Discrepancies in reverse ABO typing due to prozone

Abstract

Three group O sera manifesting prozone in reverse ABO tests are reported. All were implicated in erroneous blood typing results. One sample failed to react with A1 red cells (RBCs) in immediate-spin (IS) tests, had anti-A and -B titers of 8192 and 2048, respectively, by indirect antiglobulin technique (IAT), and was from a diabetic patient; the parenteral administration of A substance present in porcine insulin is a possible cause of hyperimmunity in this case. The second sample was from the recipient of a single unit of group B fresh-frozen plasma; the serum anti-A and -B titers were 10,240 by IAT, but only weak reactions with A1 and B RBCs were noted in routine IS reverse typing tests; the hyperimmunity in the patient concerned was likely due to crossreacting anti-A, B stimulated by B-active glycoproteins and/or glycolipids in the transfused plasma. The third serum also had anti-A and anti-B IAT titers of 10,240 but did not react with A1 and B RBCs by IS; the hyperimmunity in this case may be related to sepsis from intestinal flora carrying A-and/or B-like antigens. These antibodies lysed A1 and/or B RBCs in tests incubated at room temperature (RT) and strongly agglutinated those RBCs by IS when diluted 10-fold with saline. The absence of the prozone phenomenon in tests with RBCs suspended in diluents containing EDTA is consistent with the previously published mechanism for anti-A prozone: namely, the steric hindrance of agglutination by the C1 component of human complement. In addition, weak (.ltoreq. 1+) anti-A/-B agglutination without lysis was also seen with 5 of 231 other sera in 2-minute RT tests, but all five reached .gtoreq. 3+ by IS and lysed A1 and/or B RBCs in 5-minute RT tests. These findings raise concerns regarding the ability of the IS crossmatch to detect ABO errors, particularly if tests are not centrifuged immediately. While prozones in true IS tests are rare, failure to detect anti-A/-B due to C1 blocking occurs more frequently in tests left briefly at RT. Accordingly, use of EDTA is indicated when IS crossmatches are performed to demonstrate ABO incompatibility.