Delayed hemolytic transfusion reactions. Evidence of the need for an improved pretransfusion compatibility test.

Delayed hemolytic transfusion reactions were diagnosed with a frequency of 1 per 4,000 units of whole blood or erythrocytes transfused, which represents an increased frequency of detection for those reported in other studies. The reasons for this increase, as well as the current detection of relatively milder reactions, appear to be related to the careful monitoring of the transfusion process, along with an increased clinical awareness of the problem and more sensitive laboratory detection methods. The increased frequency of detection emphasizes the need for more sensitive pretransfusion crossmatch methods to prevent those delayed hemolytic transfusion reactions that are the result of secondary immune responses.