A more accurate blood test to detect fetal Downs syndrome launched

By Dr. Ananya Mandal, MDOct 20 2011

Pregnant mothers in the U.S. who wish to know whether they're carrying a fetus with Down syndrome now have access to a commercial genetic blood test that has a 99 per cent accuracy rate.

Down syndrome, which results in a range of mental retardation, cognitive delays and other birth defects, is caused by having an extra copy or parts of chromosome 21. Current screening tests such as amniocentesis involve inserting a needle into the woman's womb to get a sample for diagnosis. Amniocentesis is invasive and comes with a risk of miscarriage.

But as this week, Sequenom is offering women in 20 major U.S. cities a blood test that analyzes fetal DNA in the woman's blood. The company published a study this week in the journal Genetics of Medicine that suggested the test picked up 98.6 per cent of fetuses with Down syndrome with a false-positive rate of 0.20 per cent. The blood test, called MaterniT21, can be used as early as 10 weeks into a pregnancy for those at high-risk.

“The pregnant women can be safely assured that all we will be taking is just a blood sample,” said Dr. Rossa Chiu of the University of Hong Kong, who helped develop the test. “The majority of cases would not need to go on to an invasive test.”

Up to six per cent of babies are born with some sort of problem, and a substantial proportion of those are potentially amenable to prenatal diagnosis, said Dr. Mark Evans, a professor of obstetrics and gynecology at Mount Sinai Medical Center in New York. “While prenatal diagnosis conjures up the Machiavellian fears of search and destroy missions, in fact, what we do for most patients is give them good news and reassure them,” Evans said. Prenatal diagnosis doesn't aim to completely replace invasive tests like amniocentesis but to give doctors a better handle on who really needs those procedures, Evans said.

“The goal is to give families and women the best information that they can use for their own individual decisions,” said Dr. Douglas Wilson, a medical geneticist at the University of Calgary. When a woman is known to be carrying a baby with Down syndrome it will change her obstetrical care, Wilson noted.

“You will have dramatically fewer procedures,” said Dr. Stephen A. Brown, an associate professor of obstetrics and gynecology at the University of Vermont who has no financial relationship with any of the companies. “It’s a game-changer.” “It’s better than anything by far that we’ve ever seen in testing for Down syndrome noninvasively,” said Jacob A. Canick, a professor of pathology at Brown University and the senior author of the study.

Another company, Verinata Health, has said it would introduce a similar test in early 2012. Gene Security Network hopes to have a test ready later in 2012. The test is expected to cost about $1,900, about as much as amniocentesis. The company said that privately insured women would have to pay $235 out of pocket, with the company assuming the risk of getting insurers to pay the rest. Sequenom’s test has not been approved by the Food and Drug Administration. The agency has typically not regulated tests that are offered by a single laboratory, although it has said it might start doing so. In New York, the test will not be available immediately because the state has its own approval process.