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Objective: To study the possible relationship between whole-blood hydroxychloroquine (HCQ) concentrations and clinical efficacy of HCQ in patients with systemic lupus erythematosus (SLE)., Methods: Whole-blood HCQ concentrations were measured, under blinded conditions, in 143 unselected patients with SLE who had been receiving HCQ 400 mg daily for at least 6 months. The relationship of these concentrations to current disease activity and to subsequent exacerbations during 6 months of followup was investigated., Results: At baseline, 23 patients had active disease (mean +/- SD SLE Disease Activity Index 12.4 +/- 7.5). The mean whole-blood HCQ concentration in this group was significantly lower than that in the 120 patients with inactive disease (694 +/- 448 ng/ml versus 1,079 +/- 526 ng/ml; P = 0.001). Among the 120 patients who had inactive disease at baseline, the mean HCQ concentration at baseline in the 14 (12%) who had disease exacerbations during followup was significantly lower than that in the patients whose disease remained inactive. Multivariate logistic regression showed that the HCQ concentration was the only predictor of exacerbation (odds ratio 0.4 [95% confidence interval 0.18-0.85], P = 0.01). Receiver operating characteristic curve analysis showed that a whole-blood HCQ concentration cutoff of 1,000 ng/ml had a negative predictive value of 96% for exacerbation during followup., Conclusion: Low whole-blood HCQ concentrations are associated with SLE disease activity and are a strong predictor of disease exacerbation. Regular drug assaying and individual tailoring of treatment might help to improve the efficacy of HCQ treatment in patients with SLE.