A test using only a blood sample taken from a pregnant woman is more reliable than current checks in indicating the likelihood of Down's syndrome, say researchers.
In a study of 1,005 pregnancies, the new test was applied at the ten-week stage. Results were compared with the outcome of the standard 'combined test' currently offered in the UK which involves an ultrasound scan and a different maternal blood test performed between 11 and 13 weeks' gestation.
Both the new test and current screening techniques gave an indication of all subsequent cases of Down's syndrome or two other similar chromosomal conditions. However, the new test was able to offer predictions with greater certainty.
A clearer improvement on the combined test appeared when comparing false-positive results. Here the new test incorrectly indicated an elevated risk of Down's just once, or 0.1 percent of the time. The combined test had a false-positive rate of 3.4 percent.
Kypros Nicolaides, professor of fetal medicine at King's College London, who led the research, said the test was 'nearly diagnostic. It tells you almost certainly your baby has Down's or almost certainly it does not'.
The result of the combined test is a risk assessment for Down's syndrome, presented in the form of 'one in 150' or 'one in 1,000'. To be certain of whether or not the fetus is actually affected an invasive test would be necessary. Both of the invasive tests currently offered carry a one in 100 risk of miscarriage.
The new maternal blood test presents results as very high (greater than 99 percent) or very low (less than one in 10,000) risks. The greater reliability comes from analysis of the fetus' own DNA, called cffDNA, circulating in the mother's blood.
cffDNA testing is already available commercially in several countries, including the USA. Whether and how it should be available in the UK is currently being evaluated. The BBC reports that Professor Nicolaides is leading a two-year study of 20,000 pregnant women to help with the ongoing assessment. A wider programme, called RAPID, encompassing fundamental, clinical and health economics research is also underway (see BioNews 703).
But for Carol Boys, chief executive of the Down's Syndrome Association (DSA), the study showed how incorporation of cffDNA testing into the national screening programme 'is still a long way off'.
She added that the DSA's training programme to help health professionals provide 'relevant, accurate and up-to-date information about Down's syndrome' would be ready at that time.
The study was published in the journal Ultrasound in Obstetrics and Gynaecology.