Psychometric Analysis of the Abdominal Score From the Diary for Irritable Bowel Syndrome Symptoms–Constipation Using Phase IIb Clinical Trial Data

Abstract Objectives The Diary for Irritable Bowel Syndrome Symptoms–Constipation (DIBSS-C) has been developed to assess the core signs and symptoms of irritable bowel syndrome with constipation (IBS-C). This article presents the psychometric evaluation of the DIBSS-C abdominal score. Methods Data for these analyses are from a multicenter phase IIb study in IBS-C patients (NCT02559206). Subjects completed a number of assessments via handheld electronic diary throughout the study. The analyses used the intent-to-treat population and were blinded to randomized treatment group. The analyses evaluated the reliability, validity, and responsiveness of the DIBSS-C abdominal score; identified an appropriate scoring algorithm; and determined thresholds for interpreting clinically meaningful changes at the individual level. Results The correlations between the DIBSS-C abdominal symptom items (ie, abdominal pain, discomfort, and bloating) were strong (>0.75). Cronbach’s alpha for the abdominal symptom severity items was very strong (.94), indicating that the 3 abdominal symptom items produce a reliable score. The intraclass correlation coefficient for the abdominal score was 0.82, exceeding the threshold of 0.70 and indicating good test–retest reliability. Guyatt’s responsiveness statistic values all exceeded the threshold for a large effect of 0.80, so the DIBSS-C abdominal score can be considered highly responsive to change. Triangulation across 3 sets of anchor-based analyses indicated that a threshold of −2.0 points on the abdominal score is an appropriate threshold for identifying meaningful change. Conclusions Overall, this study provides evidence that the DIBSS-C abdominal score is valid, reliable, responsive to change, and interpretable for assessing treatment benefit in patients with IBS-C.