A newer blood test for DNA defects that cause Down's syndrome and similar conditions is more accurate than older tests, researchers reported Wednesday.
The test, called a cell-free DNA test, detected cases more accurately, a team reported in the New England Journal of Medicine. It also came up with fewer false positives — when a test incorrectly indicates an abnormality, Dr. Mary Norton of the University of California San Francisco and colleagues reported.
"The performance of cfDNA testing was superior to that of traditional first-trimester screening for the detection of trisomy 21 in a routine prenatal population," they wrote in their report.
It's not likely to be the last word on such tests, which are controversial and not yet regulated by the Food and Drug Administration. But the very large size of the trial — nearly 16,000 women — gives it some power. The researchers also included women of all ages, including many not considered at high risk of having a child with a chromosomal abnormality.
But it does provide an option for women to find out very early in pregnancy, Norton said. It can be done as early as 10 weeks into a pregnancy.
The test looks for free-floating DNA (cfDNA) from the fetus. It can find the extra copies of chromosome 21, which causes Down's syndrome, as well as other genetic conditions, such as trisomy 13 and trisomy 18.
Norton's team said the test caught all 38 cases of Down's syndrome. The standard screening test found just 30 of the 38 cases.
It also had a lower rate of false positives — but not a zero rate, pointed out Lyn Chitty of Great Ormond Street Hospital, a specialized children's hospital in London. "This study supports the need for invasive testing to confirm a positive cfDNA result: nine of the 47 results indicating a high risk of trisomy 21 were false positives," Chitty wrote in a commentary. There were 854 false positives using standard screening.
Not all parents opt to do anything when they discover a pregnancy may carry a chromosomal abnormality, and more and more parents are actively opting to have and celebrate babies with Down's syndrome. But the test can prepare parents for the coming challenges.
Critics note that false-positive tests can unnecessarily upset parents and could potentially cause some to abort fetuses that may not have actually had a chromosomal syndrome.
Norton said any prenatal test needs to be discussed thoroughly with a patient.
"Providers need to be attuned to patients' preferences and counsel them about the differences in prenatal screening and diagnostic testing options," she said in a statement.
"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."
The usual test for Down's looks for cells from the fetus, and usually an ultrasound is done to look for structures associated with the syndrome.
Amniocentesis is the most accurate test and detects a wide range of genetic problems but is usually not done until 18 weeks gestation, and the test itself can cause miscarriage in rare cases. Chorionic villus sampling (CVS) takes a tiny piece of the placenta and can be done as early as 10 weeks, but it has a one-in-100 risk of miscarriage.
The American College of Obstetricians and Gynecologists recommends that cell-free DNA tests be offered to women over 35 but not to women whose risk of having a baby with Down's syndrome or another chromosomal abnormality is low.
Several companies offer the cfDNA tests and charge $1,700 or more for them. Norton's group got funding from Ariosa Diagnostics, which makes one of the tests.