First-trimester fetal nasal bone audit: evaluation of a novel method of image assessment

Absence of the nasal bone has been recognized to be a strong ultrasound marker for Down syndrome and its inclusion in combined first-trimester screening would increase the sensitivity and specificity of this test. We describe the development of a method of image scoring that should allow reliable assessment of practitioners submitting themselves to peer review for nasal bone imaging.

Twenty sonographers submitted 20 images demonstrating the presence of the nasal bone for quality assurance audit. Image quality was compared with the criteria described by The Fetal Medicine Foundation. Three raters scored the images on four separate occasions. On the first two occasions all 400 images were assessed qualitatively and given a simple pass/fail score. On the third and fourth occasions, five images from each of the 20 sets were scored objectively for five criteria by each of the three raters, with a cut-off applied to the scores generated. The reliability of these image assessment techniques was compared statistically.

Through quantitative assessment, 84% of images were judged in the same manner by three raters on two separate occasions and in 94% of cases five of these six ratings drew the same conclusion. Rates of intrarater and inter-rater agreement were significantly better using quantitative rather than qualitative techniques.

This study has shown that clearly defined assessment criteria together with a quantitative scoring method improve the reliability of expert peer review. The quantitative method is recommended as the basis for future nasal bone image audit.