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Objective: The aim of the study was to compare the clinical effects and hemostatic efficiency of transfusions of platelets preserved in the frozen state for as long as 2 years with transfusions of platelets preserved in the conventional manner for as long as 5 days in patients undergoing cardiopulmonary bypass. Methods: Seventy-three patients were prospectively randomly assigned to receive transfusions of cryopreserved or liquid-preserved platelets. Nonsurgical blood loss was measured during and after the operation. Bleeding time, hematologic variables, and the bleeding time site shed blood were assayed before cardiopulmonary bypass and at 30 minutes and 2, 4, and 24 hours after transfusion. In vitro platelet function tests were conducted on platelets obtained from healthy volunteers. Results: No adverse sequelae of the transfusions were observed. Blood loss and the need for postoperative blood product transfusions were lower in the group receiving cryopreserved platelets. Lower posttransfusion platelet increments and a tendency toward decreased platelet survival were observed in patients receiving cryopreserved platelets. Hematocrit and plasma fibrinogen were significantly higher in this group, and the duration of intubation was shorter. In vitro, cryopreserved platelets demonstrated less aggregation, lower pH, and decreased response to hypotonic stress but generated more procoagulant activity and thromboxane. Conclusions: (1) Cryopreserved platelet transfusions are superior to liquid-preserved platelets in reducing blood loss and the need for blood product transfusions after cardiopulmonary bypass. (2) The reduction in blood loss in the patients receiving cryopreserved platelet transfusions after cardiopulmonary bypass probably reflects improved in vivo hemostatic function of cryopreserved platelets. (3) Some in vitro measures of platelet quality (aggregation, pH, hypotonic stress) may not reflect in vivo quality of platelet transfusions after cardiopulmonary bypass, whereas other in vitro measures (platelet procoagulant activity and thromboxane) do. (J Thorac Cardiovasc Surg 1999;117:172-84)