Antigen testing for human immunodeficiency virus (HIV) and the magnet effect: Will the benefit of a new HIV test be offset by the numbers of higher-risk, test-seeking donors attracted to blood centers?

Background: There currently is debate on whether to include new assays for viral antigens and nucleic acids in the battery of screening tests applied to all blood donations. Proposals call for implementation of p24 antigen screening tests to help identify donors who are infected with human immunodeficiency virus type 1 (HIV-1) but have not yet seroconverted (window-period donors). There is concern, however, that people at higher risk for HIV infection will be attracted to blood centers in order to obtain the results of an HIV assay that is not routinely available elsewhere. Therefore, the benefit of antigen testing may be offset by an increase in the HIV incidence rate among blood donors. Study Design and Methods: Estimates were obtained from previous reports for the HIV incidence rate among blood donors, the percentage of donations from test-seeking donors, the duration of the HIV-infectious window period, and the performance of p24 antigen screening assays. Sensitivity analyses were performed by using various percentages of test-seeking donors following implementation of antigen testing and various HIV incidence rate ratios of test-seekers to nonseekers. Hypothetical residual HIV risks were calculated for multiple scenarios and compared to the current estimate of HIV risk without antigen screening of blood donations. Results: If antigen testing reduces the window period by 27 percent (e.g., from 22 days to 16 days), and the HIV incidence rate among test-seekers is 30 times that among nonseekers, then the benefit of antigen testing will be eliminated if the percentage df test-seekers (currently estimated at 3.2%) increases to 5.7 percent. If antigen testing reduces the window period by approximately 45 percent (e,g., from 22 days to 10 days or from 13 days to 6 days) and the HIV incidence rate ratio (test-seekers vs. nonseekers) is assumed to be equal to the HIV prevalence ratio (5.3), then the percentage of donations from test-seeking donors would have to increase to 25 percent of all donations to offset the benefit of antigen screening. Conclusion: This model helps identify the key measures for assessing the potential adverse effect of increased test seeking as a consequence of the addition of p24 antigen (or other direct virus) assays to blood donor screening programs. In most scenarios, the beneficial effect of antigen screening exceeded the potential detrimental effect of attracting higher-risk, test-seeking donors. The possibility cannot be ruled out, however, that a subgroup of high-risk, test-seeking donors attracted to the blood centers by a new HIV test will offset the reduction in the residual risk of HIV infection associated with antigen screening.