New DNA test highly accurate at detecting Down Syndrome

A test that detects fetal DNA in the mother's bloodstream is more accurate than conventional screening at identifying Down Syndrome (aka trisomy 21), finds a new study led by researchers at Columbia University Medical Center and University of California at San Francisco.

The findings were published April 1 in the New England Journal of Medicine.

''This study supports the use of non-invasive prenatal testing as a first-line screening option for any pregnant woman, regardless of the patient's age or whether the patient is low, average, or high risk,'' says Ronald Wapner, MD, director of reproductive genetics at Columbia and one of the principal investigators of the study. Dr. Wapner led the study with Mary E. Norton, MD, professor of maternal-fetal medicine at the University of California, San Francisco.

Tests that detect fetal DNA in maternal blood were first introduced in 2011 but have been investigated mostly in high-risk women - those who are over age 35 or who have genetic risk factors.

This study supports the use of non-invasive prenatal testing as a first-line screening option for any pregnant woman.

The new study - with more than 18,500 participants - is the first in the general pregnant population that is large enough to compare the performance of the new DNA test with conventional screening.

The researchers tested one type of DNA test, offered by Ariosa Diagnostics, but other companies market similar tests.

The researchers found that the new test correctly identified more Down cases - with far fewer false positives - than standard screening that combines blood tests with an ultrasound exam.

The DNA test identified all 38 cases of Down Syndrome and produced only nine false positives (0.06 per cent). Standard screening identified 30 of the 38 cases of Down syndrome and produced 854 false positives (5.4 per cent).

About 3 per cent of the DNA tests did not produce a result, often because the fraction of foetal DNA in the blood sample was too low. These participants had a higher frequency of Down Syndrome cases as well as other chromosomal abnormalities, and further study is needed to determine the best approach for these patients.

Before the DNA test can be widely introduced into the clinic, the study's authors say that people's expectations of the test should be carefully considered.

The test is highly accurate at detecting Down Syndrome and other conditions with extra chromosomes, but standard screening and diagnostic testing can identify a broader array of abnormalities not detectable with the cell-free DNA test.