Uncertainty of the measurements in antenatal screening for Down's syndrome

Prenatal screening for Down's syndrome and some other chromozomal abnormalities is currently based on maternal blood tests and ultrasound examinations. Results of quantitative measurements are used for the risk evaluation. Effectivity of screening models is influenced by the selection of markers and by the quality of measurements. The article gives a basic overview of the uncertainty of measurements and shows how it could influence the whole screening process. The preanalytical phase of the biochemical tests is not discussed in this article, although it could also affect the screening process.