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BACKGROUND: Recruitment of adolescents to intervention studies is a known challenge. For randomized controlled trials (RCT) to be generalizable, reach must be assessed, which means ascertaining how many of the intended population actually participated in the trial. The aim of this study was to evaluate the reach and representativeness of an RCT evaluating the effectiveness of a complex intervention for adolescents with chronic conditions.

METHODS: A mixed methods sequential explanatory design was employed. Firstly, quantitative cross-sectional data from the RCT, patient registries and medical records were collected and analysed regarding baseline differences between participants and non-participants in the trial. Secondly, qualitative data on their reasons for participating or not were collected and analysed with content analysis to explain the quantitative findings.

RESULTS: Participants showed larger differences in effect sizes and a significantly more complex chronic condition than non-participants. No other statistically significant differences were reported, and effect sizes were negligible. Reasons for declining or accepting participation were categorized into three main categories: altruistic reasons, personal reasons and external reasons and factors.

CONCLUSIONS: Integration of quantitative and qualitative findings showed that participation in the RCT was affected by disease complexity, the perceived need to give back to healthcare and research and the adolescents' willingness to engage in their illness. To empower adolescents with chronic conditions and motivate them to participate in research, future intervention studies should consider developing tailored recruitment strategies and communications with sub-groups that are harder to reach.