Test-retest reliability of the Swedish version of the Modified Fatigue Impact Scale in people with mild to moderate multiple sclerosis.

Background: Fatigue is a subjective lack of physical and/or mental energy and is commonly perceived by people with multiple sclerosis (MS). People with MS often describe fatigue as the most troublesome MS related impairment, and it also has a negative impact on ability to initiate or maintain activities as well as work capacity and health related quality of life. The Modified Fatigue Impact Scale (MFIS) is a patient-reported outcome measurement of fatigue. The scale consists of 21 items divided into three subscales to capture the cognitive (10 items), physical (9 items) and psychosocial (2 items) dimensions of fatigue. MFIS is translated to Swedish but not evaluated for test-retest reliability for use in Swedish settings of rehabilitation or research. The aim of the study was to investigate the test-retest reliability of the total score and the scores of the subscales of the Swedish version of the MFIS in people with mild to moderate MS-disability.

Methods: Eligible study participants were people with mild to moderate overall MS-disability according to the Expanded Disability Status Scale (EDSS) - EDSS score from 2.0 to 5.5. The included participants completed the Swedish version of MFIS at two occasions, one week apart. Intraclass Correlation Coefficient (ICC) agreement was used to analyse the test-retest reliability of the total score and a quadratic weighted Cohen's kappa was used to analyse the subscale scores. Further, measurement error, smallest detectable change (SDC), and limits of agreement (LOA) were calculated for the total score of the whole sample.

Results: A total of 54 people with MS were included in the study. The mean age was 50 years and 70 % were female. The mean overall MS-disability were 3.7 according to the EDSS, 28 of the participants had mild MS-disability (EDSS 2.0 to 3.5) and 26 of the participants had moderate MS-disability (EDSS 4.0 to 5.5). Results of test-retest reliability for the whole sample of the total score of the MFIS were acceptable with an ICC value of 0.84, a measurement error of 8.2, an SDC of 22.6 and LOA of 23.3 and -22.4. The ICC values of the total score were also acceptable for the subgroups with mild MS-disability and moderate MS-disability with ICC values of 0.83 and 0.74. The kappa values for the whole sample showed excellent or fair to good test-retest reliability of the physical, cognitive and psychosocial subscales with values of 0.83, 0.84, and 0.75.

Conclusion: The results of this study show that the total score of the Swedish version of MFIS is a reliable measure and a change of 23 points indicates a true change in fatigue impact in people with mild to moderate MS-disability. The test-retest reliability of the cognitive and the physical subscales were considered as excellent, and the psychosocial subscale as fair to good. The total score of the scale can be recommended for use in Swedish populations.