Introduction: One of the most widely used methods for estimating patient adherence to drug therapy is the use of proportion of days covered (PDC), which is based on the assumption that patients who chronically purchase a drug do actually consume it.
Objectives: To investigate whether PDC reflect adherence based on steady state levels of lithium.
Methods: Using data on over 2000 patients on lithium in steady state, we correlated PDC with lithium steady state serum concentrations.
Results: In the case of lithium, PDC did not predict serum concentrations of the drug. This is probably due to poor and inconsistent adherence, which may stem from the medical condition itself.
Conclusions: The use of PDC cannot be assumed to avidly reflect drug adherence in all cases, and should be validated in different drugs and varying medical conditions.
Discussion: Steady purchasing of a medication cannot be assumed to reflect excellent adherence. Studies based on PDC must be scrutinized carefully for evidence of adherence.