7th June 2013 - Routine screening of pregnant women using a non-invasive blood test can accurately detect Down's syndrome in the first three months of pregnancy, according to new research.
A study in the journal Ultrasound in Obstetrics & Gynecology says the results suggest that the test is superior to currently available screening strategies and could redefine best standards in prenatal testing for Down's and other genetic foetal abnormalities.
The test can also be made earlier in pregnancy than the current range of checks.
The Down's Syndrome Association says the widespread use of the blood test is still some way off.
Combined tests
Current screening for Down's involves an initial two-stage check which can provide information about the chance of a baby having Down’s syndrome.
This 'combined test' involves:
A blood sample taken from the mother at around 11 to 13 weeks which is used to measure the amount of some substances that are found naturally in the mother’s blood.
An ultrasound scan carried out at around 11 to 13 weeks to measure the amount of fluid lying under the skin at the back of the baby’s neck. This is called the nuchal translucency (NT) measurement.
Those women who are found to be at higher risk of having a baby with Down's are offered a diagnostic test. Only these tests - either chorionic villus sampling oramniocentesis - can definitely detect or rule out foetal genetic abnormalities. However, unlike the screening tests, these are invasive to the pregnancy and carry a small risk of miscarriage.
Chromosome 21
About 750 babies with Down's syndrome are born in the UK each year.
The condition is caused by the presence of an extra copy of chromosome 21.
A King's College London team led by Professor Kypros Nicolaides has demonstrated the feasibility of checking for Down's syndrome by testing for foetal DNA in the mother's blood.
Fewer 'false positives'
This testing process - known as cfDNA or cell-free foetal DNA testing - was carried out on 1,005 women with an average age of 37 when they were 10 weeks pregnant. It was found to be more sensitive at detecting Down's and returned fewer 'false positives' than the current combined tests which are carried out a little later in the pregnancy.
The cfDNA test returned false positive rates of 0.1% compared with 3.4% for the combined tests.
“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,” the authors wrote.
However, abnormal results from cfDNA testing still require confirmation by diagnostic tests.
Commenting on the findings, Carol Boys, chief executive of the Down’s Syndrome Association (DSA) says in a statement: "The latest results from Dr Nicolaides and his team at King’s College show that the use of an early non-invasive blood test that could be used throughout the national screening programme is still a fair way off.
"The test is becoming more accurate. There are still small but significant anomalies that make it difficult to see how this could replace the current screening process. For pregnant women, this test has become an additional option, and the screening period has been extended by a further two weeks. Invasive procedures are still required to confirm results.
"The DSA consider it far more important at this point to focus on providing relevant, accurate and up-to-date information about Down’s syndrome, delivered by midwives and associated health professionals, who have received our targeted training prior to any screening test.
"We are currently seeking full funding to ensure that our 'Tell it Right, Start it Right' training can be rolled out nationally in readiness for the time when the non-invasive diagnostic test in early pregnancy is a reality in the UK. We do not believe that this is imminent."
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