Down's syndrome blood test more accurate than existing screening

Research shows that a new prenatal blood test for Down's syndrome outperforms current methods of screening.

The test, developed by the Roche-owned Ariosa Diagnostics, correctly identified all instances of Down's syndrome in the study, with a false-positive rate a fraction of that seen with the existing first trimester combined test.

The researchers say that the accuracy of the test could mean fewer pregnant women have to undergo amniocentesis, which carries a one in 100 risk of miscarriage.

The team, from the University of California, San Francisco, studied over 15,000 women who underwent standard screening for Down's syndrome at weeks 10 to 14 gestation, as well as the new cell-free DNA (cfDNA) test.

They report in the New England Journal of Medicine that the cfDNA test identified all 38 cases of Down's syndrome that were later confirmed by diagnostic genetic testing or examination at birth, giving a 100 percent sensitivity. This compared with the detection of 30 cases, and a sensitivity of 78.9 percent, with standard testing.

Furthermore, cfDNA testing produced a false-positive rate nearly 100 times lower than that of standard screening; there were only nine false-positives compared with 854 with standard screening.

The cfDNA test also detected nine of the 10 cases of Edwards' syndrome (trisomy 18) and both cases of Patau's syndrome (trisomy 13) in the study sample.

Current screening procedures for Down's syndrome and other aneuploidies consist of blood biochemistry and ultrasound to assess nuchal translucency. However, when a positive result is found, amniocentesis is needed to confirm the diagnosis.

The new test, marketed as Harmony, analyses the small percentage of fetal DNA that is found floating in a pregnant woman's plasma. Increased quantities of DNA from certain chromosomes, such as 21 in the case of Down's syndrome, can indicate chromosomal abnormality.

Last month, Great Ormond Street Hospital in London announced that it would begin offering a non-invasive prenatal test for Down's syndrome and other trisomies, developed through the NHS's RAPID (Reliable Accurate Prenatal Non-Invasive Diagnosis) Project.

Discussing the new research findings, Professor Lyn Chitty from the Hospital told the BBC: 'These are really exciting times; this cell-free DNA is changing prenatal care dramatically.

'I think it broadens access to testing. Really, a number of women will decline invasive testing because of the risk of miscarriage and they may well take up non-invasive prenatal testing.'

Both tests are to be assessed by the UK's national Down's syndrome screening programme and a decision over whether they are rolled out nationally in the NHS is due in June.