ABOdiscrepanties

ABO discrepantities are inconsistencies between forward (cell) typing and reverse (serum) typing in ABO blood grouping. They occur when the antigen profile on a person’s red cells does not align with the antibody profile in their plasma, making accurate blood typing challenging and potentially affecting transfusion decisions.

Causes of ABO discrepantities fall into two broad categories. Technical or pre-analytic factors include sample mislabeling, improper handling or storage, delays in testing, degraded reagents, and clerical errors. Biological or analytic factors include recently transfused blood, pregnancies, or stem cell or marrow transplant that introduce mixed populations of red cells; weak or subgroup expressions such as A subtypes; presence of unexpected antibodies (for example anti-A1 or anti-B); autoantibodies or cold agglutinins causing nonspecific agglutination; rouleaux due to elevated proteins; or rare phenotypes such as the Bombay (Oh) phenotype.

Diagnosis and resolution require a systematic approach. Recollect and verify the patient sample and identity, review transfusion history, and repeat ABO typing with different antisera or reagents. Perform forward typing with both A and B antisera and reverse typing using appropriate A and B cells, including testing for A1 and A2 subgroups. If discrepancies persist, employ additional techniques such as saline replacement to distinguish rouleaux, direct antiglobulin testing to detect coating antibodies, and adsorption/elution studies to identify autoantibodies or alloantibodies. Enzyme treatment or testing with different reagent panels and platforms may help clarify results. When unresolved, escalate to a reference or reference-compatible laboratory for further evaluation and consider crossmatching with compatible units while awaiting clarification.

ABO discrepantities have important clinical implications, as incorrect typing can lead to mismatched transfusions. Proper investigation helps ensure patient safety and appropriate blood product selection.