Prenasal thickness to nasal bone length ratio: effectiveness as a second or third trimester marker for Down syndrome

ObjectiveTo assess the value of the prenasal thickness to nasal bone length ratio (PT/NBL) for detecting trisomy 21 (T21) after the first trimester.MethodTwo examiners blinded to fetal T21 status retrospectively measured prenasal thickness (PT) and nasal bone length (NBL) of T21 and control fetuses at 15–36 weeks’ gestational age on two-dimensional images from all T21-screening ultrasounds from November 2010 to April 2013. ROC curve analysis and its diagnostic values determined the best cut-off value for the ratio. Interobserver reproducibility was assessed.ResultsGood quality ultrasound profile images were available for 26 fetuses with T21 compared to 91 normal fetuses. The median PT/NBL ratio was 1.28 for T21 and 0.73 for control fetuses (p < 0.0001). The PT/NBL ratio performed significantly better (AUC 0.99; 95%CI 0.97–1) than either PT (0.82; 0.73–0.91) or NBL (0.91; 0.85–0.98). The optimal PT/NBL ratio cut-off was 0.98, with a sensitivity of 88.5% [76.2–100%] and a specificity of 100%. Interobserver variability was low.ConclusionThe PT/NBL ratio is a strong marker for detecting T21 in the second and third trimesters, significantly more effective than either indicator alone.