New prenatal blood test picks up Down syndrome with incredible accuracy

Pregnant woman

During pregnancy, many women opt in for standard screening for chromosomal abnormalities like Down syndrome. While non-invasive in nature, these techniques are also associated with a significant amount of false positive results.

Researchers from UC San Francisco have all eyes on a new cell-free DNA blood test that’s amazingly accurate at diagnosing Down syndrome during early pregnancy (10 to 14 weeks).

According to Mary Norton, M.D., professor of clinical obstetrics and gynecology at UCSF, there are now substantial data supporting the performance of this test in detecting the condition in both high and low-risk women.

Norton led a recent study that tracked the outcomes of almost 16,000 pregnancies. Of these, 38 fetuses were eventually diagnosed with Down syndrome. The cell-free DNA blood test correctly identified all 38 cases, while traditional screening techniques flagged only 30. What’s more is that there were just nine false positive results with the cell-free DNA test, while standard screening rendered 854 false positives.

“Clearly, cell-free DNA is the best test if one is considering non-invasive screening options for Down syndrome,” says Norton.

The test works by zeroing in on a small percentage of fetal DNA that researchers say is detectable in the woman’s blood. An increased amount of DNA is often a red flag for a chromosomal condition.

What this means for Down syndrome screening

While the results are indeed significant, Norton says there is still much debate over which assessment should be the first-line screening test.

“It is important to understand that Down syndrome comprises about 50 percent of all chromosome abnormalities, and only about 8 to 10 percent of all significant birth defects and genetic causes of intellectual disability, although this varies by maternal age,” Norton adds. “So while this test is highly accurate in detecting this one condition in these low-risk patients, Down syndrome is quite rare in young women. Other causes of birth defects and intellectual disability are far more common, therefore other tests provide better coverage when considering all risks.”

She also noted that a percentage of women who undergo cell-free DNA blood tests do not obtain a result for a number of reasons. For example, there may be an insufficient presence of fetal DNA. So while the number of false positive results are exceedingly small, the number of women who need some type of follow-up testing, evaluation or counseling is much higher.

Does this test detect Down syndrome as well in low-risk women as it does in high-risk women? Norton says it does, but that there’s still gray area in terms of clinical effectiveness in the broader context of prenatal genetic testing overall.

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