The page below is a sample from the LabCE course Introduction to the ABO Blood Group System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Example of an ABO discrepancy

The composite image shown on the right illustrates the ABO typing reactions that were obtained for a patient. This particular case illustrates an ABO discrepancy. An ABO discrepancy occurs when the results of forward and reverse typing do not match. The reactions shown are described below in descending order:
Patient red cells with reagent anti-A: negative reaction.
Patient red cells with reagent anti-B: 4+ agglutination.
Patient red cells with reagent anti-D: 4+ agglutination.
Patient serum with reagent A1 red cells: negative reaction.
Patient serum with reagent B red cells: negative reaction.
This patient forward types as a group B, but reverse types as a group AB. (A group B patient should have anti-A. This patient demonstrates neither anti-A nor anti-B, similar to an AB patient).
Further workup is necessary to determine the ABO type since the forward and back typing do not match. In this case, incubation at 4°C demonstrated the presence of weakened anti-A. The patient was therefore typed as group B. This case is an example of an ABO discrepancy which was due to a "missing" anti-A antibody. This could be due to old age, severe illness, or immunosuppression.
Although evaluation of ABO discrepancies is beyond the scope of this course, it is important to note that all ABO discrepancies must be resolved before blood products can be released for transfusion.
This patient is Rh (D) positive, as evidenced by the strong agglutination of his cells with reagent anti-D antibody.