Quality assurance of nuchal translucency for prenatal fetal Down syndrome screening

To assess the quality of nuchal translucency, (NT) measurements were performed at four public institutions performing routine first trimester combined prenatal screening for Down syndrome.

The median of the NT-MoM distribution and standard deviation (SD) of the log(10) NT-MoM were determined. Sonographers and screening centres distributions were assessed for measures of central tendency (median) and dispersion (log(10) SD). Cumulative Sum (CUSUM) charts were created to assess whether screening centres and individual sonographers who had performed at least 30 NT measurements exhibited any systematic bias by checking whether their CUSUM scores exceeded predefined upper and lower control limits.

Of the 36 sonographers, only 67% (n = 24) had performed 30 or more scans. The median NT-MOM at each screening centre ranged from 1.02 to 1.09. Screening centre standard deviations ranged from 0.073 to 0.099. CUSUM charts indicated that only one screening centre remained within the predefined control limits throughout the assessment period. Analysis of variance indicated that a statistically significant difference existed between the log NT-MoM distributions of the individual sonographers (F = 10.7; p <0.0001). Inspection of the individual sonographer CUSUM charts indicated that 11 (45%) of the 24, with more than 30 NT measurements were either under or over measuring the NT.

Prospective monitoring and feedback of quality assurance assessment results of sonographers and screening centres should be routinely reported as both are responsible, if equity of screening performance is to be maintained.