An alternative approach to confirming anti-HIV reactivity: a multi-country collaborative study.

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-.