Background Hydroxychloroquine (HCQ) is a key component of therapy for Systemic Lupus Erythematosus (SLE). Medication non-adherence is reported in 40%–80% of lupus patients and results in increased morbidity, mortality, and health care utilization. HCQ levels are a sensitive and reliable method to assess medication adherence. Our study evaluated the feasibility of HCQ level measurement in a routine clinical setting and its association with disease activity in a predominantly Hispanic population.

Methods SLE patients from the Columbia University Lupus cohort, treated with HCQ for ≥6 months and reporting medication-adherence were included. HCQ levels were measured by whole blood high performance liquid chromatography. Non-adherence was defined as HCQ level <500 ng/ml. Univariable and multivariable regression models were constructed to evaluate the association between HCQ level and disease activity measured by SLEDAI-2K.

Results One-hundred and eight patients were enrolled (table 1); the mean age was 38 years, 91% were female, and 62% were Hispanic. The average SLEDAI-2K was 4.3 (0–20). Forty-one percent of patients had HCQ levels<500 ng/ml consistent with non-adherence, of whom 19% had undetectable levels. A higher SLEDAI-2K score was associated with low HCQ levels (p<0.001). This association remained significant after adjusting for age, sex, race/ethnicity, education, primary language, glomerular filtration rate, depression, smoking, major organ involvement, steroid use, and immunosuppression (p<0.004).

Conclusions HCQ levels <500 ng/ml were associated with higher disease activity and accounted for 32% of the SLEDAI variability. HCQ levels are a simple and reliable method to evaluate medication adherence in SLE. Levels <500 ng/ml should be discussed with patients and the reasons for non-adherence should be further explored and addressed.

Acknowledgements We acknowledge the patients who participated in this study.