Maintaining accurate medication lists in the medical record and ensuring patient medication adherence remains an ongoing challenge. In this cross-sectional study, researchers tested the use of a mass spectrometry assay to identify medication adherence among 1346 patients across 3 different care settings. Mass spectrometry testing revealed discrepancies between medications listed as prescribed in the electronic health record and what patients were actually taking. The authors suggest that the use of such testing may be helpful in improving both the accuracy of medication lists and medication adherence.