Effect of interrupting prenatal Down syndrome screening due to a large nuchal translucency

The aim of this study is to determine the effect of interrupting prenatal Down syndrome screening for women with large nuchal translucency (NT) measurements on the performance of combined and integrated tests.

Distribution of large NT at 11-13 weeks of gestation was determined from a screening programme (204,982 pregnancies, 509 with Down syndrome). Monte Carlo simulation was used to estimate the effects of interrupting screening on the detection rate (DR) and false-positive rate (FPR) in (i) the remaining women and (ii) all women.

At 12 weeks of gestation, 28% of affected (76/275) and 0.4% of unaffected (389/107,386) pregnancies had an NT of ≥ 3.5 mm. Among the remaining women, for an 85% DR, combined test FPR was 7.2% with interpretation of screening but 4.1% without interruption (1.8% and 0.9% respectively for integrated test). Among all women (women who interrupted screening with an NT of ≥ 3.5 mm and a 1/150 risk cut-off for others), the FPR was 0.1 and 0.2 percentage points higher for the same DR at 12-13 weeks compared with no interruption using the combined and integrated tests, respectively, and 0.3 and 0.7 percentage points higher at 11 weeks.

Interrupting combined or integrated screening is acceptable using an NT of ≥ 3.5 mm, but screening performance in screened women will seem less than expected. This should be taken into account when performing audits.