Rasch analysis of the Modified Fatigue Impact Scale (MFIS) in multiple sclerosis

BACKGROUND: The 21-item Modified Fatigue Impact Scale (MFIS) has been recommended as an outcome measure for use in multiple sclerosis and is commonly used to generate an overall score of fatigue. OBJECTIVE: To test if the MFIS total score is valid by application of the Rasch measurement model. METHOD: The MFIS was sent by post to patients with clinically definite multiple sclerosis in two centres in the UK. Data were fitted to the Rasch model. RESULTS: Analysis was based on 415 records (55% response). The 21-item scale did not fit the Rasch model mainly because of multidimensionality. The scale was found to contain a "physical" dimension and a "cognitive" dimension, consistent with the original subscale structure. Valid physical and cognitive subscales were derived after deletion of some items. CONCLUSION: The MFIS cannot be used to generate a single overall score of fatigue. The conceptual interaction between the two dimensions remains unclear, which poses problems when interpreting change scores in these individual scales. Studies in which a global MFIS score was used as either an outcome measure or selection tool may need to be re-evaluated.