Noninvasive prenatal blood group genotyping.

Determination of fetal RHD from maternal plasma is increasingly used as a valuable tool for prenatal diagnosis. A remaining pitfall which hampers its use in situations with severe consequences is the following: (a) The reliability of negative results, however, is limited by difficulties to distinguish true negative results from false negative results due to insufficient amounts of free fetal DNA (ffDNA). False negative results can result in severe complications for the fetus and have to be reliably excluded. Large studies were performed in the last 10 years to investigate the reliability of noninvasive fetal RHD typing with real-time PCR. The majority of the assays were performed without internal controls. We present a protocol for inclusion of standards to assess the presence of adequate amounts of ffDNA for prenatal genotyping in maternal blood.