The determination of the fetal D status from maternal plasma for decision making on Rh prophylaxis is feasible.

BACKGROUND: Noninvasive fetal RHD genotyping might become a valuable tool in decision making on antenatal Rh prophylaxis, which is currently in routine practice for all D- pregnancies in several countries. This study provides a large-scale validation study of this technology to address questions concerning feasibility and applicability of its introduction into clinical routine.

STUDY DESIGN AND METHODS: Real-time polymerase chain reaction (PCR) targeting RHD Exons 5 and 7 was applied for the detection of fetal-specific RHD sequences in maternal plasma. A total of 1113 women in 6 to 32 weeks (median, Week 25) of pregnancy were recruited. All of them were serologically typed as D- according to current German guidelines. DNA was extracted via a spin-column method and a novel automated approach using magnetic tips. Real-time PCR results were compared with postnatal serology and discrepancies further elucidated by DNA sequencing from a newborn's buccal swab.

RESULTS: Sensitivities of fetal RHD genotyping were 99.7 percent (spin columns) and 99.8 percent (magnetic tips), thus comparable with serology (99.5%). The detection of weak D variants was more reliable by real-time PCR. Specificities of fetal RHD genotyping were 99.2 percent (spin columns) and 98.1 percent (magnetic tips), which is lower than serology (>99.7%). Automation achieved significantly higher yields of cell-free fetal DNA.

CONCLUSION: This prospective clinical trial revealed that routine determination of the fetal D status from maternal plasma is feasible. Noninvasive fetal RHD genotyping can be considered as sensitive as the traditional postnatal serologic assay.