Medication adherence is a key factor impacting efficacy and safety of medicines, yet how it is dealt with in European registration trials is unknown. A cross-sectional analysis of European Medicines Agency (EMA) marketing authorization dossiers for new medicines approved through centralized procedures in the European Union between 2010 and 2020 was performed. Data were extracted from European Public Assessment Reports and Clinical Study Reports. Clinical trials covering five therapeutic areas were included: diabetes, respiratory conditions, cardiovascular diseases, infectious diseases, and oncology. Outcomes included adherence assessment, measurement methods, and rates. Overall, 102 medicines studied in 253 clinical trials were reviewed. All but one study reported measuring adherence. Two hundred twenty trials (87%) measured adherence using quantitative methods, while 32 (13%) trials monitored adherence but did not further quantify. Reported adherence rates were high (> 90%) across trials yet marked disparities in measurement methods and definitions were found. The most frequently used adherence measurement method was pill/dose count (single method: 52.7%; in combination: 37.7%; with patient diary/report: 17.3%; electronic methods: 1.4%; bioanalytical methods: 4.1%). Patient diary/report (6.4%) and electronic methods (2.7%) were also used as single methods. Electronic methods were more often used in respiratory and anti-infective trials, while bioanalytical methods were more frequently used in diabetes. Overall, adherence is measured in EMA registration trials, yet the methods used and the way in which adherence rates are presented vary widely between trials and therapeutic areas. To better understand and compare efficacy of medicines, standardization of adherence definitions and measurement methods is needed.
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