False positive findings are possible due to “silent alleles”, which are genes without a real function. In such cases, although the fetus was actually RhD-negative, the test results would mean that Rh prophylaxis would be given unnecessarily. This occurs in approx. 0.4%1of cases. Without the use of non-invasive fetal blood group testing, however, Rh prophylaxis would have been given anyway.
False negative results are possible, due to factors such as insufficient cell-free fetal DNA in the mother’s blood. Since the amount of cell-free fetal DNA increases as the pregnancy progresses, it is recommended that the Rhesus NIPT is carried out only after the 19th week of pregnancy. If another non-invasive prenatal test is performed (e.g. the Harmony® Test), and the fetal fraction of the blood is determined to be at least 4%, the fetal Rh status can also be determined from the 12th week of pregnancy.
Overall, the false negative rate is approximately 1 out of 2000 tests. In this case, a required prenatal Rh prophylaxis would not be administered. Given the generally low immunization risk during pregnancy (no more than 1-2% risk per pregnancy with an RhD-positive fetus), this proportion of false negative results is considered acceptable.
In the opinion of the german „Institute for Quality and Efficiency in Health Care” (IQWiG), the standard administration of Rh prophylaxis and targeted prophylaxis after testing with Rhesus NIPT give equivalent protective effects, because false negative results are also occasionally found in blood testing during childbirth2.