Researchers develop first non-invasive prenatal screening Chinese University researchers yesterday said they believe they had found the 'Holy Grail' of prenatal screening by developing the first non-invasive diagnostic test for Down's syndrome. The blood plasma test detects RNA molecules of a specific gene called PLAC4 located in chromosome 21, which is expressed only in the placenta of a baby. The mother's blood plasma is taken and then the ratio of RNA molecules is measured to determine whether she is carrying a Down's baby. The test results are available in one to two days, and doctors say it could cost HK$2,000 when available in two years, compared with HK$5,000 for the test used at present, amniocentesis. Down's syndrome is a common disorder caused by an extra chromosome 21. It afflicts one in 700 babies and can result in congenital heart defects and mental retardation. In the past, 60 to 70 babies were born in the city each year with Down's syndrome, but with genetic screening now more widely accepted, about 20 Down's babies are born a year. 'It is a journey that has taken us 10 years of work,' said Dennis Lo Yuk-ming, a geneticist who is director of the Li Ka Shing Institute of Health Sciences at Chinese University. 'This is the Holy Grail of prenatal diagnosis. [Researchers] have been looking for it for 40 years.' The experimental diagnostic test's sensitivity was 90 per cent and its specificity was 96 per cent, he said. 'This is the most accurate single marker test for Down's syndrome that is known today. What we are doing now is to push this sensitivity and specificity to 100 per cent,' Dr Lo said, adding that the test could be available clinically in two years. In 1997, Dr Lo and his team discovered for the first time in the world the presence of fetal DNA in the blood plasma of pregnant women. In 2000, he said his team was on its way to developing a DNA blood test for Down's syndrome. But Dr Lo said the DNA blood test had not been working as expected and so the team shifted its focus to RNA. The test, however, could only be applied to half of pregnant women, theoretically allowing the halving of the number of invasive tests, said Lau Tze-kin, professor at the university's department of obstetrics and gynaecology. 'It still has a limitation, but this is because it is the latest technology that requires further [fine-tuning],' Dr Lau said. The team will look for other markers to detect Down's syndrome. They believe the technique can be used to detect other chromosomal abnormalities, such as Patau and Edward syndromes, and even various cancers. It is estimated that 70 to 80 per cent of some 50,000 pregnant women each year undergo diagnostic screening for Down's syndrome as more put off having their first child until they are aged 35 and over, which increases the risk of having a Down's baby.