DNA antenatal screening for Down's syndrome

Scientist have demonstrated that it is possible to incorporate DNA analysis into antenatal screening for three serious chromosome disorders, including Down's syndrome, in a way that is far more accurate than existing methods, and safer and less stressful for mothers.The new method, called antenatal "reflex DNA screening," which screens for Down's syndrome, Edwards syndrome and Patau syndrome, detected more affected pregnancies than the test it replaced, with far fewer false-positives. Reflex DNA screening combines conventional screening with new DNA testing.

At present, women are offered screening for these three chromosome disorders at 10-14 weeks of pregnancy. The test combines an ultrasound scan and a blood test, and if it shows that a woman is at an increased risk of having an affected pregnancy, she is offered a diagnostic test, an amniocentesis or chorionic villus sampling (CVS); these are invasive tests that involve inserting a needle through the mother's abdomen into her womb to collect samples of fluid surrounding the fetus or tissue from the placenta.

With the new method, women provided a blood sample at about 11 weeks of pregnancy. The sample was divided into two; one used for the conventional screening test and one held in reserve. The first sample was used to assess the women's risk of any of the three disorders. If, on the basis of that test, the risk of having an affected pregnancy was 1 in 800 or a higher risk, the other blood sample was automatically retrieved for a DNA test without having to recall the women to provide a fresh blood sample. This avoided needlessly worrying the women by informing them that the DNA test was required because of their increased risk of an affected pregnancy.

Of the 22,812 women screened, 11 per cent had a reflex DNA test. It detected 101 out of the 106 pregnancies affected with one of the three disorders, with only four false-positives, yielding a 95 per cent detection rate (compared with 81 per cent using the existing conventional test alone), and a 0.02 per cent false-positive rate (100-fold reduction in false positives from 2.42 per cent for the combined test). Among pregnancies that tested positive and where the women were offered an invasive diagnostic test, 25 out of 26 were affected by one of the syndromes.

The reflex DNA approach has substantial benefits to the well-being of the women screened. Not only are more affected pregnancies identified, but many fewer women will be made acutely anxious by being notified that they have a positive screening result, and among those women with a positive DNA screening result, almost all will have an affected pregnancy. Reflex screening is also safer than conventional screening as it avoids nearly all invasive diagnostic tests in unaffected pregnancies and miscarriages related to these procedures.

The reflex DNA approach has beneficial resource implications by reducing the number of women who need counselling on account of a positive screening result, and by avoiding about five out of six diagnostic amniocenteses that would otherwise be indicated. The extra costs of the DNA tests are offset against savings from fewer amniocenteses and associated counselling. The overall balance of costs and savings can be chosen by adjusting the proportion of women who have a reflex DNA test.
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