Fatigue is associated with disease activity in some, but not all, patients living with rheumatoid arthritis: disentangling “between-person” and “within-person” associations

BACKGROUND: Previous research has shown an unclear and inconsistent association between fatigue and disease activity in patients with rheumatoid arthritis (RA). The aim of this study was to explore differences in “between-person” and “within-person” associations between disease activity parameters and fatigue severity in patients with established RA. METHODS: Baseline and 3-monthly follow-up data up to one-year were used from 531 patients with established RA randomized to stopping (versus continuing) tumor necrosis factor inhibitor treatment enrolled in a large pragmatic trial. Between- and within-patient associations between different indicators of disease activity (C-reactive protein [CRP], erythrocyte sedimentation rate [ESR], swollen and tender joint count [ SJC and TJC], visual analog scale general health [VAS-GH]) and patient-reported fatigue severity (Bristol RA Fatigue Numerical Rating Scale) were disaggregated and estimated using person-mean centering in combination with repeated measures linear mixed modelling. RESULTS: Overall, different indices of disease activity were weakly to moderately associated with fatigue severity over time (β’s from 0.121 for SJC to 0.352 for VAS-GH, all p’s < 0.0001). Objective markers of inflammation (CRP, ESR and SJC) were associated weakly with fatigue within patients over time (β’s: 0.104–0.142, p’s < 0.0001), but not between patients. The subjective TJC and VAS-GH were significantly associated with fatigue both within and between patients, but with substantially stronger associations at the between-patient level (β’s: 0.217–0.515, p’s < 0.0001). Within-person associations varied widely for individual patients for all components of disease activity. CONCLUSION: Associations between fatigue and disease activity vary largely for different patients and the pattern of between-person versus within-person associations appears different for objective versus subjective components of disease activity. The current findings explain the inconsistent results of previous research, illustrates the relevance of statistically distinguishing between different types of association in research on the relation between disease activity and fatigue and additionally suggest a need for a more personalized approach to fatigue in RA patients. Trial registration Netherlands trial register, Number NTR3112.