Isoagglutinins in the plasma of apheresis platelets are a concern. High titer anti-A and anti-B can cause severe hemolytic transfusion reactions. Our facility is testing donor plasma using the gel method to identify isoagglutinin titers exceeding 250. Platelet additive solution (PAS), recently introduced as a collec-tion and storage solution, replaces approximately 65 percent of the plasma in a platelet component. We intended to confirm the effect of PAS on the isoagglutinin titers. We compared the isoagglutinin titers in donor plasma from EDTA-anticoagulated whole blood (without PAS) with the plasma in apheresis platelet components with PAS. Titers were determined in a buffered gel matrix test using serial twofold dilution steps. Among 100 donors tested, 26 plasma samples exceeded a threshold titer of 250; 25 were group O and only one was group B. When samples from these 26 platelet components with PAS were tested, only one group O donor exceeded the threshold titer. Samples from plasma components with PAS consistently showed a 50 percent decrease in titer compared with the donors' plasma samples. In conclusion, nearly half of the group O donors tested exceeded a titer of 250. Only one apheresis platelet component with PAS exceeded this clinically applied threshold-a 96 percent decrease compared with the number of donor plasma samples without PAS. The implementation of PAS in apheresis platelet components prompted us to revise our component screening process, which then minimized component manipulation of out-of-group platelet transfusions.