Objective:
To report a case with unusual ABO discrepancies caused by coexistence of the loss of anti-B isoagglutinin and rouleau formation.
Clinical presentation and intervention:
A 79-year-old female diagnosed as having multiple myeloma (MM) with monoclonal IgG-λ type showed rouleau formation in peripheral blood smear. The ABO and Rh blood type before the diagnosis of MM was A+, but the following ABO grouping was interpreted as AB+. The ABO genotype revealed the subtypes A102 and O101, which confirmed her ABO phenotype as A+.
Conclusion:
This was a case of combined group I and III ABO discrepancies mimicking blood group AB.