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Robin Elise Weiss, LCCE

New Blood Test for Down Syndrome

By January 13, 2011

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Amniocentesis

A new maternal blood test is being perfected to help pregnant women avoid invasive procedures for diagnosing Down Syndrome. The new test uses the mother's blood and looks for the baby's DNA in the blood and then screens for Down Syndrome.

Currently Down Syndrome testing has about a 5% false positive rate, meaning 5% of mothers are told that their baby has Down Syndrome when the baby does not. These women are then offered an amniocentesis or chroionic villus sampling (CVS), which each carry about a 1% risk of miscarriage. Researchers say that the new test is expensive and is not ready for universal screening at this point. But it can reduce the number of mothers referred for the more invasive and risky tests by 98% when screening the women at a high risk for Down Syndrome.

So the new proposal would be, everyone does the more common AFP screening. If that test is positive, then you would have this more advanced maternal blood test and only then would you be referred for invasive testing.

Related:

Source:
Chiu RWK, et al "Non-invasive prenatal assessment of trisomy 21 by multiplexed maternal plasma DNA sequencing: large scale validity study." BMJ 2011; 342: c7401.

Comments
November 10, 2011 at 3:02 am
(1) Jenny says:

Test is now available

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” 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.

“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.

Recap : WITH THE NEW TEST ONLY 5 OUT OF 10,000 WOMEN WILL BE OFFERED AN INVASIVE PROCEDURE.

The blood test is accurate in detecting Trisomy 21, the genetic chromosomal abnormality that most commonly causes Down syndrome, 99.1 percent of the time as early as 10 weeks into a pregnancy.

Recent published data indicate that the core technology which underlies the MaterniT21 test also has the capability to detect other aneuploidies, such as trisomy 18 and trisomy 13.

If either of these aneuploidies is detected by the MaterniT21 test, Sequenom CMM will be reporting these results to the physician.

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