Unexpected neonatal ABO discrepancy revealing ABO * cisAB .05 during routine cord blood testing

Abstract Background An apparent AB blood group in a neonate born to a group O mother can raise diagnostic, transfusion, and parentage‐sensitive concerns. CisAB alleles are rare ABO variants in which a single inherited allele encodes glycosyltransferase activity producing both A and B antigens. ABO*cisAB.05 is particularly uncommon, and its detection during routine neonatal cord blood testing has not, to our knowledge, been previously reported. Case Report A term female neonate born to a group O, RhD‐positive mother and a group AB, RhD‐positive father typed as AB, RhD positive on routine cord blood testing. The direct antiglobulin test was negative by gel but weakly positive by tube anti IgG, with anti‐B in an acid eluate, providing serologic evidence of subclinical ABO incompatibility without clinically significant hemolysis. Repeat testing and reference laboratory evaluation reproduced an AB‐like, RhD‐positive phenotype, with 4+ reactivity to anti‐A, anti‐B, and anti‐A,B reagents, anti‐A1 lectin negativity, and anti‐H lectin positivity graded as 2+. Because the result appeared discordant with the maternal group, identity and specimen concordance were evaluated first. Short tandem repeat analysis confirmed maternity and cord blood–buccal swab concordance. ABO exon sequencing identified an ABO*cisAB.05/ABO*O.01.01 genotype, explaining the discrepancy. Conclusion To our knowledge, this is the first ABO*cisAB.05 case identified through routine neonatal cord blood testing. A stepwise approach beginning with repeat serology and patient/specimen identity confirmation, followed by targeted molecular testing, can resolve the discrepancy while minimizing premature parentage‐related conclusions and guiding safe transfusion planning.