Diagnostic Virology

Diagnostic virology is organised around specimen selection, direct viral detection, serology, culture, and sequencing.

The specimen must match the suspected site and clinical question. Wherever feasible, collect it as early as possible in the illness, because viral shedding and detectability for many organisms decline over time from symptom onset. This matters particularly for the arboviruses, where PCR sensitivity falls rapidly once viraemia resolves.

Respiratory swabs, nasopharyngeal swab or aspirate, are the standard specimen for the entire respiratory virus group and for suspected COVID-19 or influenza specifically.

Blood is used for viraemic-phase direct detection, many arboviruses, HIV viral load, serology across virtually every organism, and viral culture where still performed.

CSF is used for suspected meningitis or encephalitis, across the full neurotropic virus group. PCR is the standard direct detection method, given the invasiveness of repeat sampling and the clinical urgency typically involved.

Stool is used for the enteric viruses and for poliovirus surveillance. Many nonenveloped enteric viruses shed in faeces for a prolonged period even after symptom resolution, which extends the useful diagnostic window relative to blood or respiratory specimens.

Vesicle fluid is used for the vesicular viral exanthems, HSV, VZV, and hand-foot-and-mouth disease-causing enteroviruses. Collect it by unroofing an active vesicle. Diagnostic yield is highest from fresh, early lesions and declines as lesions crust over.

Tissue is used for viruses requiring histopathological confirmation:

• HHV-8-associated Kaposi sarcoma
• suspected viral haemorrhagic fever where tissue diagnosis is undertaken, generally only post-mortem given the biosafety concerns of this organism group
• confirming viral oncogenesis-associated malignancy generally

Antigen tests detect specific viral protein directly in a clinical specimen. They are generally faster and cheaper than PCR but less sensitive. They are widely used for point-of-care triage, rapid influenza, RSV, and SARS-CoV-2 antigen testing.

They are also used for specific organisms where antigen testing offers a practical advantage: rotavirus antigen detection, and NS1 antigen testing early in dengue illness.

PCR detects viral nucleic acid directly. It is the modern diagnostic mainstay across essentially every virus, with superior sensitivity to both antigen testing and traditional culture.

It is particularly essential for viruses that cannot be cultured practically or safely in a routine clinical laboratory, such as the viral haemorrhagic fever agents, which require high-containment facilities for any culture attempt. It is also essential for viruses with impractically slow or technically demanding culture requirements.

RT-PCR, reverse transcription PCR, is the technique required for RNA viruses, the majority of organisms in virology. It converts the viral RNA genome into complementary DNA before standard PCR amplification can proceed.

It differs from standard PCR, used directly for DNA viruses, only by this preliminary reverse transcription step. The two terms are frequently used loosely and interchangeably in general clinical conversation.

Viral load assays are quantitative PCR-based tests. They report the actual quantity of viral nucleic acid rather than simply a positive or negative result. They are essential for monitoring chronic viral infections over time and guiding treatment decisions:

• HIV viral load: monitoring antiretroviral therapy efficacy and confirming viral suppression
• HBV and HCV viral load: monitoring chronic infection activity and treatment response
• CMV and BK virus viral load: monitoring reactivation risk and guiding pre-emptive therapy in transplant recipients

IgM is generally the first antibody class produced after infection. In most contexts it indicates recent or acute infection. Examples are EBV and CMV mononucleosis distinction, hepatitis A, B, and E acute diagnosis, and arbovirus acute diagnosis.

IgM results need cautious interpretation. False positives can arise from cross-reactivity with other infections or nonspecific polyclonal B cell activation. IgM may also persist beyond the acute phase in some infections and some individuals. An isolated positive IgM result is therefore not always as definitive as it first appears.

IgG generally appears later than IgM and persists, often lifelong. It indicates past infection or immunity, including vaccine-induced immunity where relevant. The anti-HBs and anti-HBc IgG distinction illustrates this classically.

IgG alone, without accompanying IgM or a demonstrated rise in titre, generally cannot distinguish an infection from years ago from one acquired very recently. The paired sera approach addresses this limitation.

Seroconversion is the transition from seronegative to seropositive status, or a significant rise in antibody titre between two sequential specimens. It provides definitive evidence of genuinely recent infection, independent of the caveats for IgM and IgG interpretation.

Paired sera means collecting an acute-phase specimen, as early as possible in the illness, and a convalescent-phase specimen, typically two to four weeks later. A significant rise in titre between the two, or true seroconversion from negative to positive, is the most definitive serological evidence of acute infection available.

The cost is a substantial delay before the diagnosis can be fully confirmed. This limits the approach for acute clinical decision-making but retains real value for retrospective confirmation and for outbreak or epidemiological investigation.

Viral culture was historically the reference standard for viral diagnosis. It requires inoculating the clinical specimen onto susceptible live cell culture monolayers and observing for characteristic cytopathic effect. The pattern and timing of cytopathic effect historically gave preliminary identification even before specific confirmatory testing.

Culture is now less commonly used because:

• it is slow, days to weeks depending on the virus, and entirely impractical when rapid clinical decisions are needed
• it is technically demanding, needing specialised cell culture expertise and facilities not widely available outside reference or specialist laboratories
• PCR-based direct detection has substantially superseded it in sensitivity, speed, and practicality across virtually every clinical scenario

Culture retains a narrower role in contemporary practice:

• antiviral susceptibility phenotypic testing in specific research or complex clinical resistance scenarios, where genotypic or sequencing-based resistance prediction is insufficient or ambiguous
• novel virus characterisation during emerging outbreak investigation, where isolating live, replication-competent virus allows full characterisation in a way that nucleic acid detection alone cannot

Genotyping determines a virus's specific genetic lineage or subtype. It is relevant for two purposes:

• surveillance, such as identifying which influenza strains are currently circulating to inform annual vaccine strain selection
• clinical decisions where genotype affects treatment, historically HCV genotype-guided treatment selection before pan-genotypic direct-acting antivirals became widely available

Resistance testing is sequencing-based, genotypic detection of specific mutations known to confer antiviral drug resistance. It is established and clinically central for HIV, guiding antiretroviral regimen selection and identifying transmitted or acquired resistance. It is increasingly used for HBV and, in specific complex scenarios, other chronic viral infections.

Outbreak investigation using whole-genome sequencing of viral isolates allows precise tracking of transmission chains and identification of the source or introduction event. It also detects emerging variants with altered transmissibility or immune evasion properties.

This capability has become substantially faster, cheaper, and more widely deployed in real-time outbreak response in recent years. Public health authorities can now characterise and respond to an evolving outbreak with a speed and precision that previous eras, relying predominantly on epidemiological contact tracing alone, could not match.