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855) A 54-year-old man is going to the operating room for a liver transplant. Per your hospital protocol, the blood bank crossmatches 20 packed red cell units in preparation for the case. The patient has a history of prior alloimmunization with an antibody to the E antigen of the Rh blood group system on current and previous testing. On crossmatching 20 E antigen–negative units, 1 U is found to be incompatible. Repeat testing of this unit confirms that it is E antigen negative and that it is indeed crossmatch incompatible. Which one of the following is the most appropriate explanation for this finding, and what should be done with this unit?

This is likely a nonspecific finding due to the patient’s liver disease. This unit may be used for this patient in the operating room. In addition to the anti-E antibody, the patient has an antibody to a low-prevalence antigen. The incompatible unit can be reissued to another patient, and another unit should be crossmatched for this patient. The patient does not have an anti-E antibody and a repeat antibody screen and full red cell phenotype should be performed. Surgery should be delayed until the nature of the patient’s antibody is resolved. In addition to the anti-E antibody, the patient has a warm autoantibody. Because this is an autoantibody, the incompatible unit may be transfused in this patient. In addition to the anti-E antibody, the patient has an antibody to a high-prevalence antigen. The incompatible unit should be discarded from inventory because it contains a high-prevalence antigen.

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• It is difficult to phenotype red cells for antigens of low prevalence because commercial phenotyping reagents do not exist. Thus, it is impossible to order antigen-negative units and the crossmatch is used to rule out an incompatibility due to such an antibody.

• Other possible explanations for the scenario described in this case are that the incompatible unit was mistyped and was ABO incompatible, had a positive direct antiglobulin test, or was polyagglutinable.

• Identifying the specificity of an antibody to a low-prevalence antigen is not necessary and is often only of academic interest.

• Some antibodies to low-prevalence antigens can cause hemolytic disease of the fetus and newborn; however, routine red cell screening tests on the mother’s plasma will likely be negative. Testing the father’s red cells with the mother’s plasma can be helpful in these cases. Furthermore, reacting an eluate from the newborn to the father’s red cells can also help implicate an antibody to a low-prevalence antigen as the probable cause of hemolysis.

• When a patient has an antibody to a low-prevalence antigen, virtually all red cells are compatible. Conversely, when a patient has an antibody to a high-prevalence antigen, virtually all red cells are incompatible.

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Clinical Pathology: General Principles, Hematology & Coagulation, Transfusion Medicine

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