Diagnosis

Diagnosis

Main article: HIV test

Many HIV-positive people are unaware that they are infected with the virus.^[117] For example, less than 1% of the sexually active urban population in Africa have been tested and this proportion is even lower in rural populations.^[117] Furthermore, only 0.5% of pregnant women attending urban health facilities are counselled, tested or receive their test results.^[117] Again, this proportion is even lower in rural health facilities.^[117] Since donors may therefore be unaware of their infection, donor blood and blood products used in medicine and medical research are routinely screened for HIV.^[118]

HIV-1 testing consists of initial screening with an enzyme-linked immunosorbent assay (ELISA) to detect antibodies to HIV-1. Specimens with a nonreactive result from the initial ELISA are considered HIV-negative unless new exposure to an infected partner or partner of unknown HIV status has occurred. Specimens with a reactive ELISA result are retested in duplicate.^[119] If the result of either duplicate test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing with a more specific supplemental test (e.g., Western blot or, less commonly, an immunofluorescence assay (IFA)). Only specimens that are repeatedly reactive by ELISA and positive by IFA or reactive by Western blot are considered HIV-positive and indicative of HIV infection. Specimens that are repeatedly ELISA-reactive occasionally provide an indeterminate Western blot result, which may be either an incomplete antibody response to HIV in an infected person, or nonspecific reactions in an uninfected person.^[120]

Although IFA can be used to confirm infection in these ambiguous cases, this assay is not widely used. Generally, a second specimen should be collected more than a month later and retested for persons with indeterminate Western blot results. Although much less commonly available, nucleic acid testing (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain situations.^[119] In addition, a few tested specimens might provide inconclusive results because of a low quantity specimen. In these situations, a second specimen is collected and tested for HIV infection.

Modern HIV testing is extremely accurate. The chance of a false-positive result in the two-step testing protocol is estimated to be 0.0004% to 0.0007% in the general U.S. population.