Balancing Standardisation and Flexibility in National Complex Intervention Scale-Up: A Qualitative Study of Leadership Experiences and Management in the Implementation of Proactive Health Support in Denmark

Background Policymakers increasingly aim to expand small-scale healthcare interventions to a national level to maximise reach and impact. However, scaling complex interventions presents challenges, particularly in balancing national standardisation with flexibility in local implementation across contexts. This study investigates how leaders experience and manage the national implementation of Proactive Health Support, a large-scale self-management support intervention in Denmark. Methods The study employed a single-case study design with embedded units to examine the national implementation of the Proactive Health support intervention. Data were collected through 10 semi-structured interviews with leaders from the five Danish regions, over 25 hours of participant observation of leadership meetings and workshops, and a document analysis of 468 pages of leadership meeting minutes and reports. Results Implementing Proactive Health Support required leaders to balance the tension between fidelity and adaptation within a standardised framework. While the national framework provided stability, leaders leveraged the flexibility of implementation strategies to preserve the core intervention components while enabling local adjustments to fit diverse contexts. Reflexive practices, characterised by real-time adjustments and operating through feedback loops, both reinforcing and stabilising, were pivotal for balancing standardisation with flexibility. Vertical leadership mediated alignment between national and regional levels, while horizontal collaboration fostered shared learning across regions. Conclusion This study demonstrates the complexity of scaling healthcare interventions across diverse contexts, underscoring the non-linear and dynamic nature of such processes and the significant ongoing leadership engagement required to navigate complexity and manage uncertainty. It highlights collaborative leadership as crucial for enabling real-time responsiveness, fostering alignment across levels, and balancing fidelity and adaptation in national implementations. The findings empirically demonstrate that fidelity and adaptation are not opposing forces but complementary in practice, with adaptation in implementation strategies supporting fidelity by allowing interventions to be applied meaningfully across diverse contexts. The findings further suggest that successful national implementation relies not on strict adherence to standardised guidelines but on a co-adaptive process within a standardised yet flexible framework, where intervention and context are co-constructed through ongoing, leadership-driven negotiation. Trial registration: Part of a research programme including a registered randomised controlled trial (ClinicalTrials.gov, NCT03628469). Not applicable to this study.