Development of platelet refractoriness. The estimated percent of patients who will become platelet refractory is plotted against the time to become platelet refractory. Of the 528 patients analyzed, 143 (27%) became platelet refractory. By 17 days, 25% were estimated to become refractory, and this number would be projected to increase to 42% with continued platelet transfusions. There was no difference in the incidence of platelet refractoriness among the patients assigned to receive control PCs, UVB-PCs, F-PCs, and F-APs. Refractoriness was defined as 2 serial platelet transfusions with 1-hour posttransfusion platelet increments of less than 11.0 × 10⁹ platelets/L.