Objective: Poor adherence to treatment is very difficult to diagnose accurately. Hydroxychloroquine (HCQ) has a long elimination half-life and its concentration in whole blood can be measured easily. We evaluated the utility of a very low blood HCQ concentration as a marker of poor compliance in patients with systemic lupus erythematosus (SLE).
Methods: We determined HCQ concentrations on a blinded basis in 203 unselected SLE patients. At the end of the study, patients were informed of the results and retrospectively interviewed about their adherence to treatment.
Results: 14 patients (7%) said they had stopped taking HCQ (n=8) or had taken it no more than once or twice a week (n=6). Their mean HCQ concentration was 26 ± 46 ng/mL [0-129]. In contrast, the other patients had a mean HCQ concentration of 1079 ng/mL [205-2629]. The principal barriers to adherence were related to HCQ treatment characteristics. Adherence subsequently improved in 10 of the 12 patients whose blood HCQ concentrations were remeasured.
Conclusions: Very low whole-blood HCQ concentrations are an objective marker of prolonged poor compliance in SLE patients. Regular drug assays might help physicians to detect noncompliance and serve as a basis for counseling and supporting these patients.