Abstract
In Down's syndrome screening, the change in analyte concentrations in maternal serum with advancing gestational age is compensated for by converting concentration to multiples of the median (MoM) by using a mathematical equation describing the expected relationship. However, owing to assay drifts and shift, the equation may be incorrect, leading to deviation of the observed MoM distribution from the ideal MoM distribution. The NHS Fetal Anomaly Screening Programme has produced standards limiting acceptable deviation, and has provided the Down's Syndrome Screening Quality Assurance Service (DQASS) to monitor it. DQASS recommends monitoring by cumulative sum plot. Down's screening data for 61,368 consecutive samples (12 October 2004 to 31 December 2007) was evaluated using different median assignment techniques. A change in the paradigm for median equation derivation is described, which significantly improves the probability that medians will be correct at any point in time. Software developers need to change the way medians are derived in their programmes.
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