ABO blood group discrepancy - An unexpected news for a known blood group O positive regular blood donor.

ABO discrepancies occur when cell and serum blood grouping results do not match, posing a significant risk during transfusions. One rare cause of such discrepancies is the presence of B subgroups, including B 3 , B x , B m , and B el , which show weak or absent B antigen expression due to genetic variations. These subgroups can lead to misclassification, requiring specialized serological tests such as adsorption-elution or molecular genotyping for accurate identification. Proper recognition and documentation are essential to prevent transfusion reactions. We present a case of a 27-year-old regular blood donor, previously known as O RhD positive, who showed a discrepancy between cell and serum grouping during routine testing. Cell grouping indicated O RhD positive, while serum grouping suggested group B. Repeat testing using the conventional tube technique and adsorption-elution studies confirmed weak B antigen expression, with eluate reacting with B cells, indicating a B subgroup. Saliva testing revealed the presence of both B and H substances, confirming the individual as a secretor. Based on serological and secretor status, the donor was identified as having the B m phenotype. This case highlights the importance of detecting and resolving ABO grouping discrepancies to prevent ABO mismatched transfusions, which can lead to life-threatening reactions. Accurate blood typing, through advanced testing, and communication about the individual's unique blood group are crucial for ensuring transfusion safety. Preemptively informing individuals with unusual blood group expressions of their donor and transfusion recipient status ensures compatibility and minimizes the risk of errors in emergencies.