FETAL RHD GENOTYPING IN FETAL CELLS FLOW-SORTED FROM MATERNAL BLOOD

OBJECTIVE: The aim of this study was to determine the accuracy of noni nvasive fetal RhD genotyping by fetal cell isolation from maternal blo od. STUDY DESIGN: Candidate fetal cells from 18 pregnant women (one tw in gestation) were flow-sorted. Polymerase chain reaction amplificatio n of a 261 bp fragment of the RhD gene was performed on sorted fetal c ells. The presence of amplified product was considered predictive of t he RhD-positive genotype in the fetus. RESULTS: Sixteen of the 19 feta l RhD genotypes were correctly predicted in fetal cells isolated from maternal blood (10 were Rh positive, 6 were Rh negative). In 3 cases n o amplification products were detected in RhD-positive fetuses. The as sociation between presence of the fragment and RhD-positive genotype w as significant (p = 0.003, Fisher's exact test). CONCLUSIONS: Noninvas ive prenatal diagnosis of the fetal RhD genotype is feasible. Absence of amplification products in the reaction requires confirmation that f etal material is present. Improvements in fetal cell purity and yield should increase diagnostic accuracy, although the current protocol has a positive predictive value of 100% and a negative predictive value o f 67%.