Rapid and simple prenatal DNA diagnosis of Downʹs syndrome

Background Prenatal diagnosis of chromosomal abnormality requires cytogenetic analysis of amniotic fetal cells. The necessary culture time delays diagnosis, is expensive, and requires substantial scientific expertise. In a masked prospective study, we investigated the feasibility of PCR amplification of chromosome 21 markers for the prenatal diagnosis of Downʹs syndrome. Methods The study population consisted of 2167 pregnant women, undergoing amniocentesis for prenatal diagnosis. In this cohort at least 1.5 mL amniotic fluid was available surplus to the requirements for traditional diagnostic methods. DNA was extracted from the surplus amniotic fluid and amplified in fluorescence-based PCR reactions, with three small-tandem-repeat markers located on chromosome 21. The products of the reactions were analysed on a DNA sequencer to identify the presence of two or three copies of chromosome 21. Findings In 2083 (97•4%) of 2139 samples of amniotic fluid that were not macroscopically blood-stained, two DNA markers gave an informative and correct result, identifying 2053 fetuses as normal and 30 as having trisomy 21 Downʹs syndrome (as confirmed by cytogenetic analysis). An extra marker was informative in 32 of 41 other clear samples. Thus a total of 99•6% informative results was achieved with these three markers. Macroscopically bloodstained samples (28 [1•3%]) were unsuitable for DNA testing. They gave a typical but non-informative result. There were no false-positive or false-negative results. Interpretation The PCR-based DNA diagnostic test has great potential for improved prenatal diagnosis of Downʹs syndrome, with the advantage that results may be available within a day.