Serological Diagnosis of HIV Infection
The initial screening of anti-HIV antibodies is done by an enzyme-linked immunoassay test (EIA) using HIV antigens obtained either from infected cells or by recombinant technology. Since this is a screening test, its cut-off (particularly when used to screen blood in blood banks) is set for maximal sensitivity, since it is preferable to discard some blood units that test false positive than to transfuse contaminated units with low antibody titers that es-cape detection.
Any positive result on ELISA needs to be confirmed, first by repeating the EIA to rule out errors or technical problems. If the repeat test is positive, the result should be con-firmed primarily by Western blot (immunoblot). A Western blot is considered positive if antibodies to structural proteins (e.g., p24), enzymes (gp41), and envelope glycoproteins (gp41 or gp120) are simultaneously detected. The accuracy of the combined tests (EIA and Western blot) is better than 99.5%.