Fatigue numeric rating scale validity, discrimination and responder definition in patients with psoriatic arthritis

OBJECTIVES: This study assessed the psychometric properties of the fatigue numeric rating scale (NRS) and sought to establish values for clinically meaningful change (responder definition). METHODS: Using disease-specific clinician-reported and patient-reported data from two randomised clinical trials of patients with psoriatic arthritis (PsA), the fatigue NRS was evaluated for test–retest reliability, construct validity and responsiveness. A responder definition was also explored using anchor-based and distribution-based methods. RESULTS: Test–retest reliability analyses supported the reproducibility of the fatigue NRS in patients with PsA (intraclass correlation coefficient=0.829). Mean (SD) values at baseline and week 2 were 5.7 (2.2) and 5.7 (2.4), respectively. Supporting construct validity of the fatigue NRS, moderate-to-large correlations with other assessments measuring similar concepts as measured by Sackett’s conventions were demonstrated. Fatigue severity was reduced when the underlying disease activity was improved and reductions remained consistent at week 12 and 24. A 3-point improvement was identified as being optimal for demonstrating a level of clinically meaningful improvement in fatigue NRS after 12–24 weeks of treatment. CONCLUSIONS: Fatigue NRS is a valid and responsive patient-reported outcome instrument for use in patients with PsA. The established psychometric properties from this study support the use of fatigue NRS in clinical trials and in routine clinical practice. Robust validation of reliability for use in routine clinical practice in treating patients with active PsA in less active disease states and other more diverse ethnic groups is needed.