Down's test success

September 10, 1999

A safer, more accurate technique for screening foetuses for Down's syndrome has been devised by a team from St Bartholomew's Hospital in London.

The group, led by Nicholas Wald, chairman of the Wolfson Institute of Preventive Medicine, combined results from blood tests and ultrasound scans to give an 85 per cent detection rate.

The "integrated test", details of which have been published in the New England Journal of Medicine, will cut the number of women requiring the more hazardous diagnostic test to just 0.9 per cent and hence greatly reduce the number of unaffected foetuses who are miscarried as a result, the scientists say.

"The integrated test is far more effective than any other Down's syndrome screening method currently available and it results in a much reduced risk of losing an unaffected foetus," Professor Wald said.

Down's syndrome children have an extra chromosome that results in physical and mental abnormalities and an early death. About one in 700 babies has it, and the older the mother, the higher the risk.

The current screening technique is to use either ultrasound or blood tests to assess the risk that a woman's baby may have Down's syndrome. Those who gain a positive result are offered a diagnostic test that will determine for certain if the foetus is affected. This usually involves removing a sample of fluid from around the baby through a needle inserted into the abdomen, and it carries a

1 per cent chance of triggering a miscarriage whether or not it has the syndrome.

The new computerised test uses the results of ultrasound measurements of the foetus and the levels of a key protein in the blood taken in the first trimester of pregnancy. A month later, another set of blood tests is carried out to identify abnormal levels of four substances associated with high risk of the disease. These are combined to give a single estimate of the woman's risk of having a Down's syndrome child.

The new test gives an 85 per cent detection rate with only a 1 per cent chance of a false positive result. Standard methods catch about 70 per cent of cases. The new test's principal drawback is that its results come later than those of standard screenings.

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