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Annals of Clinical Biochemistry

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This version was published on 1 November 2008
Ann Clin Biochem 2008;45:567-570
doi:10.1258/acb.2008.008057
© 2008 Association for Clinical Biochemistry

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Original Articles

Median parameters for Down's syndrome screening should be calculated using a moving time-window method

T M Reynolds1,2 and J Aldis2


1 Clinical Chemistry Department, Queen's Hospital, Belvedere Road, Burton-on-Trent, Staffordshire DE13 0RB; 2 Northern General Hospital, Sheffield S5 7A9, UK


Corresponding author: Professor T M Reynolds. Email: Tim.Reynolds{at}Burtonh-tr.wmids.nhs.uk

Background: 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.

Methods: Down's screening data for 61,368 consecutive samples (12 October 2004 to 31 December 2007) was evaluated using different median assignment techniques.

Results: 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.

Conclusion: Software developers need to change the way medians are derived in their programmes.


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