antigen assays: an extremely high analyte level saturates the capture and detector reagents separately, so few complete sandwiches form and the result reads falsely low or even negative, a trap modern assays are designed to minimise.

These constraints are the reason serology is built as a screen-then-confirm system: a sensitive first assay to avoid missing infection, a specific second assay to avoid acting on a false positive, and clinical correlation throughout.

References and recommended reading

• Forghani B; Hodinka RL. Diagnosis by Viral Antigen Detection; Serologic Tests in Clinical Virology. In: Jerome KR, editor. Lennette’s Laboratory Diagnosis of Viral Infections, 4th edition, Chapters 8-9. Informa Healthcare; 2010. The conceptual scaffold for antigen-detection and antibody-detection assay families, the antibody response, and the interpretation of serological results.
• Greninger AL, Wang D, Storch GA, Jerome KR. Diagnostic Virology. In: Fields Virology, 7th edition, Volume 4 (Fundamentals), Chapter 16. Wolters Kluwer; 2023. The current account of serologic assays, antibody kinetics, virus-specific IgM testing, avidity, and the role of combined antigen-antibody assays.
• Jeffery K, Aarons E. Diagnostic Approaches. In: Principles and Practice of Clinical Virology, 6th edition, Chapter 1. Wiley-Blackwell; 2009. The reference for how serological and antigen-detection methods are selected and interpreted alongside the other diagnostic approaches.
• Roche Diagnostics. Elecsys HIV Duo method sheet (document 08836973501 V1.0). Roche Diagnostics; 2020. The source for the modern platform specifics: the electrochemiluminescence immunoassay (ECLIA) principle, the double-antigen bridging sandwich for antibody detection, and the high-dose hook effect.