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An experimental DNA blood screening could eventually cut down on the number of invasive tests such as amniocentesis, researchers hope. Women in high-risk groups for delivering a Down syndrome baby may have ultrasound scans and hormone level tests to determine in they may need to be assessed with invasive tests like amniocentesis or chorionic villus sampling, which have about one per cent risk of miscarriage, researchers say.

In a new study published in this week's issue of the British Medical Journal, researchers at the Chinese University of Hong Kong and King's College London analyzed genetic material from both the expectant mother and fetus using samples of the woman's blood.

Current initial screening with blood tests and ultrasound has a false positive rate of about five per cent — meaning five per cent of women tested will be told they are carrying a Down syndrome baby when they are not, according to the journal.

If these women were given the proposed DNA blood test instead, most of the invasive procedures could be avoided, the researchers said.

Down syndrome, a major developmental disorder also called trisomy 21, occurs in around one in every 800 live births. People with Down syndrome have three copies of chromosome 21 rather than two.

"If we took into consideration the results of the sequencing test, trisomy 21 could be ruled out in 98 per cent of those pregnancies," the study's authors wrote.

"This would leave just 0.1 per cent of all pregnant women needing referrals for amniocentesis or chorionic villus sampling."

Cost, accuracy questions

The findings suggest the test is feasible for clinical practice, but still too expensive and needs more study before it could be used routinely in about 10 years, Prof. Kypros Nicolaides of King's College London, who led the research team, told the BBC.

"As yet, the results are not accurate enough to inform important decisions, like whether to continue the pregnancy," added Professor Lyn Chitty of University College London, who is also testing the DNA blood tests for Down, said on the university's website.

The large study was based on 753 pregnant women in Hong Kong, the UK and the Netherlands.

Of these, 86 were found to be carrying a fetus with Down. The test detected this with 96.6 per cent accuracy.

In Canada, prenatal screening at around 11 weeks is used to determine a pregnant woman's chance of having a baby with Down or other trisomy conditions that can lead to neural tube defects like spinal bifida or congenital heart disease.

The Calgary-based Canadian Down Syndrome Society has previously said that such testing information is important, but the group stresses the importance of families receiving fair and balanced information about the challenges of a life with Down syndrome as well as the rich rewards.

Several of the study's authors said they've filed patent applications on detecting fetal DNA in mother's blood samples for prenatal screening, which have partly been licensed to test maker Sequenom and the Institut Jacques Boyd.

Source: CBC News

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