Blood Test Might Identify Down Syndrome

Tuesday, October 14, 2008

For pregnant women worried about Down syndrome and other chromosomal abnormalities in their unborn child, a new blood test could ease their minds in several ways, a small Stanford study suggests.

In current practice, older women and those with telltale signs on ultrasound or abnormal levels of certain blood proteins usually undergo a definitive test for extra chromosomes such as chorionic villus sampling (CVS) or amniocentesis.

These involve inserting a needle into the uterus to retrieve tissue or fluid rich in fetal DNA. Because such tests cause miscarriage up to 1 percent of the time, women must weigh the risks of getting the information against the stress of uncertainty.

A blood test could minimize both worries, according to Stephen Quake and his team at Stanford University. Quake says he was sensitized to the challenges when his wife underwent prenatal testing: "It was a nerve-wracking experience."

Since the discovery 10 years ago that tiny amounts of fetal DNA circulate in the mother's bloodstream, researchers have been working to extract information -- such as the baby's sex -- from it.

But distinguishing extra chromosomes in this trace DNA has been difficult, according to Dennis Lo of the Chinese University of Hong Kong, who is involved in a separate effort to develop a blood test for Down syndrome.

By applying a technique known as shotgun sequencing, Quake's team was able to measure if a chromosome is over- or underrepresented in the mother's blood as early as the 10th week of pregnancy.

Out of 18 cases they wrote about in last week's issue of Proceedings of the National Academy of Sciences, the researchers identified nine fetuses with Down syndrome and three with extra copies of other chromosomes; the six others were considered normal.

Larger studies will be needed before the test is ready for clinical use. Though it now carries an estimated $700 price tag, the researchers expect costs to drop quickly.

Deciding what to do with the information is another question. While it's difficult to determine how many babies with chromosomal abnormalities are conceived, the Centers for Disease Control and Prevention estimate that Down syndrome is found in about one in 800 live births.

-- Ishani Ganguli

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