Immunoassay methods that detect HIV antibodies have improved over the years from the earliest indirect enzyme immunoassay (EIA) method of the mid-1980s that detected only anti-HIV-1, to the current generations that are capable of detecting both anti-HIV-1 and anti-HIV-2 antibodies. Fourth generation assays are capable of detecting HIV IgM antibodies that may be produced early in infection, as well as IgG antibodies that are produced later in the infection. These assays also include labeled anti-p24 antibodies to detect the presence of p24 antigen, giving the assay even greater sensitivity and specificity. HIV p24 antigen is detectable in serum/plasma between 14 and 22 days after infection and before antibodies are present. Once seroconversion occurs and antibodies are produced, p24 antigen usually falls below detectable limits. Therefore, the combination assay is useful early in infection, if antigen is still present, and remains effective once antigen diminishes and antibodies are detectable.
Fourth generation assays produce a single positive result, if either antigen or antibody is positive. A fifth generation assay has recently come on the market. It differentiates all three HIV analyte markers; antigen, HIV-1, and HIV-2 antibodies. Fourth or fifth generation assays are now the preferred and recommended assays for initial HIV detection.