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BACKGROUND: To decrease the likelihood of transmission from donations containing West Nile virus (WNV) levels below minipool nucleic acid test (MP-NAT) detection limits, blood centers switch from MP-NAT to individual-donation testing (ID-NAT) after detection of MP-NAT–positive donations. The effectiveness of strategies to trigger or discontinue ID-NAT screening is largely unknown.

STUDY DESIGN AND METHODS: Twenty-seven strategies to trigger and discontinue ID-NAT screening were evaluated with a statistical model based on known dynamics of WNV infection and historical data on WNV prevalence among blood donations. Breakthroughs were defined as WNV immunoglobulin M antibody–negative, viremic (RNA-positive) donations that could only be identified by ID-NAT, but were screened by MP-NAT. Effectiveness (proportional reduction of breakthroughs relative to MP-NAT screening alone) and efficiency (absolute reduction of breakthroughs relative to the number of tests performed) were estimated by simulating donation years of varying outbreak severities over a range of blood collection frequencies.

RESULTS: Most strategies were effective (>75% reduction in breakthroughs) when daily donations exceeded 560. In larger centers (1008 donations daily), effectiveness of trigger-on strategies based on absolute number of MP-NAT–positive donations improved, but worsened for strategies using rate-based criteria. Effectiveness increased slightly by triggering on one MP-NAT–positive rather than two and increased substantially by increasing the duration from 7 to 14 days that no ID-NAT–positive donations are detected before resuming MP-NAT.

CONCLUSION: Most trigger strategies become effective when test results from at least 560 donations daily are considered. A 14-day ID-NAT period may improve safety relative to the increase in the number of tests performed.