February 28, 2014 — A cell-free DNA test is more accurate than a standard blood test and ultrasound to screen for Down syndrome and Trisomy 18 during pregnancy, according to a study published Wednesday in the New England Journal of Medicine, the New York Times' "Well" reports.
The test analyzes fetal DNA that floats freely in a woman's blood. It can be used beginning at 10 weeks of pregnancy, when fetal DNA from the placenta makes up about 10% of the DNA in the woman's blood.
For the study, researchers screened 1,914 pregnant women of diverse backgrounds at 21 clinics throughout the U.S. The participants received the standard blood test from their doctors that screens for biological markers linked to chromosomal disorders. Meanwhile, a lab used a cell-free DNA test to screen participants' blood samples for chromosomal indications of Down syndrome, as well as Trisomy 13 and 18, two often-fatal disorders. The researchers compared the results with how many infants were born with the conditions.
The study found that the cell-free DNA test was 10 times more accurate at predicting cases of Down syndrome and five times better at predicting cases of Trisomy 18 than the standard test. The DNA test also significantly reduced the amount of false-positive results for both disorders. Too few participants were screened for Trisomy 13 for complete comparisons to be made.
Overall, the DNA test correctly predicted Down syndrome 45.5% of the time, while the standard screening correctly predicted the condition 4.2% of the time. The test accurately predicted Trisomy 18 nearly 41% of the time, compared with a rate of 8% for the standard screening.
Researchers noted that the test was equally accurate in women in all trimesters and both high- and low-risk pregnancies.
Researchers said the test potentially could prevent nine out of 10 women who would otherwise receive the standard blood test from having to undergo more-invasive screenings to confirm the results. While positive results of the DNA test would still need to be confirmed by more invasive procedures, negative results would provide greater confidence that the disorders were absent, according to "Well" (Belluck, "Well," New York Times, 2/26).
According to the AP/Contra Costa Times, the test also could allow pregnant women to prepare sooner for possible medical issues in the fetus or to consider an abortion.
Experts noted that while the test seems promising, further research is needed (Marchione, AP/Contra Costa Times, 2/26). In addition, the DNA tests currently cost between $500 and $2,000 and are not usually covered by insurance for women who are at low risk for having a fetus with abnormal chromosomes ("Well," New York Times, 2/26).