Construction and validation of a fatigue impact scale for daily administration (D-FIS)

The fatigue impact scale (FIS) was developed previously as a symptom-specific profile measure of health-related quality of life (HRQoL) for use in medical conditions in which fatigue is a prominent chronic symptom. Thus, it was not developed to be a responsive measure of daily changes in fatigue. This study describes the development and initial validation of an adaptation of the FIS for daily use. Items for the daily fatigue impact scale (D-FIS) were selected from the pool of original FIS items through Rasch analyses of existing data. The reduced-item FIS was administered to a sample of 93 subjects with flu-like illness, 25 of whom were followed for a 21-day period. Rasch analyses were used to further reduce the scale to a minimum number of items that represented a unidimensional measure of self-reported fatigue impact. This 8-item D-FIS demonstrated good relations to flu symptom ratings and to other general health ratings. It also proved to be a responsive measure of change in reported fatigue impact for subjects who were followed longitudinally. This initial validation study indicates that the D-FIS has considerable promise as a valid measure of the subjective daily experience of fatigue.