Diagnostic Virology

Diagnostic virology answers one of two questions about a patient: is the virus present now, or has the immune system met it before. Direct detection finds the virus itself, through its antigen or its nucleic acid, and so reports on current infection. Indirect detection, serology, measures the antibody the host makes in response, and so reports on exposure and immune status. The whole discipline turns on choosing the right approach for the clinical question.

Three variables decide that choice:

• The target: antigen, antibody, or nucleic acid, each detectable in a different phase of illness.
• The stage of illness: nucleic acid and antigen appear first and mark active infection, while antibody rises later and persists, so a test can be negative simply because it was taken too early.
• The performance trade-off: a screening test is built for sensitivity so it misses few true positives, a confirmatory test for specificity so it produces few false positives, and no single assay does both perfectly.

The two articles below take the two families of method in turn. The shared ground, direct versus indirect detection, the diagnostic window, and the measures of test performance, is drawn together on this page.

→ See Molecular Virology for nucleic acid detection: the polymerase chain reaction and its real-time, multiplex and isothermal formats, viral load quantification, sequencing and next-generation sequencing, genotyping and resistance testing, point-of-care platforms, and the controls that keep results trustworthy.

→ See Serology for antibody and antigen detection: the enzyme immunoassay and its automated chemiluminescent descendants, immunofluorescence, immunoblot and neutralisation, immunoglobulin M versus immunoglobulin G and avidity, and how serological patterns are read against the diagnostic window.

Which test, when

The four broad approaches differ in what they detect and therefore in what they can tell you and when.

Approach	Detects	Reports	First positive	Main uses
Nucleic acid amplification (NAAT/PCR)	Viral genome	Current infection, quantifiable	Earliest, from symptom onset	Most acute diagnosis, viral load, genotyping and resistance
Antigen detection	Viral protein	Current infection	Early, needs abundant antigen	Rapid point-of-care tests, HBsAg, p24
Antibody detection (serology)	Host antibody	Exposure and immunity; recent vs past by IgM/IgG	After seroconversion, later	Immune status, retrospective diagnosis, viruses hard to detect directly
Viral culture	Replicating virus	Viable infectious virus	Days to weeks	Reference and phenotyping; largely superseded

Nucleic acid amplification is usually the most sensitive and the earliest to turn positive, and it alone gives a quantitative viral load and a genotype. Antigen detection trades sensitivity for speed and simplicity, so it is the basis of rapid tests. Serology answers the questions the direct methods cannot: whether a person is immune, and whether an infection is recent or past.

The diagnostic window

The order in which markers appear defines the window. Nucleic acid and antigen appear first, in active infection, while antibody follows after a lag, IgM before IgG. A test taken before its marker has appeared reads falsely negative, the commonest avoidable error in both molecular and serological diagnosis.

This is why fourth-generation HIV assays add p24 antigen to antibody, narrowing the window in which a recently infected person tests negative, and why a single early serology sample may need repeating on a later specimen.

Measuring test performance

No test is perfect, and four measures describe how it fails.

Measure	What it is	What it governs
Sensitivity	Proportion of infected people called positive	Missed cases (false negatives)
Specificity	Proportion of uninfected people called negative	False positives
Positive predictive value	Proportion of positive results that are true	Rises with prevalence
Negative predictive value	Proportion of negative results that are true	Falls with prevalence

Sensitivity and specificity are properties of the test, but predictive value depends on prevalence: a positive screening result in a low-prevalence setting is often a false positive. Diagnostic virology manages this by screen-then-confirm, a sensitive first assay to avoid missing infection, then a specific confirmatory assay to remove false positives, with every result read against the clinical picture.

Key terms

Term	Definition
Direct vs indirect detection	Finding the virus (its antigen or nucleic acid) versus inferring it from the antibody response.
NAAT	Nucleic acid amplification test, such as PCR, that detects the viral genome.
Diagnostic window	The interval after infection before a given marker is detectable, a source of false negatives.
Seroconversion	The appearance of specific antibody, moving from negative to positive.
Sensitivity / specificity	The true-positive and true-negative rates of a test.
PPV / NPV	The proportion of positive or negative results that are correct, both prevalence-dependent.
Confirmatory test	A specific second assay used to verify a reactive screen.

References and recommended reading

Greninger AL, Wang D, Storch GA, Jerome KR. Diagnostic Virology. In: Fields Virology, 7th ed., Volume 4 (Fundamentals), Chapter 16. Wolters Kluwer; 2023. The current overview of the diagnostic methods and how they are chosen.

Jeffery K, Aarons E. Diagnostic Approaches. In: Principles and Practice of Clinical Virology, 6th ed., Chapter 1. Wiley-Blackwell; 2009. The reference for selecting and interpreting tests across the diagnostic strategy.

Jerome KR, editor. Lennette’s Laboratory Diagnosis of Viral Infections, 4th ed. Informa Healthcare; 2010. The conceptual scaffold for the molecular and serological methods detailed in the two articles.