Inter-Rectus Distance Measurement Using Ultrasound Imaging: Does the Rater Matter?

Purpose: To investigate the interrater reliability of inter-rectus distance (IRD) measured from ultrasound images acquired at rest and during a head-lift task in parous women and to establish the standard error of measurement (SEM) and minimal detectable change (MDC) between two raters. Methods: Two physiotherapists independently acquired ultrasound images of the anterior abdominal wall from 17 parous women and measured IRD at four locations along the linea alba: at the superior border of the umbilicus, at 3 cm and 5 cm above the superior border of the umbilicus, and at 3 cm below the inferior border of the umbilicus. The interrater reliability of the IRD measurements was determined using intra-class correlation coefficients (ICCs). Bland-Altman analyses were used to detect bias between the raters, and SEM and MDC values were established for each measurement site. Results: When the two raters performed their own image acquisition and processing, ICCs(3,5) ranged from 0.72 to 0.91 at rest and from 0.63 to 0.96 during head lift, depending on the anatomical measurement site. Bland-Altman analyses revealed no systematic bias between the raters. SEM values ranged from 0.23 cm to 0.71 cm, and MDC values ranged from 0.64 cm to 1.97 cm. Conclusion: When using ultrasound imaging to measure IRD in women, it is acceptable for different therapists to compare IRDs between patients and within patients over time if IRD is measured above or below the umbilicus. Interrater reliability of IRD measurement is poorest at the level of the superior border of the umbilicus.