Synopsis
Select the correct HIV assay, interpret reactive and negative results within their windows, recognise acute infection that needs RNA testing, and complete confirmatory linkage safely.
- A venous laboratory fourth-generation HIV-1/2 antigen-antibody assay has a 45-day window under BHIVA/BASHH guidance.
- All rapid point-of-care tests use a 90-day window in the UK guideline; confirm the exact product and specimen rather than calling every rapid test fourth generation.
- Test immediately after a recent exposure to detect pre-existing infection, then repeat after the relevant window; do not delay PEP while waiting.
Key red flags
A reactive screening assay is preliminary and requires the national confirmatory algorithm before final diagnosis is communicated.
A negative result before the assay-specific window has closed cannot exclude acquisition from the recent exposure.
Acute retroviral symptoms after recent risk require HIV RNA even when antigen, antibody or a rapid test is non-reactive.
Current or recent PrEP, PEP or ART can suppress viraemia and delay serological evolution, so specialist testing is needed.
Pregnancy, breastfeeding, organ donation, needlestick exposure or suspected advanced disease creates an urgent linked clinical pathway alongside testing.
Fever, rash, sore throat, nodes, diarrhoea or meningism weeks after exposure raises acute HIV despite negative serology.
Oesophageal candidiasis, wasting, opportunistic infection or unusual neurological disease requires urgent testing and CD4 assessment.
Recent PEP or PrEP may reduce RNA and delay antigen or antibody evolution, creating discordant results.
Reasoning priorities
Detect p24 antigen and HIV-1 or HIV-2 antibodies in venous blood.
A non-reactive result excludes exposures at least 45 days earlier when prophylaxis and other modifiers do not apply.
Worked reasoning
A person has a negative laboratory fourth-generation test 18 days after a substantial exposure and no symptoms.
- Explain that the result is reassuring for older infection but remains inside the 45-day window for the reported exposure.
- Arrange repeat laboratory testing at or after day 45 and give an exact date, while assessing whether PEP was indicated earlier or PrEP is now appropriate.
- Ask about any subsequent exposures because each creates its own closing date and prevention need.
- Verify the final result personally and document how the actual assay, specimen and prophylaxis history were considered.