Poor adherence to treatment is difficult to diagnose accurately. Hydroxychloroquine (HCQ) has a long elimination half-life and its concentration in whole blood can be measured easily.
To evaluate the utility of a very low blood HCQ concentration as a marker of poor compliance in patients with systemic lupus erythematosus (SLE).
HCQ concentrations were determined on a blinded basis in 203 unselected patients with SLE. At the end of the study, the patients were informed of the results and retrospectively interviewed about their adherence to treatment.
14 (7%) patients said that they had stopped taking HCQ (n = 8) or had taken it no more than once or twice a week (n = 6). Their mean (SD) HCQ concentration was 26 (46) ng/ml. range (0-129 ng/ml) By contrast, the other patients had a mean HCQ concentration of 1079 ng/ml range (205-2629 ng/ml). 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.
Very low whole-blood HCQ concentrations are an objective marker of prolonged poor compliance in patients with SLE. Regular drug assays might help doctors in detect non-compliance and serve as a basis for counselling and supporting these patients.