Background: Complex diseases such as multiple sclerosis (MS) present dilemmas over the choice of patient-reported outcome measures as no single scale can inform on all types of MS impact from the patient's perspective.
Objective: To develop an outcome tool, the Patient-Reported Indices for Multiple Sclerosis (PRIMUS), to assess MS symptoms, activities, and quality of life.
Methods: PRIMUS content was derived from qualitative interviews with UK MS patients and checked by clinical experts. Semi-structured cognitive debriefing interviews assessed scale face and content validity. PRIMUS scaling properties, reliability, and construct validity were assessed by a test-retest postal survey.
Results: Cognitive debriefing interviews (n = 15) demonstrated scale clarity, relevance, and comprehensiveness. The postal survey was completed by 135 patients with MS. After removal of misfitting items and those exhibiting differential item functioning, all scales fitted the Rasch model, confirming unidimensionality. For all scales, test-retest reliability exceeded 0.80. Scale scores were related to perceived MS severity, general health, and symptoms of depression. Moderate correlations were observed between PRIMUS and Nottingham Health Profile scores.
Conclusions: Clinicians and researchers can have confidence in scores obtained by respondents on the PRIMUS. The PRIMUS will aid the assessment of the impact of MS from the patient's perspective.