Blood test for Down Syndrome could help avoid invasive procedures

Editor's comment: What are you going to do with the fetus if you learn he or she has Down Syndrome? Abort the pregnancy?

by Aimee Keenan-Greene

Researchers in the British Medical Journal say about 98 percent of  invasive diagnostic procedures like amniocentesis or chorionic villus sampling for Down Syndrome could be avoided if referrals were based on sequencing blood test results.

Scientists say multiplexed maternal plasma DNA sequencing analysis could be used to determine fetal trisomy 21 in high risk pregnancies. 

According to MedPageToday, the method involves testing the blood for free DNA sequences from chromosome 21. Between 10% and 20% of free DNA sequences in a pregnant woman are from the fetus, so an elevated proportion of chromosome 21 sequences indicates trisomy 21, the researchers argued.

Researchers studied more than 750 mothers in prenatal diagnostic units in Hong Kong, United Kingdom, and the Netherlands at high risk for fetal trisomy 21 who underwent definitive diagnosis by full karyotyping.  86 had a fetus with trisomy 21.

Multiplexed massively parallel sequencing of DNA molecules in maternal plasma was done according to two protocols with different levels of sample throughput: 2-plex and 8-plex sequencing. 

Scientists wrote:

A trisomy 21 fetus was diagnosed when the z score for the proportion of chromosome 21 DNA molecules was less than 3. Results were available from 753 pregnancies with the 8-plex sequencing protocol and from 314 pregnancies with the 2-plex protocol.  

The performance of the 2-plex protocol was superior to that of the 8-plex protocol. With the 2-plex protocol, trisomy 21 fetuses were detected at 100 percent sensitivity and 97.9 percent specificity, which resulted in a positive predictive value of 96.6 percent and negative predictive value of 100 percent.  

The 8-plex protocol detected 79.1 percent of the trisomy 21 fetuses and 98.9 percent specificity, giving a positive predictive value of 91.9 percent and negative predictive value of 96.9 percent. 

According to the Down Syndrome Society of Rhode Island, Down syndrome is a common genetic variation which usually causes delay in physical, intellectual and language development. The exact causes of the chromosomal rearrangement  is unknown.  It is one of the leading clinical causes of cognitive delay in the world - it is not related to race, nationality, religion or socio-economic status. 

The Centers for Disease Control and Prevention say the incidence of Down syndrome in the United States is 1 in every 733 live births. There are approximately 400,000 families in the United States affected by Down syndrome. 

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