Poor participant retention in randomised clinical trials, resulting in missing outcome data, can impact the validity, reliability and generalisability of results. While participants’ views on general non-retention issues have been reported elsewhere, a qualitative evidence synthesis specifically focusing on trial processes (ie, outcome data collection) impacting retention has not been undertaken to date. This is an important research question to inform targeted interventions to support retention. This review aims to address this by systematically searching and synthesising the evidence on participant reasons for trial non-completion, linked to outcome data collection.

We conducted a qualitative evidence synthesis of qualitative studies and mixed methods studies with a qualitative component, in Embase, Ovid MEDLINE, PsycINFO, Cochrane Central Register of Controlled Trials (CENTRAL), Social Science Citation Index, Cumulative Index of Nursing & Allied Health Literature and Applied Social Sciences Index and Abstracts, up to February 2025. We used Thomas and Harden’s thematic synthesis approach. The Grading of Recommendations Assessment, Development and Evaluation-Confidence in the Evidence from Reviews of Qualitative framework was used to assess confidence in the review findings.

We identified 11 studies reporting qualitative data from 14 separate trials, with findings from 105 trial non-retainers. The studies were undertaken between 2007 and 2025.

There were three types of participant non-retention behaviours reported across the studies, where participants either: (1) missed at least one clinic visit; (2) did not complete a postal questionnaire or (3) did not complete online data collection. We developed four analytical themes outlining participant-reported influences on trial non-retention, specifically related to trial processes (ie, data collection for outcome measures): fluctuating health, balancing trial burdens, navigating life as a trial participant and managing expectations of participation.

This review generates important insights into participants’ reasons for trial non-completion linked to outcome data collection. The review highlights the need for further research into supporting trial recruitment discussions that provide clear, realistic expectations for potential trial participants, as well as strategies that recognise, and where possible, address some of the influences on participants to improve outcome data completeness and ultimately improve trial retention.