The disease causing agent of AIDS (Acquired Immuno-Deficiency Syndrome) is HIV (Human Immunodeficiency Virus). The diagnosis of HIV infection depends on the demonstration of ‘antibodies’ (particles produced within the human body against the virus) to HIV and/or the direct detection of HIV or one of its components. Antibodies to HIV generally appear in the blood 4 to 8 weeks after the virus enters the human body.
The standard ‘screening test’ (initial test that is done routinely whenever there is a suspicion about a disease) for HIV is the Enzyme linked Immunosolvent assay (ELISA). This is an extremely good screening test with a sensitivity of 99.5%. Most diagnostic laboratories use a commercial ELISA kit that contains antigens from HIV.
ELISA tests are generally scored as +ve (highly reactive), -ve (non-reactive) or indeterminate (partially reactive). While the ELISA is extremely sensitive, it is not specific in detecting the HIV infection.
Anyone suspected of having HIV infection on the basis of an inconclusive or +ve ELISA result confirmed with a more specific test. The other commonly used confirmatory test in the ‘western blot.’ In a patient in whom HIV infection is suspected, the appropriate initial test is the ELISA. When ELISA test is not the confirmatory test for HIV infection, why should it be done first and not the western blot which is the confirmatory test?
It is because ELISA is less expensive and more easily available than the western blot. Because of the social and global importance AIDS is having, one should certainly go for the western blot after the Elisa to confirm the diagnosis of AIDS.
If ELISA tests result is negative, unless there is a strong reason to suspect early HIV infection (as in a patient exposed within the previous 3 months), the diagnosis is ruled out and retesting should be performed only as clinically needed.
If the ELISA is indeterminate or +ve (positive), the test should be repeated. If the repeat is –ve (negative) on two occasions, one can assume that the initial +ve reading was due to a technical error in the performance of the test and that the patient is negative for AIDS. If the repeat is indeterminate or +ve, one should proceed to the western blot. If the western blot is +ve, the diagnosis is HIV infection. If the western blot is -ve, then the ELISA test can be assumed to have been false +ve and the diagnosis of HIV infection are ruled out. If the western blot is indeterminate, it should be repeated in one month and if there is no progress in the western blot, a diagnosis of HIV is ruled out.
