Screening for Down Syndrome

Down syndrome, or trisomy 21, as it is known in official circles, is a genetic abnormality where most often,  three copies of chromosome 21 are seen in an individual rather than two.  While some such genetic abnormalities are inconsistent with life and the fetus simply fails to develop, trisomy 21 produces a constellation of physical features as well as compromised intellectual function that will affect an individual for their lifetime.  The disorder occurs on average in one of every 700-800 infants.

In this week's podcast, Rick and I talk about a test described in this issue of the British Medical Journal that is capable of detecting Down syndrome with impressive sensitivity and specificity in samples of maternal blood.

Over 750 women who were pregnant and at high risk for trisomy 21 were enrolled in this study.  Blood samples were collected from the mother and analyzed by 'massively parallel sequencing of DNA.'  One method called a two-plex model of analysis was used in just over 300 of the samples; an eight plex sequencing protocol was used on them all.

Using the two-plex protocol, researchers were able to detect trisomy 21 with 100% sensitivity and almost 98% specificity, so as Rick says, you can go to the bank on the results.  This is great news because it allows a simple blood test to be used instead of invasive amniocentesis or chorionic villus sampling, which each carry risks, although admittedly small. 

Rick and I both admit that we're intrigued by the fact that as cited in this paper, about 15% of circulating DNA in maternal plasma comes from the fetus.  The techniques described in the paper assist in detection of the increased amount of chromosome 21 DNA contributed by a fetus with Down syndrome.

Might such a strategy soon be used to screen all pregnant women?  Rick thinks not as the test is still quite expensive, but I suggest that as the technique is perfected and promulgated, the price will come down.  Another advantage in my mind is how early in the course of the pregnancy such screening can be employed.  In this study participants were on average about 13 weeks of gestational age.  I suspect that as the test is used earlier in pregnancy the earliest limit will be identified, but will likely be much sooner than either chorionic villus sampling or amniocentesis.  No doubt women who are pregnant with a fetus with Down syndrome would like to know that as soon as possible.  As detection of Down syndrome is perfected other genetic abnormalities may soon be tested for as well, likely with improved detection limits.

Other topics this week include treating middle ear infections in kids in NEJM, a lack of evidence for expert recommendations from Archives of Internal Medicine, and starting therapy for high blood pressure with two medications instead of one in the Lancet.  Until next week, y'all live well.

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Bette September 4, 2011 at 3:39 am

A good many vaulabels you've given me.


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