Determining the antigens carried on a red blood cell using serologic methods (i.e., using antibodies against specific antigens on the red cells). The “phenotype” of any blood group refers to which antigens are detectable on the RBC, and it usually (though not always) corresponds to the genes that the patient carries. Phenotyping is used in blood banking in four main settings:

  • For blood donors, to determine compatibility for a patient with an alloantibody
  • For blood donors, to identify donors with uncommon phenotypes
  • For patients, to determine which antigens the patient lacks in order to prevent future antibody formation 
  • For patients, to confirm that the patient is negative for an antigen to which he has formed an antibody

An example of the latter scenario: A patient has what appears to be an anti-Fya, anti-e, and possibly anti-K, with no evidence of an autoantibody. This patient is phenotyped for those three antigens, and is found to be Fya-negative and e-negative, but is K-positive. This would make the patient highly unlikely to have anti-K (assuming that the patient has not been recently transfused with K-positive cells).

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