The Impact of Early Antimalarial Adherence on Future Acute Care Utilization and Cost among Rheumatoid Arthritis and Systemic Lupus Erythematosus Patients: 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 01/1997 and 03/2022 with at least one year follow‐up period were identified using provincial administrative databases. Each antimalarial adherent (proportion of days covered, PDC≥0.90) was propensity score (PS)‐matched up to two antimalarial non‐adherents (0<PDC<0.90). The outcomes: hospital admissions and 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 standard errors. 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 non‐adherent PS‐matched RA/SLE individuals. The adjusted rate ratios (RRs) of hospital admissions and hospitalized days obtained for antimalarial adherent vs. non‐adherent patients were 0.89 (95% CI: 0.84‐0.94) and 0.79 (95% CI: 0.71‐0.88), respectively. The adjusted average hospitalization costs were lower (Est: ‐$549.6 CAD, 95% CI: ‐896.1, ‐203.2, p‐value: <0.01) for antimalarial adherent patients than non‐adherent patients. Falsification tests and sensitivity analyses confirmed the robustness of the results. Conclusion Adherence to antimalarial in RA and SLE patients was associated with a lower risk of hospital admission (11% reduction) and hospitalized days (21% reduction) and lower hospitalization costs ($549.6 CAD lower) compared to patients with non‐adherence.