To detect DNA defects which cause Down’s syndrome and similar conditions, a new blood test has been developed which furnishes more accurate results than other screening methods, a research team reported. Dr. Mary Norton of the University of California San Francisco and colleagues who carried out the study informed that the test, called a cell-free DNA test, also produced fewer false positives. The research has been published in the New England Journal of Medicine. The study authors wrote “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.”
Researchers conducted a large trial including nearly 16,000 women, belonging to different ages, many of whom were not at high risk of having a child with a chromosomal abnormality. Norton mentioned that the test provides an option for women to find out very early in pregnancy. The test can be done as early as 10 weeks into a pregnancy. The test looks for free-floating DNA (cfDNA) from the fetus and can find extra copies of chromosome 21, which causes Down’s syndrome. It also detects other genetic conditions like trisomy 13 and trisomy 18. Research team reported that the test detected all 38 cases of Down’s syndrome whereas the standard screening test found just 30 out of 38.
The test has a lower rate of false positives but not a zero rate. Lyn Chitty of Great Ormond Street Hospital, a specialized children’s hospital in London, said “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.” Standard screening furnished 854 false positives.
Norton highlighted that any prenatal test should be discussed thoroughly with a patient. She added “Providers need to be attuned to patients’ preferences and counsel them about the differences in prenatal screening and diagnostic testing options. 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.”
Usually, Down syndrome is tested by looking for cells from the fetus and performing an ultrasound to detect structures associated with the syndrome. Amniocentesis is a very accurate test detecting a vast range of genetic problems but it is usually not done until 18 weeks gestation. In rare cases the test can also cause miscarriage. According to the recommendation of the American College of Obstetricians and Gynecologists, cell-free DNA tests should be offered to women over 35 but not to those who are at low risk of having a baby with Down’s syndrome or another chromosomal abnormality.