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New Blood Test Offers More Accurate Screening for Down Syndrome

Katlyn Joy |17, April 2015


A new test looks very promising for allowing a better indication of whether the child being carried has a chromosomal abnormality such as Down syndrome. This test is called cell-free DNA test, or cfDNA test, and looks at the bits of baby's DNA floating in mother's blood. It measures the number of chromosomes by high-speed genetic sequencing, and should there be a higher number than expected, Trisomy 21 or Down syndrome is likely.

Researchers compared this newer test to the older ones previously available. The new cfDNA test does not have a 100 percent accuracy rate in detecting Down syndrome, but does offer a much higher accuracy rate than the older tests, at 10 times greater reliability. Significantly, it also has a far lower false positive rate than those tests, at .06 percent rather than 5.4 percent for those older tests.

False positives create a great deal of maternal stress, which can have real negative effects on pregnancy. It also leads to more invasive testing which carries a risk of miscarriage.

Researchers looked at over 15,000 women who were pregnant and eligible for chromosomal testing and compared the results of the standard tests and the new cfDNA test. The women were all carrying just one fetus and were 18 and older with a pregnancy dated between 10 and 14.3 weeks. The women were in the US, Canada and Europe. The study involved 35 different medical centers and was conducted between March 2012 and April 2013. After the testing, information was gathered about the pregnancy outcome, to determine the accuracy of testing results, and also whether the pregnancy was miscarried or aborted.

It's very good news for pregnant women," says Diana Bianchi, a pediatric geneticist at Tufts Medical Center who led the study. "It's very important because it means a significant proportion of women are not being made anxious by being told they have an abnormal test result."

It is also significant because fewer women will be sent for further testing that is more invasive and carries risk of pregnancy loss.

The study involved researchers from Sahlgrenska University Hospital in Sweden, the Perinatal Diagnostic Center in San Jose, the University of California and other American medical centers. The results were published in the New England Journal of Medicine.

Pregnant women to this point have been offered limited choices in finding out about the possibility of chromosomal abnormalities in their unborn child.

A combined blood test done during the first trimester measures levels of pregnancy related plasma protein-A, or PAPP-A, and the pregnancy hormone hcG. When abnormal levels are found, it may indicate a problem and require further testing. The second part of the combined test involves an intensive ultrasound that measures the clear space in the back of baby's neck, since babies with abnormalities often have a larger amount of fluid in the area during the first trimester. This test is a nuchal translucency screening test. Doctors will consider a woman's age, the results of the tests and determine the risk of Down syndrome.

A variation of this testing is the first and second trimester screening, which uses the same tests as mentioned above and the addition of a quad screen, which is a blood test checking four pregnancy substances; alpha fetoprotein, estriol, HCG and inhibin A.

Both of these testing methods are far less than perfect, yielding false positives with regularity. These are only screening tests, so should your results indicate a higher than expected risk of chromosomal abnormalities, you will be referred for more invasive testing such as an amniocentesis, cordocentesis or chorionic villi sampling test. These tests will determine if your child has Down syndrome, but at a risk. These tests may cause miscarriage. Additionally, these tests are often done later in pregnancy.

Down syndrome is a genetic disorder that results in intellectual disability of varying degrees and possibly accompanying physical health problems such as with the heart. Some professionals involved in the Down syndrome community are concerned that more people will choose to terminate pregnancies and Down syndrome children will begin to disappear from our population.

"People with Down syndrome are artists. They're poets. They're athletes. Their lives are happy ones and fulfilling ones. I have a sister with Down syndrome who certainly is a life coach for not only myself but for my entire family," says Brian Skotko. Skotko is co-director of the Down Syndrome Program at Massachusetts General Hospital in Boston.

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