Diagnosing HIV Infection - HIV ELISAs and Western Blots


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Diagnosing HIV Infection

HIV ELISAs and Western Blots

Antibodies to HIV are usually detected by assays known as enzyme-linked immunosorbent assays (ELISA or EIA). The wells of plastic microtiter plates are coated with recombinant viral proteins (antigens), which stick to the plastic. If serum from an infected patient is added to the well, HIV antibodies bind to the proteins and become attached to the plate. After washing away the serum the bound antibodies are detected by a second antibody that is linked to an enzyme such as alkaline phosphatase. This second antibody binds to human anti-HIV antibodies and can be detected by reacting the plates with a substrate for alkaline phosphatase that turns color when cleaved by the enzyme.

HIV antibody tests are more than 99% sensitive (that is, they detect the presence of HIV antibodies in nearly all infected patients); virtually the only infected patients who are not detected by standard HIV tests are those who are tested within the first few weeks after infection. The specificity of HIV antibody tests is also better than 99%, but some false positive tests do occur.

Therefore, all positive ELISAs are confirmed by a second test such as the western blot. Though not as sensitive as the ELISA, the western blot is more specific and allows the laboratory to identify the specific HIV proteins to which the antibodies are reacting. Only tests that are positive by ELISA and by a second, confirmatory test are reported as HIV positive.

HIV tests performed in the United States detect antibodies to both HIV-1 and HIV-2. Antibodies to most of subtypes of HIV-1 group M are detected by the current tests, but they are less reliable at detecting infection with more distantly related strains of HIV-1 belonging to groups O and N. (Group M accounts for the vast majority of HIV-1 infections; groups O and N are found predominantly in western Africa.)

4/15/01