HIV: Testing

Key points

• HIV antibody test
• most common and accurate test
• UK first-line is a FOURTH-GENERATION laboratory test on a VENOUS sample — a combined assay detecting HIV-1/HIV-2 ANTIBODY and p24 ANTIGEN (BHIVA/BASHH/BIA 2020). Point-of-care tests are largely THIRD-generation (antibody only) and therefore have a LONGER window.
• CONFIRMATORY testing follows locally determined pathways agreed with the local virology laboratory. The WESTERN BLOT is NO LONGER the UK confirmatory assay — a reactive screening result is confirmed on a separate assay, with HIV-1/HIV-2 differentiation.
• Use MOLECULAR assays (HIV RNA) where there is diagnostic uncertainty — for example suspected primary/acute HIV, or indeterminate serology in someone taking PrEP. PrEP or PEP can DELAY antibody responses, so atypical results warrant expert discussion.
• WINDOW PERIOD (the examinable number): BHIVA/BASHH/BIA 2020 revised the window for FOURTH-generation serology to 45 DAYS — a negative fourth-generation test 45 days after the last exposure effectively excludes HIV. Testing at 4 weeks detects most but not all infections, so an early negative needs a repeat test.
• The older teaching that antibodies appear at 4–6 weeks with 99% by 3 months reflects ANTIBODY-ONLY (third-generation) testing and is no longer the UK window for a fourth-generation assay. p24 antigen test
• usually positive from about 1 week to 3 - 4 weeks after infection with HIV