Laboratory tests - virology

Role

Virology-specific testing spans direct detection (PCR/nucleic acid amplification, antigen testing) and serology, with PCR now the preferred method for most acute viral diagnoses given superior sensitivity and faster turnaround compared with older viral culture techniques
Central to diagnosis and, for chronic viral infections (HIV, hepatitis B/C), to ongoing monitoring of disease activity and treatment response via quantitative viral load testing

Qualitative PCR answers "is the virus present" (diagnosis); quantitative PCR (viral load) answers "how much virus" (disease activity, treatment response, transmission risk) - these serve different clinical purposes and are not interchangeable
Viral load trends are central to monitoring antiretroviral therapy efficacy in HIV and antiviral therapy in chronic hepatitis B/C, with an undetectable viral load being the target of effective treatment

Result interpretation is context-dependent - a detectable viral load in a patient on treatment may reflect inadequate adherence, resistance, or recent treatment initiation (insufficient time to suppress), each requiring a different response
Viral load monitoring intervals are matched to the specific infection and treatment phase (more frequent early after starting/changing therapy, less frequent once stably suppressed)

Detectable viral load on established therapy -> assess adherence first (the most common cause) before assuming resistance, then consider resistance testing and regimen change if adherence is confirmed adequate
Undetectable viral load -> continue current regimen with routine monitoring

HIV viral load monitoring (adherence vs resistance distinction), hepatitis B/C viral load and treatment response, PCR vs serology for acute diagnosis, viral load and transmission risk (undetectable = untransmittable for HIV)

A detectable viral load in a patient on established antiretroviral/antiviral therapy should prompt an adherence assessment first, since non-adherence is the most common cause and is readily addressed, before assuming treatment failure and pursuing resistance testing or regimen escalation

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