OP02.02: Effect of deviation of nuchal translucency measurements on the performance of screening for trisomy 21

Abstract

To examine the effect of deviations in the median nuchal translucency (NT) thickness and the spread in measurements on the performance of screening for trisomy 21 by maternal age and fetal NT and by maternal age, fetal NT and maternal serum biochemistry. We simulated the NT and MoM values for PAPP-A and free β-hCG for 500,000 euploid and 500,000 trisomy 21 pregnancies at 12 weeks of gestation. Detection rates for trisomy 21 and false positive rates were calculated without adjustments in NT and by adding or subtracting values ranging from 0.1 to 1.0 mm to each observed measurement. In addition, the effects of variation in the scatter of NT measurements were examined by applying a multiplicative factor ranging from 0.5 to 2 to the standard deviation (SD). The detection rate of trisomy 21 for a fixed false positive rate of 3% in screening by maternal age, fetal NT and serum free β -hCG and PAPP-A was 86%. At a risk cut off of 1 : 100, the detection rate was 84% for a false positive rate of 2.7%. An under- or overestimate in the measured NT reduced the detection rate of trisomy 21 for a fixed false positive rate (e.g. overestimation by 0.5 mm: DR 83%; underestimation by 0.5 mm DR 84%). At a fixed screen positive cut-off, an underestimate in fetal NT reduced predominantly the detection rate while an overestimate in NT increased predominantly the false positive rate (e.g. risk cut off 1 : 100: overestimation by 0.5 mm: DR 78% and FPR 1.6%; underestimation of 0.5 mm: DR 91% and FPR 7.0%). A widening in the scatter of measurements had only a small impact on the detection rate but it caused a major increase in the false positive rate (e.g. widening by 50%: DR 85% and FPR 4.3%). High performance of screening necessitates appropriate measurement of fetal NT.