Testing for Down Syndrome: DNA Blood Test is More Accurate than Standard Tests

Down syndrome is a genetic disorder than results from trisomy 21, meaning that there is an extra copy of chromosome 21. Unfortunately, this condition is not rare as 1 in 700 children are diagnosed with Down syndrome making it the most common genetic disorder. The classic characteristics exhibited by individuals with Down syndrome include flattened facial features, reduced muscle tone, upward slanting eyes, small hands, and small feet. As of now the prenatal test that screens for this condition involves measuring levels of proteins and hormones in the mother’s blood as well as having an ultrasound done in order to measure the amount of fluid accumulating around the baby’s neck. These two tests together are called the first trimester combined test. However, for high risk mothers the cell-free fetal DNA (cfDNA) test is used to test for Down syndrome. This test takes the small amount of fetal DNA floating in the mother’s blood and looks to see if trisomy 21 is present. It has been proven to be highly effective at detecting Down syndrome in women who are at high risk of having a child with the condition, but its effectiveness at diagnosing Down syndrome for lower risk women is unknown. 

A study to determine how effective the cfDNA test is compared to the older test was completed by researchers at UCSF. The study originally had 18,955 participants, but after the two tests (cfDNA test and the first trimester combined test) were administered to the participants, who were between 10-14 weeks pregnant, results and observations of pregnancy outcomes were only collected for 15,841 women.  

Of the 38 cases of Down syndrome found in these fetuses the cfDNA test identified 100% of them correctly, whereas the first trimester combined test only identified 30 of the 38 cases. The cfDNA test was also much better at producing less false positive results than the first trimester combined test. Another triumph of the cfDNA test was that it determined the presence of two other genetic disorders, Edwards syndrome and Patau syndrome more accurately than the regular test.

While this test may lead to fewer false positives and as a result less invasive procedures, which could cause miscarriages it also has drawbacks. In the study, this test was not able to be used for pregnant women that had immeasurable amounts of fetal DNA in their blood. Also, it some cases women’s results could not be interpreted. Some of these women who could not be evaluated may have had babies with Down syndrome. As a result, the detection rates of the cfDNA test may have been lowered if these women were included in the study.

I think that this is an interesting concept and that it could be useful in some cases. If it could lead to fewer invasive procedures and ultimately less miscarriages that would be great. However, it seems that it only works for some women and could only be used in certain situations. For the women that can use the test I think it is great because it seems to work much better than the traditional testing. I think more research needs to be done on this test to see if the same results are obtained.

Link to Article: http://www.medicalnewstoday.com/articles/291895.php 

Additional Link: http://www.ucsf.edu/news/2015/03/124336/blood-test-trumps-accuracy-standard-screening-detecting-down-syndrome-early