New prenatal DNA test for Down's syndrome is more accurate and cheaper

A new prenatal screening method for Down's, Edwards' and Patau's syndromes is safer, more reliable and cheaper than existing screening approaches, a study has found.

Using 'reflex DNA screening', researchers were able to pick up 95 percent of pregnancies with one of the conditions, with a very small error rate of 0.2 percent, according to the study published in Genetics in Medicine. The number of false positives – where parents were told their child had Down's, Edwards' or Patau's syndrome incorrectly – was much smaller than the tests currently used.

These conditions all involve a trisomy, an extra copy of one of the chromosomes. Children born with Edwards' or Patau's syndrome rarely survive their first year, while Down's syndrome can result in significant learning disabilities.

Conventional tests for trisomies involve taking a maternal blood test and ultrasound, combined with information such as the mother's age to calculate the likelihood that the fetus has the condition. The blood markers linked to trisomy conditions include molecules called pregnancy-associated plasma protein A and free beta-human chorionic gonadotropin. If the likelihood is more than 1 in 150, a follow-up invasive test is used to take a sample of the fluid around the fetus or the placenta.

However, in the reflex DNA screening method, part of the blood sample from the initial stages is stored. If there is a more than 1 in 800 risk of a trisomy condition, the remainder of the sample is analysed for DNA from the placenta, which shows whether there are any additional chromosomes present, with no need for an invasive test.

The study monitored the pregnancies of almost 23,000 women in the UK. A total of 2,480 of these were identified as at-risk, and 105 of these tested positive for one of the three trisomies. The new method gave four false positives test results, and five affected pregnancies were not detected. This is much more successful than conventional screening methods, which have an 81 percent detection rate, and an error rate 100 times greater.

As well as being more accurate, the reflex DNA method is cheaper as it involves fewer follow-up visits and invasive procedures. It also doesn't carry the increased risk of miscarriage associated with invasive tests. 

'The reflex DNA approach has substantial benefits to the well-being of the women screened,' said Professor Sir Nicholas Wald, from Queen Mary University of London's Wolfson Institute of Preventive Medicine, who led the study.

'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.'