Typing red cells for antigens other than ABO and Rh(D) is not done for routine pretransfusion testing in which the patient lacks unexpected antibodies. However, if patients are found to have irregular antibodies, they are typed for the corresponding antigens to confirm if the antibodies are alloantibodies or autoantibodies. Also, if such a patient requires blood transfusion, all donor units to be crossmatched are antigen typed.
Note: If a crossmatch with donors was done in parallel with antibody screening, any incompatible donor units are antigen typed to confirm that they are indeed antigen positive, thus explaining their incompatibility. [Some labs may not antigen type incompatible donors due to cost constraints.]
If an incompatible donor was found to be antigen negative, then another explanation would have to be found to explain the agglutination in the crossmatch, e.g., perhaps the donor's cells have a positive DAT. Any apparently compatible donor units must also be antigen typed to be sure they really do lack the corresponding antigen and thus are safe to transfuse.
Typically, donors are antigen typed before they are crossmatched. Donor cells may be positive for the corresponding antigen and still be compatible with the patient's serum if the patient has a relatively weak antibody whose corresponding antigen exhibits dosage. In such a case, the patient's antibody may only be able to agglutinate red cells from homozygous and may be non-reactive with a donor who has the antigen only in the heterozygous state. The heterozygous form of the antigen will be able to be detected by commercial antisera, however, since it contains potent antibodies capable of agglutinating red cells with weak, heterozygous antigens.
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| Phenotyping |
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