Validation of Fatigue Impact Scale with various item sets - a Rasch analysis

Fatigue is a symptom in patients with chronic gastrointestinal (GI) and liver diseases. Different instruments have been developed to assess the severity of fatigue and the 40-item Fatigue Impact Scale (FIS) is among the most widely used. Shorter versions of FIS include the 21-item Modified Fatigue Impact Scale (MFIS), and an eight-item version for everyday use. The study aimed to assess construct validity, reliability, and sufficiency of the raw score of the original FIS with 40 items, and examine the sufficiency of the 21 items from the Modified scale and the eight items of the Daily Fatigue Impact Scale (D-FIS), all of which are embedded in the 40-item scale.

Patients with chronic GI or liver disease (n = 354) completed the FIS with 40 items. The majority (57%) was under the age of 55 years and approximately half were females (48%). Various item sets of FIS were derived which showed fit to the Rasch model.

Local dependency and multidimensionality in FIS and the 21-item Modified scale were resolved with a testlet solution but the D-FIS showed local dependency and multidimensionality and differential item functioning (DIF) still remained. Two new item sets fulfilling unidimensionality and no DIF are suggested, one with 15 items and a six-item scale for daily use. The transformation table shows score-interval scale estimates for all these item sets.

Both the FIS and the Modified scale can be used to measure fatigue albeit requiring some adjustment for DIF. The eight-item D-FIS is more problematic, and its summed score is not valid. Alternative 15- and 6-item versions presented in this paper can offer valid summed scores, and the transformation table allows transformation of raw scores and comparisons across all versions. Implications for rehabilitation The Fatigue Impact Scale and the Modified Fatigue Impact Scale can be used to measure fatigue after adjustments for differential item functioning. Alternative 15- and 6-item versions of Fatigue Impact Scale offer valid summed scores. The summed score for the Daily Fatigue Impact Scale is not valid. A transformation table with raw scores and Rasch transformed interval scale metric makes it possible to compare scores derived from the Fatigue Impact Scale, the Modified Fatigue Impact Scale and the proposed 15- and 6-item versions of Fatigue Impact Scale for research and/or clinical use.