(Reuters Health) - Pregnant women may get accurate screening results for Down syndrome – also known as trisomy 21 - and two rarer chromosomal disorders with a finger-prick blood test that reduces the need for more invasive diagnostic procedures, a UK study suggests.

With conventional prenatal testing, women are typically screened for chromosomal disorders in the embryo at 10 to 14 weeks of pregnancy with an ultrasound and a finger prick blood test. Based on results, some women identified as high-risk return to the doctor for genetic counseling and invasive diagnostic tests that require drawing fluid or tissue samples from the placenta for DNA analysis.

The study team tested an alternative approach known as reflex DNA screening. Women still get that initial ultrasound and finger prick, but they provide extra blood that’s set aside to use later for DNA analysis if they’re identified as high risk; they only return to the doctor for invasive diagnostic tests if the DNA analysis points to an abnormality.

“This simple modification has a big impact on the delivery of screening service,” said lead study author Dr. Nicholas Wald of the Wolfson Institute of Preventive Medicine and Queen Mary University of London.

“In the reflex DNA method, most women undergoing an invasive test will have an affected pregnancy,” Wald said by email.

For the study, Wald and colleagues examined data on 22,812 women who had screenings at five UK maternity hospitals for Down syndrome as well as Edwards and Patau syndromes (trisomy 18 and trisomy 13).

These disorders occur when babies have an extra copy of a chromosome. Edwards and Patau syndromes are usually fatal within a few days of birth. Down syndrome is associated with intellectual deficits, developmental delays, thyroid problems and heart disease.

All of the women in the study underwent initial screening for these syndromes, with extra blood drawn and set aside to use if DNA analysis was needed.

Overall, 11 percent of the women had at least a 1 in 800 risk of one of these syndromes, and extra blood samples for these women were used for DNA analysis.

These DNA tests detected 101 of the 106 pregnancies affected with one of the three disorders, researchers report in Genetics in Medicine.

Only four of the DNA tests had so-called false-positive results when women who weren’t carrying babies with one of the disorders were mistakenly told that they were.

This translates into a false-positive rate of just 0.02 percent, compared with a false-positive rate of 2.42 percent with conventional testing methods.

Among women who had positive results and went on to get invasive diagnostic tests, 25 of 26 patients had babies with one of the syndromes.

The results suggest that routine reflex DNA screening could substantially reduce the number of patients who needlessly undergo invasive diagnostic tests for chromosomal disorders, the authors conclude.

One drawback of reflex DNA screening may be the cost, noted Dr. Marsha Wheeler, a prenatal genetics specialist at the University of Colorado School of Medicine in Aurora who wasn’t involved in the study.

Access and affordability in the U.S. would vary based on women’s insurance coverage and the labs used for testing.

“The prenatal reflex DNA screening is effective,” Wheeler said by email. “It has a high detection rate, low need for repeating a blood draw, high positive predictive value and low false positive rate.”

SOURCE: go.nature.com/2zFsN3m Genetics in Medicine, online November 9, 2017.