DNA blood test is accurate for detecting Down syndrome

A blood test for pregnant mothers that detects an unborn child's DNA is better than standard tests at detecting Down syndrome, and returns fewer false positives, US researchers said Wednesday.

The cell-free DNA blood test, which finds small amounts of floating DNA from the fetus in the mother's blood sample, can be given to a woman when she is between 10 and 14 weeks pregnant.

Researchers studied nearly 16,000 women, and found that the cell-free DNA blood test correctly identified all 38 fetuses with Down syndrome in the group, said the study in the New England Journal of Medicine.

However, standard screening given to the same women found only 30 of the 38 cases of Down syndrome.

Standard screening consists of drawing blood to test for hormones and proteins associated with chromosomal defects, combined with an ultrasound that checks for excess fluid in the back of the fetus's neck, in an area called the nuchal fold.

Standard screening also returned far more misdiagnoses, with 854 false positive results, compared to nine false positive with the cell-free DNA tool.

When it came to identifying other, less common chromosomal abnormalities, the cell-free DNA test was more accurate than standard screening.

"Among 10 cases of trisomy 18, also known as Edwards syndrome, the cell-free DNA technique pinpointed nine and flagged one false positive," said the study led by Mary Norton, professor of clinical obstetrics and gynecology at the University of California, San Francisco.

"With standard screening, eight were identified and there were 49 false positives," it added.

Cell-free DNA testing found two cases of another disorder, trisomy 13, also known as Patau syndrome, and flagged one false positive.

Standard screening found just one case of trisomy 13 and mistakenly diagnosed 28 fetuses.

Researchers cautioned, however, that the cell-free DNA test is unable to find a range of abnormalities that can show up in standard screening.

The team also found that "a surprisingly high number" of disorders turned up in 488 pregnant women whose plasma was disqualified for testing because it had inadequate or an immeasurable quantity of fetal DNA, or other problems interpreting test results. A total of 2.7 percent of these fetuses were found to have chromosomal defects, "markedly higher than the 0.4 prevalence in the overall group," said the findings.

Since this group was excluded from the final analysis, the success rates of the cell-free DNA test may be skewed high in the report, the authors said.

"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," said Norton.