A theory-informed mixed-methods approach for early implementation assessment in clinical trials of complex interventions

Background Clinical trials of complex interventions often face challenges when translating interventions developed under research conditions into routine practice. Implementation science offers approaches to examine this research-to-practice gap, but strategies for assessing early implementation within clinical trials remain limited. This is particularly relevant to rehabilitation interventions, which are individualized, context-dependent, and delivered through interactions among patients, healthcare professionals, and organizations. Methods A hybrid type 1 design was used to evaluate a palliative rehabilitation intervention. A theory-informed methodological approach was developed to guide early implementation assessment within the clinical trial. The EPIS process model structured assessment across trial phases, while Proctor’s taxonomy guided selection of implementation outcomes. Outcome measures were mapped and reviewed, prioritizing validated measures where available. Quantitative questionnaires and qualitative topic guides were developed for patients, healthcare professionals, and institutional representatives. The approach was designed to remain adaptable to trial phase and objectives. Results The resulting mixed-methods strategy combined quantitative assessment of selected implementation outcomes with qualitative exploration of stakeholder experiences. Three questionnaires incorporated the Acceptability of Intervention Measure, Intervention Appropriateness Measure, and Feasibility of Intervention Measure, alongside context-specific items addressing additional outcomes. Three qualitative topic guides supported integration across data sources. Data were collected at time points to capture anticipatory and experience-based perceptions of implementation. The approach aligned implementation outcomes, stakeholder perspectives, trial phases, and mixed-methods data collection without treating the strategy as a fixed or universally applicable instrument. Conclusions This study describes a theory-informed, mixed-methods approach for integrating early implementation assessment within clinical trials of complex interventions. Rather than proposing a new standardized instrument, it provides a transparent framework for selecting implementation outcomes, stakeholders, measures, and data collection strategies according to trial stage and objectives. Developed for a palliative rehabilitation intervention, the approach may be adapted to other complex intervention trials requiring implementation assessment alongside effectiveness evaluation.