CONFIRMATORY VALIDATION AND CLUSTER CLASSIFICATION OF THE IRRITABLE BOWEL SYNDROME SYMPTOM SEVERITY SCALE (IBS-SSS).

BACKGROUND: Irritable Bowel Syndrome (IBS) affects about 4% of the global population and is associated with high psychological distress. The IBS Symptom Severity Scale (IBS-SSS) is the most widely used patient reported outcome measure for IBS, but its psychometric properties are under-studied. Using the largest and most globally representative IBS sample, this study provides a comprehensive psychometric evaluation of the IBS-SSS. AIM: To examine the factor structure of the IBS-SSS and identify symptom profiles using the Rome IV Global Epidemiology Survey. METHODS: A cross-sectional sample of individuals with IBS (N = 1512) completed the IBS-SSS. Confirmatory factor analyses compared one- and two-factor models across the total sample, IBS subtypes, and IBS status (with vs. without additional Disorders of Gut-Brain Interaction (DGBI)). Latent profile analyses identified symptom profiles, and their associations with IBS subtypes were examined. MANOVA tested differences in IBS-SSS domains across profiles. RESULTS: A two-factor model showed the best fit. Three profiles were identified: high (12.5%), moderate (42.7%), and low severity (44.8%). Profile membership showed weak associations with IBS subtypes, whereas IBS subtypes differed strongly across symptom domains (Wilks' Λ = 0.02, p < .001, partial η2 = .98). ROC analysis supported severity cut-offs based on the total IBS-SSS score (≤ 274 mild, 275-354 moderate, ≥ 355 severe) with 83% accuracy. CONCLUSIONS: The IBS-SSS demonstrates strong psychometric validity and effectively distinguishes symptom severity. Findings highlight the heterogeneity of IBS and support symptom-based, personalised approaches over subtype-based classification.