A new DNA-based prenatal blood test that could better identify pregnancies with Down syndrome, and reduce risky diagnostic procedures, could soon be available says Women and Infants Hospital.
Down syndrome, known as trisomy 21, is a chromosomal disorder that includes mental retardation, characteristic facial features, and possible heart defects. It affects about one in every 550 babies born annually in the U.S.
Down syndrome occurs when each cell in an individual has three rather than the usual two copies of chromosome number 21. Current prenatal screening tests for Down syndrome combine maternal age with information from the measurement of maternal serum markers and ultrasound markers in the first and second trimesters of pregnancy. These tests can detect up to 90 percent of Down syndrome cases, but they also incorrectly identify 2 – 5 percent of normal pregnancies as positive.
The test, developed by doctors here in Rhode Island, will be offered as early as 10 weeks to pregnant women at high risk for Down syndrome and reduce the “false positive” rate while maintaining the detection rate.
The study is new and online today in the journal Genetics in Medicine . It is the largest and most comprehensive done to date, says Women and Infants Hospital.
The research led by Jacob Canick, PhD, and Glenn Palomaki, PhD, of the Division of Medical Screening and Special Testing in the Department of Pathology and Laboratory Medicine at Women & Infants Hospital and The Warren Alpert Medical School of Brown University, examined nearly 1,700 pregnancies at high risk of chromosomal abnormalities, 212 of which were affected by Down syndrome.
The newer test identified 98.6 percent of the Down syndrome pregnancies, while only 0.2% of the normal pregnancies were mistakenly called positive.
On a larger scale, take a group of 10,000 pregnant women for example. There will statistically be 20 cases of Down syndrome in this group when all is said and done. If all 10,000 took the current screening tests, 500 are identified as potentially “positive” and offered invasive diagnostic testing for further chromosome analysis.
Eighteen of the 20 Down syndrome cases will be among those 500 women who have an invasive procedure. However, 482 of these 500 women who are having a normal pregnancy underwent an unnecessary invasive procedure. More significantly, two procedure-related losses would be expected among these 482 women.
With the new DNA-based test, only the 18 affected pregnancies plus five of the normal pregnancies would now be offered an invasive procedure. All 18 Down syndrome cases would still be correctly identified, and no loss.
“If this new test is used as we’ve described, nearly all women with a normal pregnancy could avoid an invasive diagnostic procedure and its associated anxiety, cost, and potential for fetal loss, ” says Palomaki.
The study also included scientists at Sequenom, Inc. and Sequenom Center for Molecular Medicine, San Diego, CA, and an independent academic laboratory at the University of California at Los Angeles.