"A DNA blood test for Down's syndrome could save nearly all pregnant women from invasive tests like amniocentesis," reported BBC News. It said that invasive testing, which slightly increases the risk of miscarriage, is carried out in 3% to 5% of pregnant women in the UK – some 30,000 each year.
The news story is based on a high quality diagnostic study comparing the accuracy of a new non-invasive technique, maternal plasma DNA sequencing, to both amniocentesis and chorionic villus sampling. The method involves testing a pregnant woman's blood for genetic signs that her baby has Down’s syndrome. Two levels of the test were compared, the more accurate of which could identify Down’s in 100% of cases. The test also had a 3.4% possibility of a false positive, i.e, there was a slight possibility that babies identified as having Down’s did not have the condition. Therefore, a positive diagnosis would potentially need to be confirmed. However, this would still result in fewer women being exposed to these invasive techniques.
These are promising findings. Further large-scale assessments of the accuracy and feasibility of this test are needed before it could be introduced into standard care.
The study was carried out by researchers from the Chinese University of Hong Kong and other institutions in Hong Kong, the Netherlands and UK. Individual authors received support from the University Grants Committee of the Government of the Hong Kong Special Administrative Region, among other foundations and organisations. The study was published in the peer-reviewed British Medical Journal.
The media has reported this new technique quite accurately.
The aim of this study was to test the efficacy of a new blood DNA sequencing test for Down’s syndrome. People born with Down’s have three, rather than two, copies of chromosome 21. The test was designed to find this abnormality (called foetal trisomy 21) in pregnant women considered to be at a high risk of having a baby with Down’s.
Validating a new test against the “gold standard” or best diagnostic test is the best way of seeing how accurate the test is. Amniocentesis or chorionic villus sampling are considered to be the best methods for detecting Down’s in the womb. Amniocentesis involves a sample of the amniotic fluid surrounding the baby. Chorionic villus sampling involves a sample of placental tissue. Both techniques use ultrasound to guide a needle to obtain a sample of the genetic material from the developing baby or placenta, which can then be tested for Down’s. Due to their invasive nature, both techniques carry a small risk of miscarriage (often quoted as about 1%).
Pregnant women carry DNA from the developing foetus in their blood. It has been proposed therefore, that DNA sequencing could be used for non-invasive detection of foetal chromosomal defects. This diagnostic study was designed to see whether the technique could accurately confirm or exclude Down’s.
Between October 2008 and May 2009 pregnant women who were going to have amniocentesis or chorionic villus sampling were recruited from eight obstetric units in Hong Kong, one unit in the Netherlands and one unit in the UK. The women had the standard diagnostic test and blood samples taken for laboratory DNA analysis using the new technique. The researchers also used archived blood samples taken from pregnant women between 2003 and 2008. The DNA of the older blood samples was therefore examined with the prior knowledge of whether the baby had Down’s syndrome or not (either at birth or through diagnostic testing during pregnancy).
In total, blood samples were available from 753 women whose babies had been given a definite diagnosis of having Down’s or not having it. A total of 86 of the 753 babies had Down’s (40 from the archived blood samples, 46 among the women who were pregnant at the time of the study).
The DNA sequencing involved a technique known as “multiplexing”. Multiplexing allows more than one plasma sample to be sequenced on a glass slide. The researchers tested two levels of multiplexing – 2-plex and 8-plex – where DNA from two or eight maternal plasma samples were sequenced in each slide. The 2-plex protocol allows more plasma DNA molecules to be analysed at once than the 8-plex. All 753 maternal samples were tested using 8-plex sequencing and 314 were also tested using 2-plex sequencing.
The researchers found that performance of 2-plex sequencing was superior to that of the 8-plex. 2-plex sequencing had 100% sensitivity for detecting Down’s foetuses; i.e. all of the babies who had Down’s were identified by the test. It also had 97.9% specificity, which means that almost all mothers of babies without Down’s tested negative, but it also gave a few false positives. Alternatively, these results can be described from the woman’s perspective. A negative predictive value of 100% means that if you had a negative blood test you could be 100% certain the baby didn’t have Down’s. A positive predictive value of 96.6% means that with a positive test there would be a 3.4% chance the baby didn’t actually have Down’s.
The results from 8-plex sequencing were less satisfactory. Although it had a higher 98.9% specificity (slightly more reliable than 2-plex for correctly excluding Down’s), its sensitivity was much less and it only identified 79.1% of babies with Down’s.
The researchers conclude that multiplexed maternal plasma DNA sequencing analysis could be used to rule out foetal trisomy 21 among high-risk pregnancies. They say that if referrals for amniocentesis or chorionic villus sampling were based on the sequencing test results, about 98% of the invasive diagnostic procedures could be avoided.
This is a high quality diagnostic study. The findings validate the accuracy of a new maternal plasma DNA sequencing test, comparing the results with confirmatory diagnosis obtained through amniocentesis or chorionic villus sampling, currently the gold standard tests for diagnosing Down’s from genetic samples.
The study also compared two levels of multiplex sequencing, 2-plex and 8-plex, finding 2-plex to have 100% accuracy at detecting babies with Down’s syndrome. However, specificity was slightly less and there was a 3.4% possibility of a false positive. This means that people who got a negative result could be confident their baby did not have Down’s. With a positive test result however, there would be the slight possibility the baby did not actually have Down’s. This would mean that a positive result would need to be confirmed by other tests.
As the researchers aptly conclude, the main value of maternal plasma DNA sequencing would be in excluding the possibility of Down’s syndrome. A positive test may still lead to amniocentesis or chorionic villus sampling being used to confirm the baby had Down’s.
These are promising findings of the use of a non-invasive and comparatively safe test for Down’s syndrome. However, the performance of this test and its feasibility for use on a large scale in standard obstetric care has not yet been assessed. Further testing of the strengths and limitations of this technique are likely to be carried out before it would be introduced into standard care.