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

This version was published on 1 November 2008

Ann Clin Biochem 2008;45:567-570
© 2008 Association for Clinical Biochemistry


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Original Articles

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}

Background: In Down’s syndrome screening, the change in analyte concentrationsin maternal serum with advancing gestational age is compensatedfor 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 beincorrect, leading to deviation of the observed MoM distributionfrom the ideal MoM distribution. The NHS Fetal Anomaly ScreeningProgramme has produced standards limiting acceptable deviation,and has provided the Down’s Syndrome Screening Quality AssuranceService (DQASS) to monitor it. DQASS recommends monitoring bycumulative sum plot.

Methods: Down’s screening data for 61,368 consecutive samples (12 October2004 to 31 December 2007) was evaluated using different medianassignment techniques.

Results: A change in the paradigm for median equation derivation is described,which significantly improves the probability that medians willbe correct at any point in time.

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

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