Down syndrome blood test could cut invasive testing
A blood test being developed to screen for Down syndrome could practically eliminate the need for more invasive testing procedures, such as amniocentesis, a new study suggests.
The test works by using something called "multiplexed maternal plasma DNA sequencing" to analyze genetic material in the blood from both the pregnant woman and her fetus. A new study in the British Medical Journal finds that the test is highly accurate in detecting Down's syndrome in unborn babies.
If the new test were used on all pregnancies at high risk of Down syndrome, "about 98 per cent of the invasive diagnostic procedures could be avoided," the authors write.
Amniocentesis and CVS (chorionic villus sampling) are the prenatal tests currently used to detect Down syndrome (technically called trisomy 21). The tests are performed between the 10th and 15th week of pregnancy and can also screen for other less common birth defects, such as trisomy 13 and trisomy 18.
The tests are performed by inserting a needle into the uterus to take a tiny sample of the amniotic fluid surrounding the fetus or the tissue that will develop into the placenta. While the tests are reliable, they carry a 0.5 to 1.0 per cent risk of sparking a miscarriage.
The tests are generally reserved for those who have already shown abnormal results on blood work conducted in the 11th week of pregnancy, or to couples who have another child with a birth defect. In Canada, about five per cent of pregnant women undergo amniocentesis or CVS.
The new blood test is much less invasive and requires just a blood sample. For this study, a research team led by Prof. Dennis Lo of The Chinese University of Hong Kong used the test on 753 pregnant women in Hong Kong, the U.K. and the Netherlands who were at high risk of having a baby with Down syndrome.
Eighty-six of the women were found to be carrying a fetus with Down's syndrome. The test was highly accurate in detecting the birth defect in the unborn babies. What's more, there were no false-negative results in the study.
Two forms of the test were used: 2-plex and 8-plex sequencing. "The performance of the 2-plex protocol was superior to that of the 8-plex protocol," the authors wrote.
"With the 2-plex protocol, trisomy 21 fetuses were detected at 100 per cent sensitivity and 97.9 per cent specificity, which resulted in a positive predictive value of 96.6 per cent and negative predictive value of 100 per cent," the write, meaning that there were no false negatives and 3.4 per cent false positives.
The authors conclude that the blood test could be used to accurately rule out Down syndrome among high-risk pregnancies before amniocentesis or chorionic villus sampling is considered, thereby reducing the number of women who would require the more invasive procedures.
Still, some doctors note there are still a few hurdles to cross before the test would be widely used.
Dr. Mark I. Evans, an obstetrician at Mount Sinai School of Medicine in New York City notes that the test can only screen for Down syndrome, but won't pick up other chromosomal birth defects.
"The bad news is that prenatal diagnosis is not just about Down syndrome," Evans told WebMD. "Down syndrome represents about 50 per cent of what we find, and this new test can help with that. But it may also give women a false sense of reassurance."
The other problem with the test is cost. Current early screening tests cost less than $100 a patient. A commercial version of this DNA test – being developed by a company called Sequenom Inc. – is expected to sell for about $700.