This is impressive! A study reported yesterday in the British Medical Journal describes a new blood test that could vastly decrease the number of invasive procedures that are currently used to confirm if a fetus has Down syndrome.
Current blood tests used for Down syndrome screening can identify up to 90-95% of Down syndrome pregnancies but about 5% of normal fetuses are incorrectly identified as having Down syndrome. That means that lots of women have follow-up tests, like amniocentesis, to definitively determine if their baby is affected. That's a problem because amniocentesis is an invasive procedure that can lead to the loss of the pregnancy. Current estimates put the risk of fetal loss due to amniocentesis at about 1 out of 300.
This new blood test measures the amount of DNA from chromosome 21 that is present in the mother's blood. Down syndrome is caused by having an extra copy of chromosome 21. We know that a pregnant woman's blood contains small amounts of her baby's DNA but that amount is very
low compared to the amount of her own DNA in her blood. Using a technique called massively parallel sequencing, this test looks for increased amounts of pieces of chromosome 21 in the mother's blood. In a Down syndrome pregnancy, the mother's blood will have more pieces of chromosome 21 because the fetus has an extra copy of that chromosome.
The study used blood collected from 753 women who were at high-risk of having a baby with Down syndrome. 86 of these pregnancies were carrying a fetus with Down syndrome and the test was able to correctly identify 100% of them. While that alone is impressive, even more exciting is that only 2.1% of the unaffected fetuses were incorrectly identified as having Down syndrome. Stated another way, in 98% of the cases Down syndrome could be ruled-out sparing the need for invasive follow-up testing. Like some of the existing blood tests used to screen for Down syndrome, this new test can be performed in the first trimester of pregnancy.
Although the test is not yet available, there is a company that is working on making it available soon.