Blood component transfusion is usually performed because of decreased production, increased utilization/destruction or loss, or dysfunction of a specific blood component (red cells, platelets, or coagulation factors).

1. Anemia
1. Red cell mass: The primary reason for red blood cell (RBC) transfusion is to maintain an adequate oxygen-carrying capacity of the blood. Healthy individuals or individuals with chronic anemia can usually tolerate a hematocrit (Hct) of 20% to 25%, assuming normal intravascular volume. The measured values for the Hct and hemoglobin (Hgb) are roughly in a 3:1 numerical ratio. Modern analyzers directly measure the Hgb concentration by light absorbance and calculate the Hct depending on the product of the mean corpuscular volume and the red cell count.
2. Anemia may be caused by decreased production (marrow suppression), increased loss (hemorrhage and diagnostic phlebotomy), destruction (hemolysis), or dilution. Acute blood loss generally does not change the relative concentration of RBCs immediately (because other intravascular volume is lost at the same rate) but the infusion of intravenous (IV) fluid may contribute to a dilutional effect.
3. Anemia in the critically ill is common. The exact Hgb level that should prompt RBC transfusion remains controversial and should be individualized to the patient. Multiple large randomized trials conducted among patients who are critically ill suggest that a “restrictive” transfusion policy (maintaining Hgb 7-9 g/dL) is as good as a “liberal” strategy (10-12 g/dL). For an individual patient, the decision to transfuse should take into consideration oxygen delivery to tissues rather than be based exclusively on Hgb or Hct values.
4. For a patient who is not bleeding, each unit of packed RBCs is expected to raise the Hct by 3% and the Hgb by 1 g/dL. The volume of RBCs to transfuse can also be estimated depending on desired change in RBC volume as follows:

   

where BV is blood volume, which may be estimated at 70 mL/kg actual body weight in male adults and 65 mL/kg in female adults. Higher values may be used in infants (80 mL/kg) and neonates (85 mL/kg). The Hct of transfused blood is approximately 55 ± 5%.

2. Thrombocytopenia: Spontaneous bleeding is unusual with platelet counts more than 10 000/µL, but in the immediate postoperative period, platelet counts of more than 20 000 to 50 000/µL are sometimes recommended. Thrombocytopenia may be due to decreased bone marrow production (eg, chemotherapy, tumor infiltration, liver disease) or increased consumption (eg, trauma, sepsis, medications, autoantibodies, inflammatory disorders), sequestration in the spleen, or dilution.