HIV antibodies/antigen combination assay

Last updated: Thursday, 25, October, 2007
Item Process
Specimen

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

Method

Immunoassay for detection of both HIV-1 and HIV-2 antibodies and HIV p24 antigen. Specialised laboratories confirm positive immunoassay results by immunoblot methods.

Reference Interval

Positive is significant, if confirmed by HIV antigen assay for p24 or immunoblot for antibodies.

Application

Diagnosis of HIV infection. Screening of blood, tissue or organ doners. Following exposure, ther seronegative "window" period may be from 3 weeks to several months. Use of the combination assay decreases the time to detection of recent infection by 3 - 5 days compared with assays for antibodies alone. Occasional subtypes of HIV may be undetectable by some assays.

Interpretation

Positive indicated HIV infection and must be confirmed by immunoblot or p24 assay and by testing on a seperate 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 AntigenHIV-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 atibody should undergo p24 antigen testing of Proviral DNA assay.