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. 2010;17(1):8-12.
doi: 10.1258/jms.2010.009107.

Distribution of nuchal translucency in antenatal screening for Down's syndrome

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

Distribution of nuchal translucency in antenatal screening for Down's syndrome

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

Erratum in

  • J Med Screen. 2010 Jun;17(2):106

Abstract

Objective: To determine whether the standard deviation of nuchal translucency (NT) measurements has decreased over time and if so to revise the estimate and assess the effect of revising the estimate of the standard deviation on the performance of antenatal screening for Down's syndrome.

Setting: Data from a routine antenatal screening programme for Down's syndrome comprising 106 affected and 22,640 unaffected pregnancies.

Methods: NT measurements were converted into multiple of the median (MoM) values and standard deviations of log(10) MoM values were calculated in affected and unaffected pregnancies. The screening performance of the Combined and Integrated tests (that include NT measurement) were compared using previous and revised estimates of the standard deviation.

Results: The standard deviation of NT in unaffected pregnancies has reduced over time (from 1998 to 2008) (e.g. from 0.1329 to 0.1105 [log(10) MoM] at 12-13 completed weeks of pregnancy, reducing the variance by about 30%). This was not observed in affected pregnancies. Compared with results from the serum, urine and ultrasound screening study (SURUSS), use of the revised NT standard deviations in unaffected pregnancies resulted in an approximate 20% decrease in the false-positive rate for a given detection rate; for example, from 2.1% to 1.7% (a 19% reduction) at a 90% detection rate using the Integrated test with first trimester markers measured at 11 completed weeks' gestation and from 4.4% to 3.5% (a 20% reduction) at an 85% detection rate using the Combined test at 11 completed weeks.

Conclusions: The standard deviation of NT has declined over time and using the revised estimates improves the screening performance of tests that incorporate an NT measurement.

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Figures

Figure 1
Figure 1
Relative frequency distributions of nuchal translucency (NT) multiple of the median (MoM) values in Down's syndrome and unaffected pregnancies according to gestational age. Solid line is previous distribution and dashed line is revised distribution. Median MoM in Down's syndrome pregnancies at vertical line
Figure 2
Figure 2
Probability plots of nuchal translucency in unaffected pregnancies according to week of gestation
See this image and copyright information in PMC

References

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