Psychometric properties of the incontinence utility index among patients with idiopathic overactive bladder: data from two multicenter, double-blind, randomized, Phase 3, placebo-controlled clinical trials

BACKGROUND Overactive bladder is a prevalent and burdensome condition. Generic utility measures may fail to reflect its full impact on patients' health status. The Incontinence Utility Index (IUI) is a community-based preference index derived from the Incontinence Quality of Life Questionnaire (I-QOL) developed to value health states related to urinary symptoms in patients with neurogenic detrusor overactivity. This study assessed the measurement properties of the IUI in patients with idiopathic overactive bladder (OAB). METHODS Data were used from two clinical trials which recruited patients with OAB whose symptoms were inadequately managed with ≥ 1 anticholinergic medication. Psychometric evaluation included: Differential Item Functioning (DIF) analysis, concordance between I-QOL and IUI (Intraclass correlation coefficient [ICC], criterion and convergent validity according to relevant patient reported outcomes and clinical variables (Spearman's correlation coefficient, rho), responsiveness, and agreement between utility measures (ICC and Bland-Altman method). RESULTS A total of 1,105 idiopathic OAB patients were included. Mean age (range) was 60.4 years (18-90), 87.8% (n = 970) were female. DIF was identified in 3 items, none of which are contained in the IUI. ICC (CI95%) was 0.944 (0.936-0.950). Statistically significant differences (p < 0.001) were found in IUI scores for patients improving according to the Treatment Benefit Scale (TBS). Moderate to strong correlations (rho > |0.6|) were found in the expected direction with daily incontinence, urgency episodes and disease-specific domains of King's Health Questionnaire (KHQ). Low to moderate correlations (rho:<|0.6|) were found with Short Form version 2 (SF-12v2) summary components. A large effect size was found for patients reporting improvement (0.98-1.21) or great improvement (1.87-2.56) in the TBS, as well as in patients responding to treatment (1.19-2.40). Across utility measures, directional trends were consistent with OAB symptom profile, however, a lack of agreement in absolute values was observed. CONCLUSIONS The IUI presents good psychometric properties for valuing the impact of UI-related problems in idiopathic OAB patients. TRIAL REGISTRATION ClinicalTrials.gov: NCT00910845 and NCT00910520.