New Down Syndrome Blood Test, cfDNA, Is Safe And More Accurate Than Ultrasound Screen

pregnancy
Using a cell-free DNA blood test to screen pregnant women for Down syndrome is not only safe but also more accurate than the current standard. Photo courtesy of Shutterstock.

Many pregnant women fear getting an accurate though invasive test for Down syndrome because of the miscarriage risk. Using a cell-free DNA blood test to screen pregnant women for Down syndrome is not only safe but also more accurate than the current standard, a new study finds. A cell-free DNA (cfDNA) test is able to detect fragments of the baby's DNA floating through the mother's bloodstream.

Down syndrome, sometimes referred to as Trisomy 21, is the most common chromosome abnormality and cause of learning disabilities in children. This genetic disorder occurs when abnormal cell division results in extra genetic material from chromosome 21. Causing lifelong intellectual disability, Down syndrome varies in severity, with some people experiencing added physical and health problems.

Generally, doctors screen a woman at risk for having a Down syndrome baby based on her age and the results of a nuchal translucency scan (also called a nuchal scan or the NT test). This test uses an ultrasound to measure the translucent (clear) space in the tissue at the back of a baby's neck. Babies with abnormalities tend to accumulate more fluid in the neck during the first trimester, so this space would be larger than average. The screen correctly identifies 70 to 90 percent of fetuses with Down syndrome, with a false positive rate of about two to five percent. When the results of this test are positive, mothers may opt to have amniocentesis, which has a 98 to 99 percent accuracy rate.

Here, though, many women hesitate.

Amniocentesis requires the insertion of a needle directly into the mother's uterus to withdraw a sample of the amniotic fluid. The risk of a miscarriage caused by this procedure ranges somewhere between one in every 400 to one in every 200. (According to American Pregnancy, the rates are closer to one in 400 in facilities that routinely perform this procedure.)

If the early screening test, the NT scan, were more accurate, fewer women would require the invasive test. This, then, was the point of the new study.

The researchers asked, Is a cfDNA test more effective as a screen than the current standard tests? The study, conducted by the American research team, analyzed and compared results from two screen tests — the NT scan versus the cfDNA — given to 15,841 women at various health facilities around the world.

The cfDNA test “had higher sensitivity, a lower false positive rate, and higher positive predictive value than did standard screening,” the researchers wrote in their published study. More importantly, the false positive rate during the study for cfDNA was just 0.06 percent as compared to 5.4 percent for standard screening — one less worry for pregnant mothers.

Source: Norton ME, Jacobsson B, Swamy GK, et al. Cell-free DNA Analysis for Noninvasive Examination of Trisomy. NEJM. 2015.

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