Fourth generation antigen and antibody test, negative from 45 days
Expansion
The test: a fourth generation laboratory assay detecting both HIV antibody and p24 antigen.
- Window period: a negative result at 45 days after exposure excludes infection from that exposure. Point of care tests have a longer window, up to 90 days
- Any reactive result must be confirmed on a second sample before the diagnosis is given
- Antigen appears first, at about 2 to 3 weeks, then antibody, so the p24 component shortens the window
Who to test: anyone with an indicator condition (tuberculosis, pneumocystis, unusual or severe infection, oral candidiasis, seborrhoeic dermatitis, shingles in the young, unexplained lymphadenopathy, weight loss, cytopenias, Kaposi’s sarcoma, cervical dysplasia), anyone presenting to sexual health, drug and antenatal services, and in areas of high prevalence anyone registering with a general practice or admitted acutely.
Consent: verbal consent only is needed, as for any other blood test. Written consent and formal pre-test counselling are not required and act as a barrier.
Monitoring markers
- CD4 count: the measure of immune damage and the risk of opportunistic infection. Below 200 cells per microlitre defines a high risk of pneumocystis; below 50, of cytomegalovirus and Mycobacterium avium
- Viral load: the measure of disease activity and infectivity, and the target of treatment. Undetectable equals untransmittable
Seroconversion illness presents as a glandular fever like syndrome with fever, rash, pharyngitis, lymphadenopathy and sometimes meningism, at 2 to 6 weeks. Antibody may still be negative, so if suspected, request an HIV RNA viral load rather than relying on the standard test.