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Diana Bianchi discusses noninvasive prenatal testing, including false positives and testing average-risk women, at ACOG's Annual Meeting.
Since coming to market 18 months ago, noninvasive prenatal testing using cell-free fetal DNA has been incorporated into practices at an astronomical rate.
“I’m as surprised as anyone else how fast it’s taken off,” said Diana Bianchi, MD, Executive Director of the Mother Infant Research Institute at Tufts Medical Center, “[but] the reason that this is taking off is that it’s a better test.”
Bianchi delivered the Edith Louise Potter Memorial Lecture at the 61st Annual Clinical Meeting of the American Congress of Obstetrics and Gynecology (ACOG), titled, “Are You Ready for Noninvasive Prenatal Testing?”
The tests, currently offered by four companies, are more than 99% accurate in detecting Trisomy 21 (Down Syndrome), with consistent results in both commercial labs and CLIA labs. They also detect Trisomy 18 with similar accuracy, as well as Trisomy 13.
Still, some are concerned about false positives, which occur in 0.5% of cases.
“The false positive cases are getting a lot of attention in the literature and in the press ,” said Bianchi, but cautioned that the tests are not ‘false positives,’ but ‘discordant positives.’
“These are true biological positives, but with explanations other than a fetal anomaly” she said. “[The test] is unraveling some other things that are there.”
Bianchi notes that what is called ‘fetal DNA’ is actually placental DNA, and that the test screens DNA from the mother in addition to the fetus. Accordingly, the test sometimes picks up on maternal and placental anomalies. There have been at least two documented cases in which the discordant positive was the result of unidentified cancer in the mother. In other cases, the result has stemmed from demise of a twin, placental abnormality, or an undiagnosed sex chromosome abnormality in the mother.
Bianchi and her team are applying for a grant to study discordant cases.
Currently, the test is only recommended for high-risk women. Initially, researchers suspected that the predictive value of the test might be lower in a low-risk woman because the fetal fraction of the mother’s blood must be at least 4% for the test to be accurate. They speculated that the blood of higher-risk women might have a higher fetal fraction. However, research has indicated that the test performs equally well in average risk women. But there are still logistical concerns to making the test the standard of care.
“How are you in your very busy practices going to be able to provide pre- and post- test counseling [to all women]?” said Bianchi. “It’s not straightforward, and there are lots of issues to discuss.”