Genotyping confirms inheritance of the rare At(a−) type in a case of haemolytic disease of the newborn

The At(a) blood group antigen (now AUG2 in the Augustine system) is a high‐frequency antigen with negative phenotype At(a−) found only in individuals of African ancestry. In a twin pregnancy, the fifth pregnancy in a woman of African origin, serological investigations confirmed that the mother was At(a−) and anti‐At(a) was detected. DNA samples were exome sequenced and alignment was performed to allow variant calling. It was confirmed that the single nucleotide polymorphism, rs45458701, within the SLC29A1 gene encoding the ENT1 protein, recently reported to be a basis of the At(a−) phenotype was also the basis of the phenotype in this family. The reagents for serological analysis required to identify the rare blood type present in this mother are held in only a few reference laboratories worldwide. This case highlights the utility of genetic methods in resolving complex investigations involving blood grouping and demonstrates that genotyping of variants associated with blood types present in specific ethnic groups may be the fastest method available for identification of the basis of fetomaternal incompatibilities.