Impact of Early Antimalarial Adherence on Future Acute Care Utilization and Cost Among Patients With Rheumatoid Arthritis and Systemic Lupus Erythematosus: A Population‐Based Study

OBJECTIVE: To assess the association between early antimalarial adherence and future acute care utilization and cost in a population-based cohort of incident rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). METHODS: All patients with incident RA/SLE and new antimalarial use in British Columbia, Canada, between January 1997 and March 2022 with at least one year of follow-up were identified using provincial administrative databases. Each antimalarial adherent patient (proportion of days covered [PDC] ≥ 0.90) was propensity score (PS) matched up to two antimalarial nonadherent patients (0 < PDC < 0.90). The outcomes hospital admissions, hospitalized days, and hospitalization cost were assessed in the following year. We estimated the impact of early antimalarial adherence on these outcomes, accounting for potential confounders, using quasi-Poisson and linear models with robust SEs. We conducted falsification tests and sensitivity analyses to examine robustness to healthy adherer effect and unmeasured confounders. RESULTS: We identified 8,768 antimalarial adherent and 16,479 antimalarial nonadherent PS-matched individuals with RA/SLE. The adjusted rate ratios of hospital admissions and hospitalized days obtained for antimalarial adherent versus nonadherent patients were 0.89 (95% confidence interval [CI] 0.84-0.94) and 0.79 (95% CI 0.71-0.88), respectively. The adjusted average hospitalization costs were lower (estimate: CAD -$549.64, 95% CI -$896.05 to -$203.22, P < 0.01) for antimalarial adherent patients than nonadherent patients. Falsification tests and sensitivity analyses confirmed the robustness of the results. CONCLUSION: Adherence to antimalarials in patients with RA and SLE was associated with a lower risk of hospital admission (11% reduction) and hospitalized days (21% reduction) and lower hospitalization costs (CAD $549.64 lower) compared to nonadherence.