Maternal Cell Contamination in Postnatal Umbilical Cord Blood Samples Implies a Low Risk for Genetic Misdiagnoses

ObjectiveMaternal cell contamination (MCC) poses a risk for misdiagnosis in prenatal genetic testing, and is examined in accredited diagnostic laboratories However, the awareness of possible MCC in perinatal/postnatal genetic testing, mainly of umbilical cord blood (CB), is lower.MethodWe investigated the rate of MCC in DNA from both umbilical CB samples and umbilical cord samples that were sent to our diagnostic laboratory for diagnostic testing between 1995 and 2021 (n = 236).ResultsMCC was detected in 4% of umbilical CB samples, and in one umbilical cord sample. Particularly tests enriching for a specific variant are very sensitive for low amounts of MCC, as we emphasize here with a false positive diagnosis of myotonic dystrophy type 1 in a newborn.ConclusionsOverall, with appropriate collection and use, umbilical CB and umbilical cord samples are suitable for genetic testing based on the low rates of MCC and misdiagnosis. These findings do however underline the importance of routine MCC testing in umbilical CB samples and umbilical cord samples for both requesting clinicians and diagnostic genetic laboratories. What is already known about this topic?Maternal cell contamination (MCC) poses a risk of misdiagnosis in prenatal genetic testing. Umbilical cord blood (CB) is a safe and non-invasive alternative for venal puncture blood withdrawal in newborns.What does this study add?MCC poses a low risk of misdiagnosis in perinatal genetic testing performed from umbilical CB. Polymerase chain reaction-based tests for repeat expansions are very sensitive to contamination.