HIV infections are usually diagnosed by the detection of HIV antigens and antibodies in a blood sample. The HIV p24 antigen is a structural component of the viral particle and can usually be detected around 2-3 weeks after infection before declining to undetectable levels. HIV antibodies are produced by an infected individual in response to the viral infection. They are usually not detectable until 4-6 weeks after exposure, but then generally remain detectable.
An accurate HIV diagnosis also requires other testing in addition to an antigen/antibody test.
Non-reactive results from an antigen/antibody HIV test indicate a negative result and do not require any additional tests unless an acute infection (very recent exposure) is suspected. In that case, an HIV nucleic acid test may be required.
Reactive results from an antigen/antibody HIV test must be then tested with an HIV-1 / HIV-2 antibody differentiation immunoassay:
- Reactive results from an antibody differentiation immunoassay confirm an HIV diagnosis.
- Non-reactive results in an antibody differentiation immunoassay should be tested with an HIV-1 nucleic acid test.
- If reactive in the nucleic acid test (and reactive in the antigen/antibody screen and non-reactive in the differentiation immunoassay), it indicates an acute HIV-1 infection.
- If negative in the nucleic acid test (and reactive in the antigen/antibody screen and non-reactive in the differentiation immunoassay), it indicates a false-positive result in the initial antigen/antibody screen; hence the individual is negative for HIV.