hematology and oncology
Blood Collection
Pretransfusion Testing
Donor blood testing includes ABO and Rho(D) antigen typing Antibody screening Testing for infectious disease markers (see table Infectious Disease Transmission Testing ) Compatibility testing Tests the recipient’s RBCs for antigens A, B, and Rho(D) Screens the recipient’s plasma for antibodies against other RBC antigens Includes a cross-match to ensure that the recipient’s plasma is compatible with antigens on donor RBCs Compatibility testing is done before a transfusion; however, in an emergency, testing is done after releasing blood from the blood bank. Testing can also help in diagnosing Complications of Transfusion . The addition of a cross-match to ABO/Rh typing and antibody screening increases detection of incompatibility by only 0.01%. Therefore, many hospitals do computerized electronic cross-matches rather than physical cross-matches in a test tube in patients who have negative antibody screening. If the recipient has a clinically significant anti-RBC antibody, donor blood is restricted to RBC units negative for the corresponding antigen; further testing for compatibility is done by combining recipient plasma, donor RBCs, and antihuman globulin. In recipients without clinically significant anti-RBC antibodies, an immediate spin cross-match, which omits the antiglobulin phase, confirms ABO compatibility. Emergency transfusion is done when not enough time (generally < 60 minutes) is available for thorough compatibility testing because the patient is in hemorrhagic Shock . When time permits (about 10 minutes is needed), ABO/Rh type-specific blood may be given. In more urgent circumstances, type O RBCs are transfused if the ABO type is uncertain, and Rh-negative blood is given to females of child-bearing age if the Rh type is uncertain; otherwise, either Rh-negative or Rh-positive O blood can be used. “Type and screen” may be requested in circumstances not likely to require transfusion, as in elective surgery. The patient’s blood is typed for ABO/Rh antigens and screened for antibodies. If antibodies are absent and the patient needs blood, ABO/Rh type specific or compatible RBCs may be released without the antiglobulin phase of the cross-match. If an unexpected antibody is present, full testing is required. ABO typing of donor and recipient blood is done to prevent transfusion of incompatible RBCs (see figure Compatible RBC types ). As a rule, blood for transfusion should be of the same ABO type as that of the recipient. In urgent situations or when the correct ABO type is in doubt or unknown, type O Rh-negative packed RBCs (not whole blood—see Rh typing determines whether the Rh factor Rho(D) is present on (Rh-positive) or absent from (Rh-negative) the RBCs. Rh-negative patients should always receive Rh-negative blood except in life-threatening emergencies when Rh-negative blood is unavailable. Rh-positive patients may receive Rh-positive or Rh-negative blood. Occasionally, RBCs from some Rh-positive people react weakly on standard Rh typing (weak D, or Du, positive), but these people are still considered Rh-positive. Antibody screening for unexpected anti-RBC antibodies is routinely done on blood from prospective recipients and prenatally on maternal specimens. Unexpected anti-RBC antibodies are specific for RBC blood group antigens other than A and B [eg, Rho(D), Kell (K), Duffy (Fy)]. Early detection is important because such antibodies can cause serious Acute hemolytic transfusion reaction (AHTR) or Indirect antiglobulin testing (the indirect Coombs test) is used to screen for unexpected anti-RBC antibodies (see figure Indirect antiglobulin (indirect Coombs) test ). This test may be positive in the presence of an unexpected blood group antibody or when free (non-RBC–attached) antibody is present in The indirect antiglobulin (indirect Coombs) test is used to detect IgG antibodies against red blood cells (RBCs) in a patient's plasma. The patient's plasma is incubated with reagent RBCs; then Coombs serum (antibodies to human IgG, or human anti-IgG) is added. If agglutination occurs, IgG antibodies (autoantibodies or alloantibodies) against RBCs are present. This test is also used to determine the specificity of an alloantibody. Direct antiglobulin testing (the direct Coombs test) detects antibodies that have coated the patient’s RBCs in vivo (see figure Direct antiglobulin (direct Coombs) test ). It is used when immune-mediated hemolysis is suspected. Patients’ RBCs are directly tested with antihuman globulin and observed for agglutination. A positive result, if correlated with clinical findings and laboratory indicators of hemolysis, suggests autoimmune hemolytic anemia, drug-induced hemolysis, a transfusion reaction, or hemolytic disease of the newborn. The direct antiglobulin (direct Coombs) test is used to determine whether red blood cell (RBC)-binding antibody (IgG) or complement (C3) is present on RBC membranes. The patient's RBCs are incubated with antibodies to human IgG and C3. If IgG or C3 is bound to RBC membranes, agglutination occurs—a positive result. A positive result suggests the presence of autoantibodies to RBCs. A false-positive can occur and does not always equate with hemolysis. Thus, results should always be correlated with the clinical signs and symptoms. Antibody titration is done when a clinically significant, unexpected anti-RBC antibody is identified in the plasma of a pregnant woman or in a patient with Cold agglutinin disease . The maternal antibody titer correlates fairly well with the severity of hemolytic disease in the incompatible fetus and is often used to guide treatment in hemolytic disease of the newborn along with ultrasonography and amniotic fluid study. Donated blood products are tested for the presence of a number of infectious agents. 