An acute hemolytic transfusion reaction occurs when immunologic incompatibility between the donor and the recipient results in lysis of red blood cells. Most hemolytic transfusion reactions are due to the transfusion of ABO-incompatible packed red blood cells secondary to clerical or system errors. The severity of the reaction is relative to the amount of incompatible blood received, the type of incompatibility, and the length of time before treatment is initiated. These include chills, anxiety, dyspnea, rash, nausea, fever or rise in temperature of 1° C or more from baseline, hypertension or hypotension, headache, and chest and flank pain. In a deeply sedated or anesthetized patient, hemoglobinuria, hypotension, and coagulopathy may be the only signs. A hemolytic transfusion reaction is usually manifested during the transfusion and can occur after receiving as little as 10 mL of incompatible blood.

An increased temperature during a transfusion may be the first sign of a hemolytic transfusion reaction, or it may be a sign of bacterial contamination of the blood product. Fever with hypotension is characteristic of bacterial contamination. A Gram stain of the blood product is helpful to confirm the diagnosis. Bacterial contamination may occur from contamination at the phlebotomy site during blood collection, from an unrecognized infection in the donor, or from improper storage. The risk of infection has been reported as 1 in 2,000 to 10,000 units for platelets and from 1 in 250,000 to 1,500,000 for packed red blood cells