Construct validity and test-retest reliability of the Fatigue Severity Scale in people with chronic neck pain

Abstract

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.