[Model calculations for HIV screening of blood and plasma donors with a combination of 2 screening tests: test strategies, validity, costs and effectiveness].

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.