Fetal RHD genotyping in maternal plasma at 11-13 weeks of gestation.

OBJECTIVE To examine the feasibility of fetal RHD genotyping at 11-13 weeks' gestation from analysis of circulating cell-free fetal DNA (ccffDNA) in the plasma of RhD negative pregnant women using a high-throughput robotic technique. METHODS Stored plasma (0.5 ml) from 591 RhD negative women was used for extraction of ccffDNA by a robotic technique. Real-time quantitative polymerase chain reaction (PCR) with probes for exons 5 and 7 of the RHD gene was then used to determine the fetal RHD genotype, which was compared to the neonatal RhD phenotype. RESULTS In total there were 502 (85.7%) cases with a conclusive result and 84 (14.3%) with an inconclusive result. The prenatal test predicted that the fetus was RhD positive in 332 cases and in all of these the prediction was correct, giving a positive predictive value of 100% (95% CI 96.8-100). The test predicted that the fetus was RhD negative in 170 cases and in 164 of these the prediction was correct, giving a negative predictive value for RhD positive fetuses of 96.5% (95% CI 93.7-99.2). CONCLUSION The findings demonstrate the feasibility and accuracy of non-invasive fetal RHD genotyping at 11-13 weeks with a high-throughput technique.