Academic journal article Bulletin of the World Health Organization

An Alternative Approach to Confirming Anti-HIV Reactivity: A Multi-Country Collaborative Study

Academic journal article Bulletin of the World Health Organization

An Alternative Approach to Confirming Anti-HIV Reactivity: A Multi-Country Collaborative Study

Article excerpt

The confirmation of positive screening assay reactions for antibodies to human immunodeficiency virus type 1 (anti-HIV-1) by Western blot is expensive and often gives indeterminate results. We therefore. carried out a collaborative study to investigate the confirmation of screening assay reactions using a second screening assay. For this purpose, seven laboratories prospectively tested sequential specimens, using at least one additional screening assay, until about 50 confirmed anti-HIV-1-positive specimens had been identified in each test centre. The reactions of 16 assays were analysed in pairs (assay A and assay B), using assay B on specimens reactive in assay A: A+/B+ reactions were considered positive and A-, negative anti-HIV results. These outcomes were compared with those obtained using confirmatory Western blot. In all, 7950 specimens were tested, and 359 were reported as positive by the laboratories. Within the test centres, eight screening assay pairings gave rise to no false-positive or false-negative results, and these combinations were at least as accurate as a single screening assay followed by Western blot. From 6.3% to 8.3% of the Western blot results were indeterminate.

The number of specimens examined was too small to justify recommending for general use named pairs of screening assays; the choice of these would, in any case, depend on local conditions. However, individual laboratory managers may wish to investigate the large potential savings to be made by confirming HIV infection using a second screening assay on initially reactive specimens. If the more sensitive screening assay is used first, the sensitivity of this approach may be improved by further investigation of specimens that react as A+B-.


Serological tests for antibodies to human immunodeficiency virus (anti-HIV) are now widely used to screen blood donors, to confirm clinical suspicion of infection, and for surveillance purposes. The sensitivity and specificity of commercially available enzyme-linked immunosorbent assay (ELISA) kits and rapid/simple, instrument-free assays have been well documented [1].(a, b) In order to compensate for any lack of specificity in the screening assays, and because of the serious prognostic implications of positive results, a two-tier system of testing has been developed in which sera that are reactive on initial testing are retested with a second (supplemental) test.(c) Although Western blot is the test commonly used for this purpose, it is expensive, is not easy to read or standardize, and often gives indeterminate results. Its cost, use, and interpretation pose difficulties for some laboratories, and there is therefore a need for alternative confirmatory procedures [2, 3].(c)

The present study compared the accuracy of using a second screening assay (either ELISA or a simpler test) with that of using Western blot to confirm a positive reaction in an initial screening assay. The study was carried out with the collaboration of seven laboratories in Europe, Australia, and Canada. Altogether, results from ordered pairs of 16 screening assays (68 combinations) were studied to determine whether a second screening assay would be an adequate substitute for a Western blot to confirm positive screening reactions. The outcomes of using either another screening assay or Western blot to confirm reactivity were compared.


In January 1989 seven participating laboratories were invited to test prospectively consecutive serum specimens referred to them until 50 confirmed anti-HIV-positive specimens had been identified in each. Most of the sera had not previously been screened, but in some centres some of the sera had already been tested elsewhere. All the specimens were tested using the assays routinely employed in the centre plus additional assays suggested by the WHO Global Programme on AIDS. Specimens that gave a positive reaction in any assay were tested using Western blot. …

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