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. 2010;17(1):13-8.
doi: 10.1258/jms.2010.009108.

Evaluation of a proposed mixture model to specify the distributions of nuchal translucency measurements in antenatal screening for Down's syndrome

J P Bestwick  1 , W J Huttly, N J Wald
Affiliations

Evaluation of a proposed mixture model to specify the distributions of nuchal translucency measurements in antenatal screening for Down's syndrome

J P Bestwick et al. J Med Screen. 2010.

Abstract

Objectives: A mixture model of crown-rump length (CRL)-dependent and CRL-independent nuchal translucency (NT) measurements has been proposed for antenatal screening for Down's syndrome. We here compare the efficacy of the mixture model method with the standard method, which uses NT multiple of the median (MoM) values in a single distribution. Settings A routine antenatal screening programme for Down's syndrome comprising 104 affected and 22,284 unaffected pregnancies.

Methods: The ability of NT to distinguish between affected and unaffected pregnancies was compared using the mixture model method and the standard MoM method by using published distribution parameters for the mixture model of NT and parameters derived from these for the standard MoM method. The accuracy of the two methods was compared for NT and maternal age by comparing the median estimated risk with the prevalence of Down's syndrome in different categories of estimated risk.

Results: Using NT alone observed estimates of discrimination using the two methods are similar; at a 70% detection rate the false-positive rates were 12% using the mixture model method and 10% using the MoM method. Risk estimation was marginally (but not statistically significantly) more accurate using the standard MoM method.

Conclusions: The mixture model method offers no advantage over the standard MoM method in antenatal screening for Down's syndrome, is more complicated and less generalizable to other data-sets. The standard MoM method remains the method of choice.

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Figures

Figure 1
Figure 1
Mixture model distributions of nuchal translucency (NT) in mm and distributions of NT multiple of the median (MoM) values in Down's syndrome and unaffected pregnancies at 11, 12 and 13 completed weeks' gestation. Truncation limits shown (vertical lines) are those specified by Wright et al.
Figure A1
Figure A1
Two Gaussian distribution curves of hypothetical screening marker x (a), mixture distribution curve (b) and histogram of the marker that might be observed in affected individuals (c)
See this image and copyright information in PMC

References

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