Testing for HIV antibodies requires at least two different assays:
1. a screening test and
To exclude inadvertent mix-ups of samples, a second blood sample from the same patient should generally be tested. Only then should the diagnosis of HIV infection be communicated to the patient in cases of unexpected seropositivity.
Most screening tests are based on the ELISA principle (enzyme linked immuno sorbent assay) or other, closely related test formats (UNAIDS, 1997b). Screening tests must be extremely sensitive to minimize the chance of yielding a false-negative result. This means that they have to be able to also detect low-avidity and low-level antibodies found e.g. early in the course of a primary infection. If the result of a screening test is reactive (positive), this has to be confirmed by (at least) one confirmatory assay. One should never "diagnose" HIV infection on the basis of a reactive ("positive") screening assay alone!
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