The development and validation of the Unidimensional Fatigue Impact Scale (U-FIS)

Background: The multidimensional assessment of fatigue is complicated by the interrelation of its multiple causes and effects. Objective: The purpose of the research was to develop a unidimensional assessment of fatigue (U-FIS). Methods: Data collected with the Fatigue Impact Scale (FIS) were subjected to Rasch analysis to identify potential problems with the scale. Additional items for the U-FIS were generated from interviews with UK MS patients. The U-FIS was tested for face and content validity in patient interviews and included in a validation survey to determine dimensionality (Rasch model), reliability and validity. Results: The original FIS was not unidimensional when subscale items were combined. The modification of the FIS and addition of a number of items allowed the development of a 22-item unidimensional scale (U-FIS) that was reliable (Cronbach Alpha 0.96; test-retest 0.86,) and valid given correlations with the Nottingham Health Profile and ability to distinguish between MS severity groups. There was no significant difference in U-FIS scores according to MS type. Conclusion: It is valid to conceptualize the functional impact of fatigue as unidimensional. The U-FIS is a reliable and valid questionnaire that will allow the measurement of this construct in clinical studies.