Measuring fatigue in polio survivors: content comparison and reliability of the Fatigue Severity Scale and the Checklist Individual Strength.

Objectives: To compare the content of the Fatigue Severity Scale and the subscale "subjective experience of fatigue" of the Checklist Individual Strength, and (ii) to assess the reliability of both questionnaires in polio survivors.

Design: Repeated-measures at a 3-week interval.

Subjects: Consecutive series of 61 polio survivors.

Methods: Concepts contained in the questionnaire items were linked to the International Classification of Functioning, Disability and Health (ICF), using standardized linking rules. Reliability analyses included tests of internal consistency, test-retest reliability and measurement error.

Results: Questionnaires differed in the extent to which they measured other than fatigue-related aspects of functioning (represented ICF components: "Body functions": 50% and 80%, "Activities and Participation": 30% and 0%, for the Fatigue Severity Scale and Checklist Individual Strength, respectively). Internal consistency and test-retest reliability were considered acceptable, while measurement error was large (Cronbach's α: 0.90 and 0.93, intraclass correlation coefficient: 0.80 and 0.85, smallest detectable change: 28.7% and 29.4% for the Fatigue Severity Scale and Checklist Individual Strength, respectively).

Conclusion: Considering the acceptable clinimetric properties, we conclude that both the Fatigue Severity Scale and the Checklist Individual Strength can be applied in research on post-poliomyelitis syndrome when measuring fatigue. However, because the 2 questionnaires differ in content they cannot be used interchangeably.