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In 1994, the American Society of Anesthesiologists established the Task Force on Blood Component Therapy to develop evidence-based indications for transfusing red blood cells, platelets, fresh-frozen plasma, and cryoprecipitate in perioperative and peripartum settings. The guidelines were developed according to an explicit methodology. The principal conclusions of the task force are that red blood cell transfusions should not be dictated by a single hemoglobin "trigger" but instead should be based on the patient's risks of developing complications of inadequate oxygenation. Red blood cell transfusion is rarely indicated when the hemoglobin concentration is greater than 10 g/dL and is almost always indicated when it is less than 6 g/dL. The indications for autologous transfusion may be more liberal than for allogeneic (homologous) transfusion. The risks of bleeding in surgical and obstetric patients are determined by the extent and type of surgery, the ability to control bleeding, the actual and anticipated rate of bleeding and the consequences of uncontrolled bleeding. Prophylactic platelet transfusion is ineffective when thrombocytopenia is due to increased platelet destruction. Surgical and obstetric patients with microvascular bleeding usually require platelet transfusion if the platelet count is less than 50 times 10(9)/l and rarely therapy if it is greater than 100 times 10(9)/l. Fresh-frozen plasma is indicated for urgent reversal of warfarin therapy, correction of known coagulation factor deficiencies for which specific concentrates are unavailable, and correction of microvascular bleeding when prothrombin and partial thromboplastin times are >1.5 times normal. It is contraindicated for augmentation of plasma volume or albumin concentration. Cryoprecipitate should be considered for patients with von Willebrand's disease unresponsive to desmopressin, bleeding patients with von Willebrand's disease, and bleeding patients with fibrinogen levels below 80-100 mg/dL. The task force recommends careful adherence to proper indications for blood component therapy to reduce the risks of transfusion. (Key words:Practice guide-lines: anemia: blood component therapy; coagulopathy; cryoprecipitate; fresh-frozen plasma; red blood cells; transfusion.)