A study published in 2014 analyzed the blood samples from 1914 women from 21 different centers in the United States who were subjected to both standard and cell-free DNA testing-the scientifically specific term for what is in shorthand referred to as NIPT. Other prenatal tests produce far more false positives than CVS but are nonetheless advertised to pregnant women and their partners as highly accurate.
Positive results from early Non-Invasive Prenatal Screening (NIPS or NIPT) confirmed by CVS “need confirmation through amniocentesis or ultrasound scans to prevent termination of a normal pregnancy.” “An abnormal CVS result should not be considered fully diagnostic,” they wrote. They advised caution when using CVS as a diagnostic to confirm less accurate prenatal screening results because CVS’s accuracy was established on tests of older women at greater risk for non-mosaic abnormalities. That confidence, reinforced by medical professionals, sometimes leads them to choose abortion. In a co-signed letter to the editor in the American Journal of Obstetrics and Gynecology, three medical doctors affirmed that “PPVs are more valuable to clinicians than detection rates.” False Positives Aren’t Discussed Well, or Often Enoughįalse positive rates are mentioned much more often in popular prenatal literature than the PPV, though, and can give unfounded confidence to couples who agree to these tests. But it is not unlikely that for some women, the odds of a positive CVS test being wrong could be one in four. For chorionic villus sampling, the PPV is 72.6-78.3 percent, according to a 1992 study in Denmark.Ī prenatal test’s PPV can be higher or lower depending on other factors, such as maternal age.
To really examine the accuracy of prenatal testing, you have to look at the positive predictive value (PPV)-the percentage of all positive tests that reflect true positives, or, put another way, the probability that a positive test result reflects an actual abnormality in the child. Non-mosaic false positives accounted for an additional 0.11 percent of CV samples. But here’s what’s being missed: a false positive rate of 0.15 percent (or multiple times that, based on the study) doesn’t mean there’s a 0.15 percent chance a positive result is false-it means that 0.15 percent of CV samples of normal babies will yield a false positive. The study’s authors stated that mosaicism occurs in 1 percent of CV samples, a proportion 10 times greater than what’s observed in newborns. Because of the frequency of chromosomal mosaicism in CVS and its attendant need for further testing, a discussion of mosaicism should be included in counseling prior to CVS. Most instances of mosaicism detected in CVS are not associated with a fetal abnormality and should be evaluated by further prenatal testing, i.e., amniocentesis or fetal blood sampling. The false positive in this case resulted from mosaicism, where the cells of the placenta showed abnormalities that the baby’s cells did not. According to reporting by the Irish Times, the test is “99% accurate,” and “in 0.15 per cent of cases, a false positive can occur.” Some may be shocked that a healthy baby could be aborted “by mistake.” A 0.15 percent false positive rate? That’s tiny, right? How could they have known? As Live Action reported, the mother underwent chorionic villus sampling (CVS), in which cells from the placenta are sampled to screen for chromosomal abnormalities.
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