A discrepancy in ABO typing occurs when the front and back types do not agree. Resolving the discrepancy requires an evaluation of both the front and back typings.
Back type: testing the patient's serum against reagent red blood cells
Causes of an unexpectedly weak or absent agglutination of reagent red blood cells (back type often type AB)::
(1) hypogammaglobulinemia or agammaglobulinemia
Causes of unexpected reactions with reagent red blood cells:
(1) causes of rouleaux (myeloma, etc)
(2) cold autoantibody
(3) alloantibody to an antigen on the reagent red blood cells
(4) antibody to additive formulated with the reagent red blood cells
(5) bacterial contamination of the reagent red blood cells
(6) anti-A1 in a patient with A subtype (A2, etc)
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