Assessment of sonographer nuchal translucency measurement performance--central tendency and dispersion

To assess center and sonographer nuchal translucency (NT) measurements that were performed as part of routine prenatal screening for Down syndrome.

Sonographers were assessed for measures of central tendency and dispersion by comparing them with expected population median and dispersion parameters. NT measurements were converted to multiples of the expected NT values according to CRL (NTMoM) and transformed to their log(10) equivalent (log(10) NTMoM). Central tendency and measurement were assessed by checking whether the median of the NTMoM distribution and SD of the log(10) NTMoM distributions were, respectively, within 5% or 10% of the expected median of 1 and SD of 0.1. Assessment was performed using both the Center specific and Fetal Medicine Foundation (FMF) reference NT for Crown rump length (CRL).

The median NT MoM was 0.95 MoM using the FMF reference and 1.01 MoM when assessed using our center specific reference median. The difference between the center and FMF derived NT MoMs was statistically significantly (p < 0.0001). NTMoM medians increased over time at a rate of 0.0099 MoM per year while log(10) NT MoM measurement dispersement was similar to the 0.1 value expected and decreased by 0.0048 per year.

Centers should routinely monitor the quality of NT measurements used to estimate Down syndrome screening risk and should provide individualized feedback to sonographers of their measures of central tendency and dispersion to ensure consistent and improved performance. NT reference medians adopted from other populations should be assessed and validated against a centers own measurement distribution.