Test-retest reliability and convergent validity of the fatigue impact scale for persons with multiple sclerosis

OBJECTIVE. The test-retest reliability and the convergent validity of the Fatigue Impact Scale (FIS) were evaluated using secondary data from 54 persons with multiple sclerosis (MS). METHODS. This reliability and validity study used FIS data from before and after two control periods to evaluate test-retest reliability. Convergent validity of the AS with the Fatigue Severity Scale and with subscales of the SF-36 Health Survey was evaluated using data collected before the first control period. RESULTS. No significant differences between before and after AS measurements and intraclass correlation coefficients ranging from .68 to .85 indicate that the FIS has good test-retest reliability except for the physical subscale. The expected moderate correlations between the FIS and several subscales of the SF-36 support its convergent validity. In contrast, the unexpected low correlation between the FIS and Fatigue Severity Scale does not support convergent validity. CONCLUSION. The AS has adequate reliability and validity and is recommended to evaluate the effectiveness of fatigue management interventions such as energy conservation education for persons with MS.