Transfusion therapy is an essential part of hematology practice, allowing for curative therapy of diseases such as leukemia, aplastic anemia, and aggressive lymphomas. Nonetheless, transfusions are associated with significant risks, including transmission of infections and transfusion-related reactions. Controversy remains about key issues in transfusion therapy, such as triggers for red cell transfusions. This article reviews the available blood products (Table 1) and indications for transfusion along with the associated risks, and also discusses specific clinical situations, such as massive transfusion.
Whole blood is the product of 1 unit of donated blood plus anticoagulant/preservative, and by definition contains 1 unit of plasma and red cells. Whole blood can be stored for 5 weeks. Although it was the standard product in the past, whole blood is rarely used since 1 unit of donated blood can now be fractionated into 1 unit of red blood cells (RBC), 1 unit of platelets, and 1 unit of fresh frozen plasma (FFP). Thus, the use of whole blood for just a single transfusion represents a waste of resources. There are 2 exceptions. One is autologous blood donations, which are whole blood units. Second, whole blood is increasingly being used in massive transfusions for trauma patients, with the rationale being that all essential blood components are being transfused at once.1
PACKED RED CELLS
The remaining red cell mass after most of the plasma is removed is called the “packed” red cell unit (hematocrit = 70%–80%), and so red cells are often called “packed” red cells, or PRBC. A preservative is added to improve the flow of blood and to provide “nutrients” for the red cells, and this reduces the hematocrit to approximately 60%. The volume of a red cell unit is approximately 340 mL. In the average adult, 1 unit of RBC raises the hematocrit by 3%. The indications for transfusion of red cells are to increase red cell mass, and thus oxygen delivery, in patients who are compromised by their anemia.
Several randomized trials have helped define the indications for red cell transfusions and justify lower hematocrit thresholds for initiating transfusion. The TRICC (Transfusion Requirements in Critical Care Investigators for the Canadian Critical Care Trials Group) trial showed that in critical care patients (30-day mortality, 18.7%–23.3%), a conservative transfusion strategy of waiting until the hematocrit was below 21% had the same outcomes as transfusing at a threshold of 24%.2 The TRACS (Transfusion Requirements After Cardiac Surgery) trial showed that a hematocrit target of 24% had the same benefit as a target of 30% in patients who had undergone cardiac bypass surgery.3 For patients with acute myocardial infarction, the outcomes were worse with aggressive transfusion at a hematocrit of 30% compared to 24%.4 In patients with upper gastrointestinal bleeding, a hemoglobin transfusion trigger of 7 g/dL was associated with a lower mortality than a trigger of 9 g/dL (5% versus 9%).5 Finally, the FOCUS (Transfusion Trigger Trial for Functional Outcomes in Cardiovascular Patients Undergoing Surgical Hip Fracture Repair) trial showed that in older patients (average age 80 years) who had undergone hip fracture surgery, transfusions based on symptoms and not a fixed trigger of 30% had the same outcomes but considerable savings in blood products.6 Based on these trials, decisions regarding when to transfuse patients should be based on symptoms and not “numbers.” Young patients, especially those with reversible anemias, can tolerate low blood counts and should not be transfused based on an arbitrary number.
Several types of platelet products exist. One unit of platelet concentrate is derived from 1 unit of donor blood. Plateletpheresis from volunteer donors is also used to harvest platelets, with the resulting product referred to as plateletpheresis platelets. One unit of single-donor (pheresis) platelets is equivalent to 6 platelet concentrates. Finally, HLA-matched platelets are single-donor pheresis units that are obtained from an HLA-matched donor. This product should be ordered only if there is evidence of HLA antibodies (see Platelet Alloimmunization section).
The dose of platelets for the average patient is 6 units of platelet concentrate or 1 pheresis unit. In theory, 1 unit of platelet concentrate can raise the count by 5 to 7 × 103/µL, but often this response is blunted by concurrent illness or bleeding. In patients who appear to have a poor response, the platelet count can be checked 15 minutes after platelet infusion. No rise or a minimal rise (< 2 × 103/µL) in the platelet count is suggestive of platelet refractoriness, while a good 15-minute response but poor 24-hour count is more suggestive of consumption—fever, sepsis, drug, or splenomegaly—and not refractoriness.
The indication for platelet transfusion depends on the clinical situation. For patients with immune thrombocytopenia, platelets should not be transfused unless there is life-threatening bleeding. For stable patients with marrow aplasia from chemotherapy, a cut-off of a morning platelet count of less than 10 × 103/µL has been shown to be as safe as higher levels for prophylactic transfusions.7 For patients with active bleeding, the platelet count should be kept above 50 × 103/µL. Patients with acquired or inherited platelet dysfunction may benefit from transfusion no matter the platelet count.