New DNA test for Down’s syndrome

Doctors have developed a more accurate test for Down’s, Edwards and Patau syndromes.

The test picked up nearly all affected pregnancies and slashed the number of women who wrongly tested positive, sparing them from needless follow-up tests.

Five NHS maternity units used ‘reflex DNA screening’ between April 2015 and August 2016, during which nearly 23,000 pregnancies were checked for Down’s, Edwards and Patau syndromes. All of the conditions are caused by the presence of an extra chromosome in the baby’s cells.

The new procedure, according to a report in Genetics in Medicine, detected 101 of 106 pregnancies affected by the disorders, or 95 per cent, compared with 81 per cent for the conventional test used in hospitals. The rate of false positives, where babies were wrongly identified as having a condition, fell 100-fold with reflex DNA screening to two in 10,000. Compared with regular screening, the new procedure avoided 530 invasive tests to diagnose the disorders.

Reflex DNA screening draws on the same blood and ultrasound tests as conventional screening tests, but if the risk of an affected pregnancy is higher than one in 800, then some of the blood already taken from the mother is sent for DNA analysis. This looks for fragments of DNA that have leaked from the placenta, which reveal whether the baby has extra chromosomes. The woman is only alerted to risk if the DNA tests positive.

The NHS plans to introduce a similar screening procedure in 2018 that will be offered to about 10,000 women a year who are considered to have a greater risk of giving birth to a baby with one of the conditions.

The doctors are now talking to other hospitals to see if they want to adopt the procedure. The five units that took part in the project are Barts Health NHS Trust, The Royal London, Whipps Cross and Newham, Kingston, and Liverpool Women’s Hospital.

Nicholas Wald, director of the Wolfs Institute of Preventive Medicine at Queen Mary, said: “We don’t have to approach women to come back for a DNA test. This enormously reduces the false positive rate and the number of women needlessly made anxious as a result.”

Joseph Aquilina, a consultant obstetrician at Barts Health NHS Trust, said: “Not only is the screening method better than current practice, but I have more time to devote to other clinical needs, as do the nursing and midwifery staff involved.”