Strategy for managing maternal variant RHD alleles in Rhesus D negative obstetric populations during fetal RHD genotyping

Objectives: Fetal RHD screening programs that aim to reduce unnecessary antenatal anti-D prophylaxis are being introduced into clinical practice. Strategies to manage women serologically typed as Rhesus D negative who have maternal RHD variants are needed. This study describes maternal RHD allelic variants detected in nonselected and alloimmunised Rhesus D negative obstetric populations and explores a mathematical approach to identify these variants. Methods: Fetal RHD status was defined by testing cell-free fetal DNA in maternal plasma. Women at risk of an RHD variant were identified by selection for C and E haplotypes or by recognition of low polymerase chain reaction cycle threshold on fetal RHD typing. Maternal RHD alleles were defined by SNP profiling or sequencing. Results: The prevalence of RHD variants in nonselected and alloimmunised groups was 1% (6/603) and 5.5% (6/110), respectively (p<0.001). An inverse association between RHD cycle threshold values and gestational age was described by a linear model (p<0.001). Standard residual values with a Z score threshold of -3.00 would have detected all maternal variants with one (1/713) false positive. Conclusions: The prevalence of maternal RHD variants was significantly higher in alloimmunised cases. The causative mechanism for this needs further investigation. Mathematical modeling simplifies the detection of maternal RHD variants.