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One-Day Test For Down's Syndrome Created LONDON, ENGLAND -- July 3, 1998 -- Down's syndrome is one of the most common congenital chromosome disorders, affecting about one in every 600 new-born babies. Currently, the most common prenatal test for Down's syndrome involves an amniocentesis, in which amniotic fluid -- the liquid that surrounds the fetus -- is collected. The entire test requires considerable laboratory expertise and takes about 15 days to complete. In this week's Lancet, however, Loveena Verma and colleagues from Birmingham, UK, report a new test for Down's syndrome that can be done in a single day. Children born with Down's syndrome typically have short stature, undersized heads and mild to severe mental retardation. The disorder occurs when the child inherits three copies of chromosome 21 instead of just two. Because there are three copies of the chromosome in the disorder, Down's syndrome is also called trisomy 21. The new test by the investigators involves a technique called polymerase chain reaction (PCR), which makes it possible to take very small amounts of DNA and, by making copies of it, to produce enough for analysis very quickly. The investigators tested amniotic samples from 2,167 women using PCR to detect a set of genetic markers-short segments of DNA that appear only in chromosome 21. In normal cells, with the normal complement of two copies of chromosome 21, the test should detect only two sets of the marker segments, one from each of the chromosomes. But in cells from fetuses with trisomy 21, the test should detect the presence of three sets of the markers from the three copies of chromosome 21 present. Using the 2,083 samples that were eligible (not heavily blood-stained) and two DNA markers only, the test correctly indicated that 2,053 of the fetuses were normal and that 30 had Down's syndrome. The investigators showed that three DNA markers will make the test informative and give the right answer in 99.6 percent of cases. "PCR-based DNA identification of Down's syndrome is a rapid and reliable technique for prenatal diagnosis," the researchers write. E-mail this page to a friend or colleague! To print, use this version