To compare how costs relate to disability, disease activity, and health-related quality of life (HRQOL) in rheumatoid arthritis (RA).
Antitumor necrosis factor (anti-TNF)-treated patients with RA in southern Sweden (n = 2341) were monitored 2005-2010. Health Assessment Questionnaire (HAQ), 28-joint Disease Activity Score (DAS28), and EQ-5D scores were linked to register-derived costs of antirheumatic drugs (excluding anti-TNF agents), patient care, and work loss from 30 days before to 30 days after each visit (n = 13,289). Associations of HAQ/DAS28/EQ-5D to healthcare (patient care and drugs) and work loss costs (patients < 65 yrs) were studied in separate regression models, comparing standardized β coefficients by nonparametric bootstrapping to assess which measure best reflects costs. Analyses were conducted based on both individual means (linear regression, comparing between-patient associations) and by generalized estimating equations (GEE), using all observations to also account for within-patient associations of HAQ/DAS28/EQ-5D to costs.
Regardless of the methodology (linear or GEE regression), HAQ was most closely related to both cost types, while work loss costs were also more closely associated with EQ-5D than DAS28. The results of the linear models for healthcare costs were standardized β = 0.21 (95% CI 0.15-0.27), 0.16 (0.11-0.21), and -0.15 (-0.21 to -0.10) for HAQ/DAS28/EQ-5D, respectively (p < 0.05 for HAQ vs DAS28/EQ-5D). For work loss costs, the results were standardized β = 0.43 (95% CI 0.39-0.48), 0.27 (0.23-0.32), and -0.34 (-0.38 to -0.29) for HAQ/DAS28/EQ-5D, respectively (p < 0.05 for HAQ vs DAS28/EQ-5D and for EQ-5D vs DAS28).
Overall, HAQ disability is a better marker of RA costs than DAS28 or EQ-5D HRQOL.