Expectant parents and physicians concerned about Down’s syndrome (foetal trisomy 21) and other trisomies want tests to be fast and non-invasive, but most of all they want them to be accurate. The polymerase chain reaction (PCR) can be used to look at DNA circulating in the blood (cell-free DNA or cfDNA), and has potential to create a test to detect the extra chromosomes with higher sensitivity, fewer false positives, and a greater predictive value than standard tests.
A team of researchers, from the US and Sweden, compared cfDNA testing with standard screening during the first trimester in routine prenatal populations at 35 centres across the world, to look at their accuracy for Down’s syndrome, as well as for trisomy 18 (Edwards’ syndrome)and trisomy 13 (Patau’s syndrome). As well as standard aneuploidy screening (with ultrasound measurement of the fluid in the nuchal folds and biochemical tests), the 15,841 women also underwent the PCR-based cell-free DNA test between 10 and 14 weeks of pregnancy. While the women were told the results of the standard tests, the cfDNA test results remained blinded.
The results, which were published in the New England Journal of Medicine, showed that cfDNA blood test was more sensitive than measuring nuchal folds and carrying out biochemical tests, with a lower false positive rate, and higher positive predictive value than standard screening. The cell-free DNA screening is also less invasive than amniocentesis, offered to women at higher risk.
The cell-free DNA blood test correctly identified 38 foetuses with Down’s syndrome, confirmed by examination of the newborn baby, or prenatal or postnatal genetic analysis. The standard screening picked up 30 of these 38 foetuses. The cell-free DNA analysis also resulted in fewer false positives, with just nine false positives, compared with 854 with standard screening. This could reduce worries for parents who decide to carry a suspected Down’s syndrome baby to full term, or cut the risk of terminations of healthy babies.
In other trisomies, the cell-free DNA screening picked up nine of the 10 cases of trisomy 18, with one false positive, compared with eight cases and 49 false positives for the standard screening. For trisomy 13, the cell-free DNA test identified both cases and flagged one false positive, while standard screening identified one case and flagged 28 false positives.
However, cell-free testing may not be able to replace standard screening completely. According to the researchers, standard screening can also identify risk for a broad array of abnormalities that are not detectable on cell-free DNA testing. Some samples also had to be excluded because of inadequate or immeasurable quantity of foetal DNA, or assay failure or high sequencing variance that can lead to difficulties interpreting results.
According to Mary Norton of the University of California at San Francisco, use of the cell-free DNA test could reduce the number of false positives than current screening, and therefore cut the number of invasive tests and associated miscarriages.
“Providers need to be attuned to patients’ preferences and counsel them about the differences in prenatal screening and diagnostic testing options. Those women who do opt for cell-free DNA testing should be informed that it is highly accurate for Down syndrome, but it focuses on a small number of chromosomal abnormalities and does not provide the comprehensive assessment available with other approaches. Counselling should also include information about the risks associated with failed tests and the pros and cons of pursuing invasive testing if no results are obtained.”
BJS Technologies’ xxpress PCR, a fast and accurate thermal cycler with a small footprint, has potential for near patient use and so could be used for applications such as this, allowing results to be returned to the patient more quickly.