All pregnant women—not just those with high-risk pregnancies—should be offered a new, DNA-based screening test for Down syndrome, say researchers.
In a study published in the New England Journal of Medicine on Wednesday, DNA-based tests outperformed standard screening methods, which include ultrasound imaging and biochemical test of a mother’s blood.
A pregnant woman’s blood contains bits of DNA from her fetus along with her own. By sequencing this mix of DNA, scientists can determine whether or not her fetus has an abnormal number of chromosomes, such as the extra copy of chromosome 21 that causes Down syndrome. Doctors are already using this finding to screen women who are at high-risk for having pregnancies with this kind of complication (say because they are over the age of 35, see “Medical Society Approval for Non-Invasive Prenatal Testing”). The new method is safer than traditional tests for counting fetal chromosomes in high-risk pregnancies. Traditionally, that had required amniocentesis or other invasive methods of gathering fetal chromosomes, which carry a small risk of miscarriage or damage to the fetus (see “A Safer Test for Down Syndrome”).
Several companies now offer the non-invasive, DNA-based test to doctors with high-risk patients, including MIT Technology Review’s Smartest Company of 2014, Illumina (see “50 Smartest Companies 2014: Illumina“ and “A Brave New World of Prenatal DNA Sequencing”).
But so far, these tests are not used to screen the broader population of pregnant women. That’s still done with ultrasound imaging or biochemical tests that aren’t very good at actually predicting whether a baby has Down syndrome. When such screening tests flag a pregnancy as having Down syndrome, there’s only a four percent chance the fetus actually does have the chromosome abnormality, says Diana Bianchi, executive director of the Mother Infant Research Institute at Tufts Medical Center. Confirming the result is usually done with the invasive testing methods.
Now, in the NEJM study, Bianchi and collaborators at Illumina (which funded the work) report that the blood-based DNA test can detect Down syndrome in any kind of pregnancy, not just high risk. Bianchi says that was not obvious to the field, as some people suspected that the biology of high-risk pregnancies may have led to more fetal DNA in a mother’s blood than in low-risk pregnancies.
Compared to the biochemical and ultrasound screening methods, the DNA test did a better job at correctly predicting chromosome problems in fetuses in the general population: a positive screening result for Down syndrome was correct 46% of the time.
For now, if women want the DNA-based tests, they will have to pay for it themselves as insurers don’t yet cover it for low-risk pregnancies. The test have list prices range from $1,200 to $2,700 says Richard Rava, chief scientific officer of Verinata, the Illumina division responsible for the prenatal test.
Those prices are significantly higher than they few hundreds of dollars that the biochemical and ultrasound tests each cost. However, given that those tests often incorrectly flag a pregnancy as having Down syndrome, there’s the potential for cost savings for insurers. Fewer false positives would mean fewer unnecessary and risky amniocentesis procedures that are required to follow up the screening tests.
“If this were to be used as a primary screen, it could result in a 90 percent reduction of invasive procedures,” says Rava.
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