Reliability of shear wave elastography measurements of the linea alba: an intraclass correlation and Bland-Altman analysis

Diastasis recti abdominis (DRA) is a common postpartum condition characterized by an increased separation between the rectus abdominis muscles along the linea alba, which may affect the stability of the abdomino-lumbo-pelvic complex and quality of life. Two-dimensional ultrasound is an established technique for assessing inter-rectus distance, whereas, more recently, shear wave elastography (SWE) has emerged as a promising method for quantifying tissue stiffness, although it is not yet fully established in clinical practice. However, before its routine use, it is necessary to establish the reliability of SWE measurements, considering factors such as anatomical location and examiner experience. This study aimed to determine intra- and inter-rater reliability of shear wave elastography for assessing the linea alba stiffness above and below the umbilicus in women with and without diastasis rectus abdominis.

An observational, cross-sectional study was conducted. Twenty-nine participants were included and two licensed examiners with different levels of elastography experience performed independent assessments on the same day, acquiring three SWE measurements above (AB5), at (UBO), and below (BL2) the umbilicus using a standardized protocol. Reliability was assessed using ICCs and Bland-Altman analysis.

Intra-rater reliability was higher when three repeated measurements were averaged. For Examiner 1, mean-of-three ICCs ranged from 0.839 (95% CI 0.703-0.919) to 0.914 (95% CI 0.842-0.957), whereas for Examiner 2 they ranged from 0.662 (95% CI 0.371-0.831) to 0.800 (95% CI 0.630-0.899). Inter-rater reliability varied by anatomical location. For single examiner-specific mean values, ICC(A,1) was 0.439 (95% CI 0.086-0.692) at AB5, 0.517 (95% CI 0.187-0.741) at UBO, and 0.601 (95% CI 0.313-0.789) at BL2. For the mean of both examiners, ICC(A,2) was 0.610 (95% CI 0.159-0.818), 0.681 (95% CI 0.315-0.851), and 0.751 (95% CI 0.476-0.882), respectively. Bland-Altman analysis indicated minimal systematic bias and acceptable agreement: AB5, + 0.07 m/s (95% LoA - 2.74 to + 2.88); UBO, + 0.09 m/s (95% LoA - 2.63 to + 2.82); and BL2, - 0.21 m/s (95% LoA - 2.04 to + 1.63). Mean absolute errors ranged from 0.70 to 1.13 m/s, with no evidence of proportional bias.

Shear-wave elastography (SWE) demonstrated acceptable-to-good reproducibility under standardized conditions, although reliability varied according to examiner and anatomical location. Averaging three repeated measurements improved intra-rater reliability, while inter-rater reliability was highest at BL2 and lowest at AB5. These findings require confirmation in larger studies.