Subtherapeutic hydroxychloroquine concentration is associated with increased disease activity and greater organ damage in lupus.

Objective: To investigate the effects of the serum HCQ concentration on clinical manifestations, disease activity and organ damage in a longitudinal cohort of SLE patients.
Methods: The 338 SLE patients were assessed with respect to their demographic data, clinical and laboratory findings, Physician's Global Assessment (PGA), adjusted mean SLEDAI-2000 (AMS) and SLICC Damage Index (SDI) annually for 5 consecutive years. Patients were divided into two groups according to their serum HCQ concentration at baseline: subtherapeutic (<500 ng/ml) and therapeutic (≥500 ng/ml) groups. The impact of the HCQ concentration on the clinical outcomes was evaluated in a longitudinal analysis using a generalized estimating equation (GEE).
Results: Of the 338 patients, 287 (84.9%) were in the subtherapeutic group at baseline. This group had a higher incidence of newly developed LN (P = 0.036) and had been prescribed higher mean and cumulative doses of prednisolone (P = 0.003 and P = 0.013, respectively) than the therapeutic group. In multivariable analyses based on GEE, the subtherapeutic group had a higher AMS score (β = 1.398, 95% CI 0.607, 2.189; P < 0.001), higher PGA score (β = 0.328, 95% CI 0.215, 0.441; P < 0.001) and higher SDI score (β = 0.366, 95% CI 0.061, 0.671; P = 0.019) across all 5 years.
Conclusion: The subtherapeutic HCQ concentration was associated with the development of new-onset LN, and had significant associations with disease activity and cumulative organ damage in SLE patients over time.
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