RESUMEN
<p><b>OBJECTIVE</b>To explore the mechanism of ABO discrepancy in a patient by ABO genotyping and the reasonable blood transfusion strategy.</p><p><b>METHODS</b>Routine serological test was carried out to analyze ABO blood group. The presence of the blood group determinants on the red blood cells were determined by adsorption-elution test. Exons 1-7 and adjacent introns of the ABO gene were amplified by PCR and sequenced.</p><p><b>RESULTS</b>The patient showed ABO forward and reverse typing discrepancy. ABO forward typing defined as B, however, the reverse typing indicated that the patient was AB subtype. Absorption-elution test confirmed weak A antigens on the patient's red blood cells. The ABO gene sequencing showed a T>C exchange at position in exon 7 which resulted in a isoleucine to threonine substitution at codon 256. The ABO blood group genotype was ABO*Ael05/B101.</p><p><b>CONCLUSION</b>The 767 T>C substitution in the gene of α-1,3-N-acetyl galactose is the molecular mechanism leading to the decrease expression of A antigen of the Ael05 subtype.</p>