Test gets high marks in Netherlands trial
This large observational cohort trial from the Netherlands examined the accuracy of identifying RhD-positive fetuses using cell-free DNA isolates in maternal plasma. Over the 15-month study period, fetal RhD testing was conducted during Week 27 of gestation, and results were compared with those obtained using neonatal cord blood at birth. If the fetal RhD test was positive, providers administered 200 µg anti-D immunoglobulin during the 30th week of gestation and within 48 hours of birth. If fetal RhD was negative, providers were told immunoglobulin was unnecessary.
More than 32,000 RhD-negative women were screened. The cell-free DNA test showed fetal RhD-positive results 62% of the time and RhD-negative results in the remainder. Cord blood samples were available for 25,789 pregnancies (80%).
Sensitivity, specificity. The sensitivity for identifying fetal RhD was 99% and the specificity was 98%. Both negative and positive predictive values were 99%. Overall, there were 225 false-positive results and nine false-negative results. In the nine false negatives, six were due to a lack of fetal DNA in the sample and three were due to technical error (defined as an operator ignoring a failure of the robot pipetting the plasma or other technical failures).
The false-negative rate (0.03%) was lower than the predetermined estimated false-negative rate of cord blood serology (0.25%). In 22 of the supposed false positives, follow-up serology or molecular testing found an RhD gene was actually present, meaning the results of the neonatal cord blood serology in these cases were falsely negative. If you recalculate with these data in mind, the false-negative rate for fetal DNA testing was actually less than half that of typical serologic determination.
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