Clinical quality measures (CQMs) aim to identify gaps in care and to promote evidence-based guidelines. Official CQM definitions consist of a measure's logic and grouped, standardized codes to define key concepts. In this study, we used the official CQM update process to understand how CQMs' meanings change over time. First, we identified differences between the narrative description, logic, and the vocabulary specifications offour standardized CQMs' definitions in subsequent versions (2015, 2016, and 2017). Next, we implemented the various versions in a quality measure calculation registry to understand how the differences affected calculated prevalence of risk and measure performance. Global performance rates changed up to 5.32%, and an increase of up to 28% new patients was observed for key conditions between versions. Updates to definitions that change a measure's logic and choices to include/exclude codes in value set vocabularies changes measurement of quality and likely introduces variation by implementation.