Laboratory tests - serology

Role

Serological testing (antibody detection) establishes past exposure/immunity (IgG) or recent/acute infection (IgM, or paired acute/convalescent titre rise) for organisms not easily cultured or where culture is slow - widely used for viral, some bacterial (syphilis, Lyme disease), and parasitic infections
A mainstay of diagnosis for infections where direct organism detection is impractical, alongside its use in immunity/vaccination status assessment

IgM typically indicates recent/acute infection (rises early, wanes over weeks-months) while IgG indicates past exposure or immunity (rises later, persists long-term) - this kinetic distinction underlies interpretation of most serological panels, though the specific window periods vary by organism
Paired acute and convalescent titres (4-fold rise) can confirm acute infection when a single result is equivocal, particularly useful when IgM assays are unreliable for a specific organism

Window period limitations - serology can be falsely negative very early in infection before antibody response develops, meaning a negative result soon after a high-risk exposure does not exclude infection and may need to be repeated after the appropriate window
Cross-reactivity between related organisms (e.g. flaviviruses) can cause false-positive results, sometimes requiring confirmatory/reference testing

Confirmed acute infection by IgM/seroconversion -> proceed to organism-specific treatment/management and, where relevant, public health notification and contact tracing
Equivocal or unexpected results -> repeat testing after an appropriate interval or confirmatory reference laboratory testing rather than acting on a single ambiguous result

Window period false negatives (particularly relevant to STI/HIV testing timing), IgM/IgG kinetics, paired titres for confirming acute infection, cross-reactive false positives
A negative serology result obtained too soon after exposure (within the window period) does not exclude infection - repeat testing after the appropriate interval is required before excluding the diagnosis, a frequently tested point particularly relevant to post-exposure and STI screening scenarios

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