Rh genotyping: Avoiding false-negative and false-positive results among individuals of African ancestry

High homology, variant alleles, and silent alleles have made the development of completely reliable genotyping assays for the RHD and RHC alleles difficult. An RHD pseudogene (RHD) possessing a 37-bp insertion within exon 4 is common among serologically RhD-negative individuals of African descent and generates false-positive results in previously reported RhD genotyping assays. Genotyping RhC is problematic due to exon 2 homology between RHD and RHC; however, an RHCspecific 109-bp insertion within intron 2 has been reported useful for genotyping. Primers flanking the exon 4 insertion point were used for detection of RHD and RHD among a total of 231 serotyped individuals: 134 African American, 85 Caucasian, and 12 RhD serotype-negative/genotype-positive, D-sensitized women. Primers flanking the RHC-specific intron 2 insertion were used to genotype 282 serotyped individuals (128 African American, 154 Caucasian) and were compared to RHC genotyping using the exon 1 RhC-specific nt48 cytosine polymorphism. Complete correlation was observed between genotyping with the RHD primer pair and serotyping among 219 individuals and 10/12 previous RHD false-positive enotyping results were resolved. RHD was detected in 19% (n = 4/21) of RhD seronegative African Americans and 4.4% (n = 5/113) of RhD seropositive African Americans. When using the 109-bp intron 2 insertion for genotyping of RHC, a 23.9% (n = 11/46) false-negative rate was observed among African American RhCc serotyped heterozygotes. Utilization of the exon 1 nt48 cytosine for indirect genotyping of RHC yielded a 7.2% (n = 4/55) and 56.3% (n = 45/80) false-positive rate among Rhcc Caucasians and African Americans, respectively. We conclude that these additional reactions, though not sufficient alone, can be useful supplements to existing Rh genotyping assays.