Immunoassays that are currently available detect antibodies to both HIV-1 and HIV-2, using antigens specific for both viruses. Some tests also include group O-specific antigens to increase sensitivity to group O infections. Test results that are at or above the cutoff value are considered initially reactive for antibodies to HIV-1 or HIV-2.
Initial (screening) tests are highly sensitive and will produce few false-negative results, if seroconversion has occurred. However, immunoassays may lack specificity.
All current HIV screening assays used in the U.S. will detect both HIV-1 and HIV-2. It is important to note that most assays require repeat testing of an initial positive. If the repeated test is also positive it should be handled as a positive result. If the repeat is negative, a third 'tie-breaker' test is needed, which will be defined by the assay manufacturer or the laboratory director.