A novel approach to estimate the minimally important difference for the Fatigue Impact Scale in multiple sclerosis patients.

PURPOSE: The Fatigue Impact Scale (FIS) has been used extensively to assess the impact of fatigue on health-related quality of life (HRQOL) in multiple sclerosis (MS). The objective of this study was to estimate the minimally important difference (MID) of the FIS to facilitate the interpretation of the scale in patients with MS. METHODS: Data came from a cross-sectional study of 184 patients with MS. Anchor-based estimates of the MID were evaluated using patients' ratings of their own health and a clinical rating of MS severity. Using the proportional odds model, estimates of the MID were evaluated by finding FIS score differences that corresponded to a 50% increase in the odds of poorer health. Convergence between distribution- and anchor-based estimates was assessed. RESULTS: Nineteen items met the selection criteria for anchors. Triangulation of the anchor- and distribution-based approaches indicated that the MID of the FIS ranged between 10 and 20 points, approximately. CONCLUSION: A common metric of meaningful difference of FIS was defined across anchors measuring a broad range of HRQOL domains. The MID estimates in the current study can be used for sample size calculation in the planning of future studies and to aid researchers and clinicians in interpreting FIS score differences in patients with MS.