Single‐exon approach to non‐invasive fetal RHD screening in early pregnancy: An update after 10 years' experience

Background and Objectives Anti-D prophylaxis, administered to RhD-negative women, has significantly reduced the incidence of RhD immunization. Non-invasive fetal RHD screening has been used in Stockholm for more than 10 years to identify women who will benefit from prophylaxis. The method is based on a single-exon approach and is used in early pregnancy. The aim of this study was to update the performance of the method. Materials and Methods The single exon assay from Devyser AB is a multiplex kit detecting both exon 4 of the RHD gene and the housekeeping gene GAPDH. Cell-free DNA was extracted from 1 ml of plasma from EDTA blood taken during early pregnancy, weeks 10-12. The genetic RHD results were compared with serological typing of newborns for a determination of sensitivity and specificity. Results In total, 4337 pregnancies were included in the study; 44 samples (1%) were inconclusive either due to maternal RHD gene variants (n = 34) or technical reasons (n = 10). Of the remaining 4293 pregnancies, a total number of nine discrepant results were found. False positive results (n = 7) were mainly (n = 4) due to RHD gene variants in the child. False-negative results were found in two cases, of which one was caused by a technical error. None of the false-negative cases was due to RHD gene variants. Overall, the sensitivity of the method was 99.93% and specificity 99.56%. Conclusion The single-exon assay used in this study is correlated with high sensitivity and specificity.