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

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.