HIV serology

10 mL blood in plain tube. Informed consent must be obtained from the patient prior to ordering the test.

Immunoassay for detection of both HIV-1 and HIV-2 antibodies. Specialised laboratories confirm positive immunoassay results by immunoblot methods.

Positive is significant, if confirmed by immunoblotting.

Diagnosis is of HIV infection. Screening of blood, tissue or organ donors. Following exposure, the seronegative "window" period may be from 3 weeks to several months. Occasional subtypes of HIV may be undetectable by some EIA methods.

Positive indicates HIV infection and must be confirmed by immunoblot and by testing on a separate sample. False positives are rare, and can be further assessed by immunoblot, tests for HIV antigen or, preferably, HIV-1 RNA and repeat HIV antibody testing; see HIV p24 antigen, HIV-1 RNA. In terminal disease, HIV antibodies may be negative. HIV-1 is common worldwide; HIV-2 is less common and is largely confined to Africa.

Patients where there is a high clinical suspicion and are negative for HIV antibody should undergo p24 antigen testing or Proviral DNA assay.