A test that analyzes fetal DNA found in a pregnant woman’s blood proved much more accurate in screening for Down syndrome and another chromosomal disorder than the now-standard blood test, a new study has found. The promising results may change how prenatal screening for genetic diseases is done, though the test is costly and generally not yet covered by insurance for women at low risk.
The study, published on Wednesday in The New England Journal of Medicine, found that the fetal DNA test was 10 times better in predicting cases of Down syndrome than the standard blood test and ultrasound screening, and five times better in predicting the other disorder, Trisomy 18. It also greatly reduced the number of false-positive results.
It could prevent many women who would otherwise get the standard blood test from needing to confirm positive results with invasive tests like amniocentesis or chorionic villus sampling, which can be stressful, much more costly and carry small risks of miscarriage. “Nine out of 10 women who are currently being referred for further testing would not need invasive tests,” said the lead author of the study, Dr. Diana Bianchi, the executive director of the Mother Infant Research Institute at Tufts Medical Center’s Floating Hospital for Children.
A positive result on the DNA screening would still need to be confirmed with invasive tests, because in more than half the cases in which the newer test predicted a disorder, there was no chromosomal abnormality. But a negative result would provide confidence that these two major chromosomal disorders are absent.
“It’s a better mousetrap, there’s no doubt about that,” said Dr. Michael Greene, director of obstetrics at Massachusetts General Hospital and co-author of an editorial about the study. “If the test is normal, the overwhelming probability is that your fetus is normal. There will be far fewer women who will be encouraged to have invasive testing, and, as a result, far fewer miscarriages.”
The screen analyzes blood from women who are at least 10 weeks pregnant. At that point, about 10 percent of DNA in the blood will be fetal DNA from the placenta, Dr. Bianchi said.
Dr. Bianchi is a paid advisory board member for Illumina, one of the laboratories that performs the test, which is known as the cell-free DNA test because the fetal DNA floats freely in the mother’s blood, not inside a cell. Illumina, which is based in San Diego, financed the study.
Dr. Greene, an associate editor at The New England Journal of Medicine, said that the study “would sort of be purer from an academic perspective if somebody else pays for it,” but that companies are “the ones that are going to finance the research to get it to where it’s marketable.”
He added, “We can’t have our cake and eat it too.” He said the journal had rejected other cell-free DNA studies, but considered this one well done.
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