HIV Window Period — MFDS Part 1 MCQ

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ModerateMicrobiologyHIV Window PeriodMFDS Part 1

An adult has had a single possible exposure to HIV and has not taken pre-exposure or post-exposure prophylaxis. A venous blood sample will be tested using a laboratory-based fourth-generation HIV-1/2 antigen/antibody assay. After what interval from the exposure is a negative result considered outside the standard window period?

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Correct answer: C — 45 days

The correct answer is C: 45 days. A laboratory-based fourth-generation assay detects both HIV p24 antigen and antibodies, allowing infection to be identified earlier than with antibody-only tests. Although many infections become detectable within 2–4 weeks, the UK standard window period used to reliably exclude infection is 45 days after exposure when antiretroviral prophylaxis has not altered test timing. Fourteen days is too early to exclude infection, while 28 days represents the point by which many—but not virtually all—infections are detectable. Ninety days may apply to some antibody-only or rapid-testing pathways, and 180 days reflects obsolete testing schedules rather than current fourth-generation laboratory testing.

Reference: Hampshire and Isle of Wight Healthcare Sexual Health, Primary Care HIV Toolkit, Test type and window period, 2026. https://www.letstalkaboutit.nhs.uk/professionals/resources/primary-care-hiv-toolkit