Down’s syndrome risk estimates demonstrate considerable heterogeneity despite homogeneity of input

Ann Clin Biochem 2004;41:464-468
© 2004 Association for Clinical Biochemistry


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Original Articles

Tim Reynolds,
Andy Ellis and
Rick Jones

Clinical Chemistry Dept, Queen’s Hospital, Burton-on-Trent, UK and Division of Sciences, Wolverhampton University, Wolverhampton, UK;
UKNEQAS for Down’s syndrome screening Edinburgh, Scotland, UK;
School of Medicine University of Leeds Leeds, UK

Background: Due to concerns about screening quality, Down’ssyndrome screening laboratories were surveyed to identify therange of variation in risk-calculation software.

Methods: All UK Down’s syndrome screening laboratories weresent the confidential survey via the National External QualityAssessment Scheme. This covered the software used, its origin,the risk calculation methodology and the Gaussian populationparameters. Laboratories were also given multiples of the median(MoM) data from five representative cases and asked to calculateDown’s syndrome risks on their systems.

Results: Most parameter sets could be traced to published literature. The range of risk results for identical patient data was wide; the largest differences between lowest and highest risk for a test MoM set were: {alpha}-fetoprotein (AFP)+human chorionic gonadotrophin (hCG), 1:95 to 1:388 = 408%; AFP+hCG+urine estriol (UE3), 1:2011 to 1:7000 = 348%; AFP+free-β-hCG, 1:280 to 1:1681 = 600%; AFP+free-β-hCG+UE3, 1:340 to 1:13000 = 3823%. Two explanationsfor this variation – the prior age risk result (half-year correction)and the population parameters – are described.

Conclusions: (a) All laboratories should apply half-year correctionfor maternal age risk calculation. (b) Triple-test parametervariations result in huge variation of risk estimates, and asingle national risk threshold of 1 in 250 will be unlikelyto improve screening equity unless attention is also paid tostandardizing population parameters. Down’s syndrome screeninghas been controversial since it was introduced, and the possibilitythat more central control (including listing acceptable populationparameters) may be necessary to ensure comparability of resultsis certain to ensure that this controversy continues.

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