Current practice in the measurement and interpretation of intervention adherence in randomised controlled trials: A systematic review

BACKGROUND: Ideally all participants in a randomised controlled trial (RCT) should fully receive their allocated intervention; however, this rarely occurs in practice. Intervention adherence affects Type II error so influences the interpretation of trial results and subsequent implementation. We aimed to describe current practice in the definition, measurement, and reporting of intervention adherence in non-pharmacological RCTs, and how this data is incorporated into a trial's interpretation and conclusions. METHODS: We conducted a systematic review of phase III RCTs published between January 2018 and June 2020 in the National Institute for Health Research Journals Library for the Health Technology Assessment, Programme Grants for Applied Research, and Public Health Research funding streams. RESULTS: Of 237 reports published, 76 met the eligibility criteria and were included. Most RCTs (n = 68, 89.5%) reported adherence, though use of terminology varied widely; nearly three quarters of these (n = 49, 72.1%) conducted a sensitivity analysis. Adherence measures varied between intervention types: behavioural change (n = 10, 43.5%), psychological therapy (n = 5, 83.3%) and physiotherapy/rehabilitation (n = 8, 66.7%) interventions predominately measured adherence based on session attendance. Whereas medical device and surgical interventions (n = 17, 73.9%) primarily record the number of participants receiving the allocated intervention, a third (n = 33, 67.3%) of studies reported a difference in findings between primary and sensitivity analyses. CONCLUSIONS: Although most trials report elements of adherence, terminology was inconsistent, and there was no systematic approach to its measurement, analyses, interpretation, or reporting. Given the importance of adherence within clinical trials, there is a pressing need for a standardised approach or framework.