Factor structure and psychometric properties of the Intolerance of Uncertainty Scale-12 (IUS-12) in multiple sclerosis

Intolerance of uncertainty (IU) is a transdiagnostic risk factor that has been robustly implicated in several internalising psychopathologies, particularly depression and anxiety. Although emerging evidence emphasises the presence and impact of IU in multiple sclerosis (MS), it is unclear whether current measures of IU are appropriate for people with MS. In this study, we describe the psychometric features of the short-form Intolerance of Uncertainty Scale (IUS-12) in a mixed sample of 2117 participants (244 MS, 1873 non-MS controls). Specific aims were to (i) assess the internal reliability and dimensionality of the IUS-12; (ii) test measurement invariance across MS and non-MS groups; and (iii) determine construct validity by correlating the IUS-12 with measures of psychological distress and anxiety, test-retest reliability and temporal stability over a six-month interval, and clinical utility of the IUS-12 in detecting clinically significant symptoms of generalised anxiety disorder, posttraumatic stress disorder, and health anxiety. A bifactor model, comprising a General IU factor and two group factors (Prospective IU and Inhibitory IU), best represented the IUS-12. Measurement invariance was found across MS and non-MS participants. The IUS-12 had good construct validity and moderate test-retest reliability (intraclass correlation coefficient [ICC] = .62) over the six-month survey period. Given the central role of uncertainty in the lived experience of MS, these findings support the use of the IUS-12 as a psychometrically sound measure of IU in this population. Routine assessment incorporating the IUS-12 may help inform the development of targeted psychological interventions for people with MS.