New Down Syndrome Test: More Accurate, More Limited

Cell-free DNA blood testing for Down syndrome had fewer false positives than standard screening but tested for fewer conditions

(RxWiki News) A new test might improve the accuracy of Down syndrome diagnoses.

A study from the University of California in San Francisco (UCSF) found that a blood test could be more effective in diagnosing Down syndrome than standard screening techniques. Although very accurate and with fewer false positives, the new test was more limited than standard screening, the authors of this study noted.

The research team said doctors should counsel their patients to be sure they understand the risks and benefits of the new test, called cell-free DNA testing.

Mary E. Norton, MD, a professor of clinical obstetrics and gynecology at UCSF, led this study.

"Providers need to be attuned to patients' preferences and counsel them about the differences in prenatal screening and diagnostic testing options," Dr. Norton said in a press release. "Those women who do opt for cell-free DNA testing should be informed that it is highly accurate for Down syndrome, but it focuses on a small number of chromosomal abnormalities and does not provide the comprehensive assessment available with other approaches. Counseling should also include information about the risks associated with failed tests and the pros and cons of pursuing invasive testing if no results are obtained.”

Down syndrome is a genetic disorder that can cause cognitive problems and other medical disorders. It is more likely to occur in the pregnancies of older women.

Women over the age of 35 are often thought to be at high risk of having children with Down syndrome and are more likely to be offered screening for genetic disorders. Unlike standard screening, in which an invasive procedure called amniocentesis is required, this new test uses blood from the mother.

Tiny particles of the baby’s DNA can be found in the mother’s blood, and the new test is designed to examine the baby’s DNA.

Standard screening, however, can identify more abnormalities than the blood test.

Dr. Norton and colleagues examined pregnancy outcomes on nearly 16,000 women. Thirty-eight of the women had a baby with Down syndrome.

The DNA blood test correctly identified all 38 cases, which were confirmed after birth by other tests. Standard screening in the same women found only 30 of the 38 cases.

Dr. Norton and team noted that the DNA blood test had a very low incidence of false positives. A false positive can lead to more invasive testing or even the termination of a pregnancy.

DNA blood tests had only nine false positives — compared to 854 false positives for standard screening.

DNA blood testing was also more accurate in detecting two other genetic abnormalities: Edwards syndrome and Patau syndrome. Standard screening, however, can check for a wide array of other abnormalities that the DNA blood test cannot.

This study was published in the April issue of The New England Journal of Medicine.

Ariosa Diagnostics (which makes the DNA blood test) funded this research with the Perinatal Quality Foundation. Conflict of interest disclosures were not available at the time of publication.

Review Date: 
April 6, 2015