Assessment of medication adherence in patients with schizophrenia: the Achilles heel of adherence research.

Measuring medication adherence in patients with schizophrenia is difficult and lacks a gold standard. Consequently, a great number of different methods and instruments have been proposed. Although it has been assumed that they all measure medication adherence, this study demonstrates that instruments differ significantly. Using data from an international multisite study (N = 329), we found that 3 questionnaires, designed to measure medication adherence and typical for instruments used in studies in patients with schizophrenia, do not agree in labeling patients as nonadherent. Further, they seem not to measure the same trait, are related to different established risk factors of nonadherence, and are only weakly related to these established risk factors of nonadherence. If these results are representative of the validity of other measures used in adherence research, this may have serious consequences for the interpretation of, and explanations for discrepancies found in the literature. Researchers should be aware of this problem and continue to combine objective and subjective methods in the hope of increasing the reliability and validity of measures of adherence.