Dodd RY, Xu M, Stramer SL, et al. - In view of the challenges posed by the coronavirus disease 2019 (COVID-19) pandemic on the adequacy of the blood supply, many blood collection organizations implemented and publicized routine testing of donations for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies to attract new donors and support the collection of convalescent plasma. Researchers here investigated if testing of donations for SARS-CoV-2 antibodies was linked with changes in donor characteristics and reactivity of donated blood. Collection of nearly 40% of blood in the US by the American Red Cross is done from 44 states. On June 15, 2020, the American Red Cross initiated testing of all donations using the Ortho VITROS anti–SARS-CoV-2 S1 Total Ig assay detecting total immunoglobulin. Among 953,926 tested donations, 17,336 were reactive; 4,786 (28%) were from first-time donors and 12,550 (72%) from repeat donors for anti–SARS-CoV-2 rates of 2.99% among first-time donors and 1.58% among repeat donors. Findings suggest that the proportion of first-time donors increased after the introduction of antibody testing, and younger and racial and ethnic minority donors more frequently had reactive donations. In addition, there was an increase in reactivity rates with time. This rise may be due to donors with higher rates of prior exposure donating to obtain antibody test results, particularly first-time donors, but may also indicate raised exposure in the general population or raised identification of the need for convalescent plasma.