Hemolytic transfusion reactions

A hemolytic transfusion reaction (HTR) is the accelerated clearance or lysis of transfused red cells because of immunologic incompatibility. Although they may at times have similar clinical presentations, HTR is distinguished from autoimmune hemolysis or nonimmune causes of shortened survival of transfused red cells. HTR may occur when antigen-positive red cells are transfused to a patient with a preexisting alloantibody, or when a recently transfused patient makes a new alloantibody. The great majority of HTRs are a result of red blood cell (RBC) transfusion. However, HTRs may also result from transfusion of plasma-containing blood components, such as fresh frozen plasma or platelets. HTRs are classified as acute (AHTR) or delayed (DHTR) reactions, based on whether they occur within 24 hours or after 24 hours of the implicated transfusion. A more important distinction is between intravascular and extravascular hemolytic reactions. Intravascular hemolysis is characterized by hemoglobinemia and hemoglobinuria. In contrast, extravascular hemolysis lacks these dramatic signs, but is characterized by shortened survival of transfused red cells along with the accumulation of hemoglobin breakdown products. Generally, intravascular hemolysis is seen with AHTR, and precipitated by both IgM and IgG antibody isotypes, whereas extravascular hemolysis is usually seen in DHTR, and is principally of the IgG isotype. This distinction is not absolute, however. Occasional acute reactions result in extravascular hemolysis, and some intravascular hemolytic reactions are delayed. Accelerated clearance of incompatible red cells is an essential feature of HTRs. Transfusion may also stimulate the production of alloantibody without hemolysis. This phenomenon has been termed delayed serologic transfusion reaction (DSTR), and it needs to be differentiated from DHTR. DSTR is clinically benign, and is identified during routine type and screen without evidence of hemolysis, but it predisposes to later HTR. Earlier reports of hemolytic reactions have not necessarily differentiated clearly between serologic and true hemolytic reactions.