Evaluation of strategies for Downs Syndrome screening

Downs Syndrome is a chromosomal disorder affecting approximately 1 in 700 live births in unscreened populations. Definitive prenatal diagnostic tests are available but they are invasive and can cause miscarriages. Non-invasive tests earlier in pregnancy have the potential for identifying pregnancies that are at sufficient risk to warrant the invasive diagnostic test being performed. A model has been developed to evaluate possible screening strategies. Although at first sight this problem seems amenable to traditional decision analysis, our alternative method has proved more fruitful. The method can be used to derive closed form analytical expressions for a number of relevant outcome measures. This modelling allows the study of the effect of the timings of the two tests within pregnancy, optimisation of screening options and investigation of effects related to diagnostic accuracy.