Laboratory investigations - HIV serology and viral load assays

Overview

Modern HIV testing combines antibody and antigen detection in a single assay, with viral load (RNA PCR) used for confirmation, monitoring, and detecting infection during the pre-antibody "window period"
Fourth-generation combination antigen/antibody assays have substantially shortened the diagnostic window period compared with older antibody-only tests

4th-generation combination assay detects both p24 antigen (detectable from ~2-4 weeks post-exposure, before antibodies develop) and HIV antibody (detectable from ~3-12 weeks) - combining both shortens the window period compared with antibody-only testing
A reactive screening test is followed by a differentiation/confirmatory assay to distinguish HIV-1 from HIV-2 and confirm true positivity, given the high sensitivity (and correspondingly imperfect specificity) of screening assays

HIV RNA (viral load) PCR can detect infection earlier still (from ~1-2 weeks post-exposure) - used when acute HIV infection is suspected despite a negative/indeterminate antibody-antigen test, and is the standard for diagnosing infection in infants born to HIV-positive mothers (maternal antibody crosses the placenta, confounding antibody-based testing)
CD4 count stages immune competence and guides opportunistic infection prophylaxis thresholds; viral load is the primary marker of treatment response once on antiretroviral therapy

Undetectable viral load on treatment = untransmittable (U=U) - a cornerstone of modern HIV management and public health messaging, changing both clinical counselling and the framing of HIV as a chronic, manageable condition
Post-exposure prophylaxis (PEP) and pre-exposure prophylaxis (PrEP) are guided by risk assessment and, for PrEP, require confirmed HIV-negative status before starting

Opportunistic infection risk stratification (CD4 count), antiretroviral treatment monitoring (viral load), window period counselling after a potential exposure

A negative test within the window period does not exclude infection - repeat testing at the appropriate interval (or RNA PCR if acute infection is suspected) is needed before a true exposure can be confidently excluded

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