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A tool for estimating the residual risk of HIV transfusion transmission despite screening of blood donation samples with nucleic acid testing (NAT). The central concepts underlying the approach is the "infectious window period" (IWP) and HIV incidence in the donor population -- reflecting the probability that a random donation will be in the infectious window period. The infectious window period is a function of the viral growth rate during early ramp-up viraemia, the sensitivity of the screening assay and the volume transfused. Multiplying the infectious window period by incidence yields the probability (per transfusion) of a transfusion transmission.