ABO discrepancies revealing rare weak A subgroups: a 2-case series

ABO discrepancies caused by weak A subgroups are uncommon but clinically important because they can lead to mistyping and inappropriate transfusion management.

We present the cases of 2 patients at an academic medical center who were historically typed as group O but were found to harbor weak A subgroups upon detailed investigation.

In case 1, the patient's red blood cells demonstrated weak reactivity with anti-A reagent on forward typing, with anti-A1 and anti-B identified on reverse typing. In case 2, the patient's red blood cells were nonreactive with anti-A reagent on forward typing but did agglutinate with anti-A,B reagent. Reverse typing showed weak reactivity against A1 and A2 cells, with strong reactivity against B cells, consistent with an anti-A antibody. In both cases, molecular genotyping performed at a reference laboratory returned results consistent with an Aw or Ael allele.

These cases illustrate the serologic spectrum of weak A phenotypes, highlight the variable sensitivity of ABO reagents in detecting minimal antigen expression, and underscore the possible utility of molecular characterization when serologic results are ambiguous.