HIV window period. Following HIV infection, HIV RNA becomes detectable by standard NAT after ∼9 to 10 days (green). HIV antibody is detectable by immunoassay after ∼21 days (red). Blood donations made in the HIV NAT window period are responsible for most of the residual risk of transfusion-transmitted HIV. Adapted from Busch¹⁰  with permission from the author and publisher.