How imprecise may the measurement of fetal nuchal translucency be without worsening first-trimester screening?

Objective: First-trimester screening (FTS) has a trisomy 21 detection rate of about 90%. Despite profound training, the practically reached measurement quality of nuchal translucency (NT) is probably not optimal. This study investigated the impact of measurement errors on FTS. Methods: The data on 10,116 combined FTSs were obtained in a multicenter study. Risk assessment was performed by the JOY software following the Nicolaides risk calculation principles. To investigate the impact of measurement errors, the NT values were artificially altered and the adjusted risks were recalculated. Test performance parameters were obtained and compared with the correct measurements. Results: In this study 85 fetuses were genetically affected. The screening was wrongly inconspicuous in 12 cases and in 479 cases the FTS offered false-positive results. An assumed NT error of 8 +/- 0.1 mm already causes a highly significant change in the false-positive rate. A difference of -0.2 mm leads to a visible change in false negatives. Discussion: This study demonstrates that even the smallest deviations will significantly affect the false-negative rate. The detection of really diseased fetuses is influenced at a -0.2-mm measurement error. Therefore the NT measurement has to be as precise as possible. Copyright (C) 2008 S. Karger AG, Basel.