HIV

An HIV screening assay is designed to identify specimens that might contain evidence of HIV. It is deliberately sensitive: missing an infection is more harmful than sending a small number of uninfected people for additional testing. A reactive screening result is therefore a preliminary positive, not a diagnosis. Fourth-generation laboratory assays are the usual starting point. They detect HIV-1 p24 antigen as well as antibodies to HIV-1 and HIV-2. P24 antigen can appear before a person has produced detectable antibody, so this approach finds many infections earlier than an antibody-only test. A useful comparison is a smoke alarm. It should activate when there may be smoke, but the alarm itself does not establish the source, size, or location of a fire. Likewise, a reactive HIV screen signals the need for a diagnostic sequence. It cannot independently establish HIV infection, identify HIV-1 versus HIV-2, or distinguish acute infection from a false-reactive result. False-reactive screens are more likely when testing populations with a low prevalence of HIV because even a highly specific test produces occasional false positives. This is why a clinician...