A definitive blood test for Down syndrome may finally be on the near horizon, allowing pregnant women to know with near certainty whether their fetus has the genetic disorder without facing the small miscarriage risks associated with drawing amniotic fluid.
The test, which analyzes DNA material from the fetus circulating in the mother's blood, was able to accurately detect all 86 fetuses with Down syndrome in 753 pregnant women deemed to be at high-risk due to previous screening test results looking at other blood markers, according to a study published this week in the British Medical Journal. The test did, though, mistakenly identify Down in about 2 percent of fetuses which were later found not to have the genetic condition.
While various versions of genetic blood tests for Down have been in testing for a few years, this is the first large-scale study to show proof of concept: that such a test can work in the real world -- at least in women at high risk of having a Down baby.
The false positive rate could be higher in lower-risk women, which means that any positive test result might still need to be verified with a definitive result -- a drawing of amniotic fluid to look at the fetus's chromosomes themselves -- before a woman makes a decision about whether to continue the pregnancy.
Given that more testing needs to be done in lower risk populations, parents-to-be will probably need to wait a few more years before such a test becomes widely available. In the meantime, a combination blood test/ultrasound screening technique can predict with 95 percent accuracy whether a fetus has Down, which occurs in as many as 1 in 800 babies.
The combination screening involves a blood test in the first trimester to measure various blood markers predictive of Down risk, as well as an ultrasound to detect excess fluid buildup at the back of a fetus's neck, a sign of Down. The American College of Obstetricians and Gynecologists recommends that all pregnant women, regardless of their age, be offered it instead of amniocentesis as an initial screening.
Those women found to be at high risk of carrying a fetus with Down can then opt for amnio to know for certain.
The current screening recommendations, adopted by ACOG in 2007, still fail to detect 5 percent of Down cases in women who undergo screening -- which would be avoided by the genetic blood test. But it has already reduced the number of miscarriages that were caused by routinely performing amniocentesis on all women over age 35.
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