To determine the benefit of including nasal bone assessment in addition to standard first-trimester markers (nuchal translucency, free beta human chorionic gonadotropin and pregnancy-associated plasma protein A) as a screening test for Down syndrome, using a strict criterion for classification of nasal bone absence.

Nasal bone assessment was conducted in 2411 patients with crown-rump length between 45 and 84 mm, including 15 patients with Down syndrome. A patient was considered to have an absent nasal bone only if there was no evidence of present nasal bone. Unlike other studies, nasal bone was classified as present when there was evidence of a thin echogenic line under the skin. Simulation studies were conducted to assess the detection rate and false-positive rate of a combined first-trimester screening protocol including nasal bone assessment.

There were 9 of 2396 (0.4%) unaffected cases with absent nasal bone (95% confidence interval 0.2%, 0.7%) and 8 of 15 (53.3%) Down syndrome cases (95% confidence interval 26.6%, 78.7%). Using a 1 in 250 risk cut-off, the detection rate of standard first-trimester screening was 87%, with a false-positive rate of 4.3%. Incorporating nasal bone measurement improved the detection rate of Down syndrome to 90% and reduced the false-positive rate to 2.5%.

The use of a strict criterion to determine nasal bone absence leads to fewer cases classified as absent and may simplify the implementation of nasal bone as a marker for firs- trimester screening, resulting in lower false-positives and higher detection, compared with other current screening protocols.