Screening for HIV infection should be performed as a matter of routine health care. The diagnosis of HIV infection depends upon the demonstration of antibodies to HIV and the direct detection of HIV or one of its components or any one the above. Antibodies to HIV generally appear in the circulation 2 to12 weeks following infection.
The standard blood screening test for HIV infection is the ELISA (enzyme linked immuno assay) or EIA (enzyme immuno assay). ELISA extremely good screening test with a sensitivity of more than 99.5%. EIA is an extremely sensitive test, but it is not optimal with regard to specificity. This is true and a problem in low-risk individuals, like volunteer blood donors, of whom only 10% of EIA positive individuals are subsequently confirmed to have HIV infection (by confirmatory test i.e. Western blot). Among the factors associated with false positive EIA tests are antibodies to class II antigens, auto antibodies, hepatic disease, recent influenza vaccination, and acute viral infections. Because of low specificity, anyone suspected of having HIV infection based upon a positive or inconclusive EIA (intermediate or partially reactive) result must have the result confirmed with a more specific test like the Western blot.
Most of the diagnostic laboratories use a commercial EIA kit that contains antigens from both HIV-1 and HIV-2 and can detect both the types of HIV infection. These are continuously updated and they use both natural and recombinant antigens. EIA tests are generally scored as positive (highly reactive), negative (non reactive), or indeterminate (partially reactive). The latest EIA (fourth generation) can detect antibodies to HIV with detection of the p24 antigen of HIV.
Western blot is the most commonly used confirmatory test. In a patient with a positive or indeterminate EIA and a negative Western blot, one can conclude with certainty that the EIA reactivity was a false positive. Western blot demonstrating antibodies to products of all three of the major genes of HIV (gag, pol, and env) is conclusive evidence of infection with HIV. Multiple HIV antigens of different, well characterized molecular weights elicit the production of specific antibodies. All of the HIV antigens can be separated on the basis of molecular weight, and antibodies to each component can be detected as discrete bands on the Western blot. If no band appears it can conclusively conclude that the test is negative.
U.S. Food & Drug Administration (FDA) in 1993 have established criteria for a positive Western blot test, which state that result is considered positive if antibodies exist to two of the three HIV proteins: p24, gp41, and gp120/160. By these criteria about 10% of the blood donors tested positive by EIA only are positive for HIV. Western blot patterns of reactivity that do not fall into the positive or negative categories are considered “indeterminate.”
When the Western blot results are indeterminate, the following tests can help in the diagnosis of HIV infection. (1) HIV RNA by PCR (polymerase chain reaction), this test make use of PCR amplification of cDNA generated from viral RNA. (2) HIV RNA by bDNA, this is done by measuring the levels of particle associated HIV RNA in a nucleic acid capture assay employing signal amplification. (3) HIV RNA by NucliSens, this is done by isothermic nucleic acid amplification with internal controls. (4) Immune complex dissociated p24 antigen capture assay, this test is done by measuring the levels of HIV-1 core protein in an EIA based format following dissociation of antigen-antibody complexes by weak acid treatment. The last test is very cheap in compare to the other three.
Tags: Confirmatory test Western blot, EIA, ELISA
