08:41am Tuesday 07 July 2020

New, more accurate test for Down's syndrome developed

The research is published in two papers today in the journal Ultrasound in Obstetrics & Gynecology.

Down’s syndrome, also referred to as trisomy 21, is a genetic disorder caused by the presence of all or part of an extra copy of chromosome 21 in a person’s DNA. Current screening for Down’s syndrome and other trisomy conditions includes a combined test done between the 11th and 13th weeks of pregnancy, which involves an ultrasound screen and a hormonal analysis of the pregnant woman’s blood. Methods such as chorionic villus sampling (CVS), which involves taking cell samples from the placenta, and amniocentesis (using a sample of amniotic fluid), are also used to detect abnormalities but they are both invasive and carry a risk of miscarriage.

Several studies have shown that non-invasive prenatal diagnosis for trisomy syndromes using fetal cell free (cf) DNA from a pregnant woman’s blood is highly sensitive and specific, making it a potentially reliable alternative that can be done earlier in pregnancy.  

Kypros Nicolaides, Professor of Fetal Medicine at King’s College London and Head of the Harris Birthright Research Centre for Fetal Medicine at King’s College Hospital, and colleagues have now demonstrated the feasibility of routine screening for trisomies 21, 18, and 13 by cfDNA testing. Testing done in 1005 pregnancies at 10 weeks had a lower false positive rate and higher sensitivity for fetal trisomy than the combined test done at 12 weeks. Both cfDNA and combined testing detected all trisomies, but the estimated false-positive rates were 0.1 percent and 3.4 percent, respectively.

‘This study has shown that the main advantage of cfDNA testing, compared with the combined test, is the substantial reduction in false positive rate. Another major advantage of cfDNA testing is the reporting of results as very high or very low risk, which makes it easier for parents to decide in favour of or against invasive testing,’ said Professor Nicolaides.

A second Ultrasound in Obstetrics & Gynecology study by the group, which included pregnancies undergoing screening at three UK hospitals between March 2006 and May 2012, found that effective first-trimester screening for Down’s syndrome could be achieved by cfDNA testing contingent on the results of the combined test done at 11 to 13 weeks. The strategy detected 98 percent of cases, and invasive testing was needed for confirmation in less than 0.5 percent of cases.

The authors conclude that screening for trisomy 21 by cfDNA testing contingent on the results of an expanded combined test would retain the advantages of the current method of screening, but with a simultaneous major increase in detection rate and decrease in the rate of invasive testing.

Notes to editors
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‘Implementation of maternal blood cell-free DNA testing in early screening for aneuploidies’ Ultrasound in Obstetrics & Gynecology; Published Online: June 7, 2013

‘First-trimester contingent screening for trisomy 21 by biomarkers and maternal blood cell-free DNA testing’ Ultrasound in Obstetrics & Gynecology; Published Online: June 7, 2013

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