Determination of clozapine in serum of patients with schizophrenia as a measurement of medication compliance

Although antipsychotic drugs have been effective in reducing symptoms of schizophrenia, issues with adherence to these agents continue to be a barrier to the implementation and delivery of a successful treatment plan. An estimated 25% of patients with schizophrenia are partially adherent or non-adherent within 7-10 days of beginning therapy. There are some ways to evaluate the pharmacotherapy adherence of the patients: evaluation of the disease symptoms and/or the side effects of the drugs, questionnaires to evaluate quality of life, patient attitude toward his (her) drugs and pill counts. Although these methods represent a good option, they are subjective; for example, if the patients lie this leads to false results. Drug monitoring of patients' biological fluids can be a useful tool to evaluate adherence by relating the serum or plasma levels of drugs with pharmacotherapy compliance. The aim of this study was to determine if serum clozapine levels are a suitable method for evaluating patient adherence to clozapine therapy. Clozapine concentration was determined in serum of 26 volunteer patients who were using this drug as pharmacotheraphy for 6 months to 5 years (steady state conditions at 7-10 days of treatment with the drug). The analysis was done for 6 months, with three samples taken for each patient during this time, relating clozapine serum concentration of lower than therapeutic range with pharmacotherapy non-adherence of patients. Moreover, we compared the evaluation of the pharmacotherapy adherence from serum levels of the drug, with the evaluation of the pharmacotherapy adherence from an indirect tool to evaluate symptoms of disease. Twelve patients were found non-adherent by clozapine serum concentration (46.15%), whereas eight patients were found non-adherent using clinician questionnaire (30.76%). After to evaluate some factors (cigarettes, co-medication, inter-individual variability) that could give different results of adherence from evaluation of serum concentrations and from indirect methods, we can conclude that the clozapine serum concentration is a more objective method of evaluating pharmacotherapy adherence of patients receiving this drug.