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Construct validity and test-retest reliability of the Fatigue Severity Scale in people with chronic neck pain
Hiroshi Takasaki1, Julia Treleaven
1NHMRC Centre of Clinical Research Excellence-Spinal Pain, Injury and Health, Division of Physiotherapy, School of Health and Rehabilitation Science, The University of Queensland, Brisbane, QLD 4072, Australia. h.takasaki@uq.edu.au
Objective:
To investigate an appropriate scoring system and unidimensionality using Rasch analysis, discriminant validity, and reliability of the Fatigue Severity Scale (FSS) in people with chronic neck pain.
Design:
Cross-sectional.
Setting:
Tertiary institution.
Participants:
Patients with chronic neck pain (n=100) and asymptomatic controls (n=40).
Interventions:
Not applicable.
Main Outcome Measure:
The FSS.
Results:
Twenty-six of the 100 participants with chronic neck pain agreed to complete the FSS again within 1 week after the first administration for the assessment of the test-retest reliability. Two items obviously threatening unidimensionality were eliminated, and the 7-item FSS was developed through Rasch analyses. The 7-item FSS demonstrated the appropriateness of its 7-point scale and adequate internal consistency (Rasch-generated reliability, .83-.91). The 7-item FSS had a negligible floor effect (1%) and ceiling effect (2%). The item-person map demonstrated limited distribution of item difficulty in comparison with the distribution of person ability. The chronic neck pain group demonstrated significantly (P<.001) higher scores in the 7-item FSS than the control group, indicating discriminant validity. The 7-item FSS also demonstrated adequate test-retest reliability with a mean interval of 4.1 days (n=26) for each item (quadratic-weighted κ=.83-.94), and as a whole (intraclass correlation coefficient=.95). A 0-to-100 scale table reflecting Rasch scores was developed, and the minimum detectable change was 9.5 in the 0 to 100 scale.
Conclusion:
The 7-item FSS appears unidimensional and reliable, and can be used quickly in clinical practice to gain a basic understanding of fatigue symptoms in people with chronic neck pain. Further, it is possible to modify the 7-item FSS to enhance discriminant ability within people with chronic neck pain by adding additional items, enlarging the distribution of item difficulty.
