BACKGROUND AND PURPOSE: The Unidimensional Fatigue Impact Scale (U-FIS) was developed for use in a multiple sclerosis population. The aim was to determine whether the U-FIS is a valid tool for measuring the impact of fatigue in Crohn's disease (CD).

METHOD: CD patients completed the U-FIS as part of a validation study of the Crohn's Life Impact Questionnaire (CLIQ). Data were analyzed according to Rasch measurement theory (RMT).

RESULTS: Two hundred sixty-one completed U-FIS questionnaires were available for analysis. After rescoring the items to resolve disordered thresholds, all 22 items showed acceptable RMT fit. However, there was considerable local item dependency (LID).

CONCLUSION: The U-FIS did not provide unidimensional measurement in a sample of CD patients due to high levels of LID. Combining the three FIS outcomes into a single measure was not justified.