Improving the evaluation of therapeutic interventions in multiple sclerosis: development of a patient-based measure of outcome.

OBJECTIVES: To develop a patient-based, disease-specific measure of the health impact of multiple sclerosis (MS) for use in clinical trials and clinical practice. DATA SOURCES: People with MS. Members of the MS Society of Great Britain and Northern Ireland. METHODS: Standard psychometric methods were used to develop the Multiple Sclerosis Impact Scale (MSIS-29) in three stages. Stage 1 (item generation): questionnaire items were generated from 30 patient interviews on the impact of MS on their lives, expert opinion and literature review. Stage 2 (item reduction and scale generation): the questionnaire developed in stage 1 was administered by postal survey to 1530 randomly selected members of the MS Society. Standard item reduction techniques were used to develop a rating scale from the pool of questionnaire items. Stage 3 (psychometric evaluation): the questionnaire was evaluated for data quality, scaling assumptions, acceptability, reliability and validity in a separate postal survey of 1250 MS Society members. Responsiveness was evaluated in 55 people admitted to hospital for rehabilitation and intravenous steroid treatment of MS relapses. RESULTS: Stage 1 resulted in a 129-item questionnaire. Stage 2 resulted in a 29-item rating scale measuring the physical and psychological impact of MS. The MSIS-29 satisfied all recommended psychometric criteria for rigorous measurement. Data quality was excellent: missing data were low, item test-retest reliability was high and scale scores could be generated for over 98% of respondents. Item descriptive statistics, item convergent and discriminant validity, and factor analysis supported summing items to produce two summary scores. MSIS-29 physical and psychological scale scores showed good variability, low floor and ceiling effects, good internal consistency and test-retest reliability. Correlations with other measures and confirmation of hypotheses about group differences provided evidence for the validity of the MSIS-29 as a measure of the physical and psychological impact of multiple sclerosis. Effect sizes provided preliminary evidence for responsiveness. CONCLUSIONS: The 29-item MSIS-29 is a rigorous new measure of the physical and psychological impact of MS. All psychometric criteria were satisfied and there is preliminary evidence of responsiveness. The MSIS-29 is particularly appropriate for use in clinical trials to evaluate therapeutic effectiveness from the patient's perspective. Further critical evaluations of the MSIS-29 completed by people with neurologist-confirmed MS in different settings are suggested. Head-to-head comparisons of the psychometric properties of the MSIS-29 and other outcome measures for MS will help to determine the relative advantages of different instruments so that the choice of measures for studies can be evidence based.