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Laboratory diagnosis of HIV infection is based on the detection of HIV antibodies. There is a delay between infection and seroconversion (i.e. the appearance of specific HIV antibodies) corresponding to a window period which may last several weeks. Reduction of the length of the window period might be achieved by using p24 antigen and/or viral nucleic acid detection assays. We have evaluated a new screening assay (VIDAS HIV DUO) designed for the concurrent detection of HIV antibodies and p24 antigen, using sequential samples of 34 HIV-1 seroconverting patients and 236 HIV-positive samples confirmed by Western blot. Results were compared with those obtained with two third-generation anti-HIV antibody screening assays. The 236 confirmed HIV-positive samples including 3 HIV-1 subtype O and 2 HIV-2 samples were found to be positive by the 3 assays. All 128 sequential samples from 34 seroconverting patients were found to be positive using the DUO assay, including 20/20 pre-seroconversion samples (p24 antigen-positive and anti-HIV negative). The new screening assay was positive before a third-generation anti-HIV screening assay gave positive results in 14/34 patients (41%). Using the DUO assay, the mean reduction of the window period was 4 days.

Objective: The strategies for combining two screening tests for HIV infections in blood or plasma donors are formulated in biometric terms and analyzed with respect to their value, i.e. their validity, cost and effectiveness., Design: Biometrical modeling using assumptions on the validity of the single tests, the conditional correlations between them, as well as on the cost of testing and the consequences of false-negative or false-positive test results., Results: If the test combination is defined as positive whenever at least one of the single tests is positive, then this rule (the 'believe the positive' rule, BTP), due to its lower specificity, has extremely low positive predictive values. In case of high prevalence rates of the infection (e.g. 1:1,000), the BTP rule leads to lower total cost than single testing, unless the latter has very high sensitivity (e.g. 99%). For smaller prevalence rates (< 1:50,000), which are more typical of the selected group of blood or plasma donors, combination testing is of little value because the extra cost of detecting one additional infection (compared with single testing) may reach several 100 million DM., Conclusion: The cost for detecting additional cases of HIV infection by using combination instead of single testing in HIV screening is so high that this decision requires a public consensus.

The importance of an optimal diagnostic standard for the prevention of transfusion-associated AIDS (TAA) was recently enforced by a decision of our Supreme Court. Although only 18 transfusion-associated HIV infections following transfusion of approximately 30 million blood units were officially reported since 1985, HIV-1 p24 antigen testing of blood donors was introduced in Hamburg and by the Bavarian Red Cross in summer 1992. So far, no p24 antigen-positive, HIV antibody-negative blood donor has been found (May 1993).