Measurement properties of the FATIGUE-PRO in patients with SLE in real-world settings.

OBJECTIVE

Fatigue is a common, multifaceted symptom in SLE which is difficult to treat. There has been a lack of broadly accepted SLE-specific assessment tools for fatigue. This study assessed the psychometric properties of the FATIGUE-PRO, a new patient-reported outcome measure, in patients with SLE in real-world clinical outpatient settings (Birmingham, UK, and Düsseldorf, Germany).

METHODS

Patients with SLE aged ≥18 years from cross-sectional, observational studies in Birmingham and Düsseldorf completed the three FATIGUE-PRO domains (physical fatigue, mental and cognitive fatigue and susceptibility to fatigue). Psychometric evaluation was completed using Rasch Measurement Theory and classical test theory methods. Association between the FATIGUE-PRO and other clinical outcome assessments was evaluated using Spearman's rank order correlations.

RESULTS

Overall, 200 patients were recruited from Birmingham and 234 from Düsseldorf. The FATIGUE-PRO demonstrated consistently adequate measurement properties. Items demonstrated high levels of targeting (82-95%) and the response scale functioned as intended for most items across all domains (82-100% ordered thresholds). Domains were cohesive overall, with minimal item misfit, and demonstrated little local dependency. Principal component analysis of residuals was supportive of unidimensionality of all three domains. All domain scores were reliable and demonstrated high internal consistency reliability (0.90-0.97). Moderate-to-high correlations, supportive of construct validity, were observed between the FATIGUE-PRO scores and other patient-reported outcomes, including Lupus Quality of Life. Low correlations were seen with outcome measures such as Brief Index of Lupus Damage and clinician-reported SLE Disease Activity Index-2000.

CONCLUSIONS

The FATIGUE-PRO was demonstrated to be fit-for-purpose in real-world clinical outpatient settings, with strong measurement properties, supporting its use in the assessment of fatigue in SLE.