Evaluating the implementation of a mental health joint response with young people and families: protocol for the care responders study, a realist and health economic evaluation.

Introduction: Mental health crises among children and young people are increasing in frequency and complexity, yet emergency responses often lack appropriate tailoring for young people and responders cite a need for further workforce training and support. This study will integrate immersive theatre, co-production methods, health economic analysis, research data, and a realist synthesis to develop a holistic programme theory informing joint responses to crisis care for children and young people. It will incorporate novel triangulation across these methods and, where appropriate, draw on the recently published Realist Economic Evaluation Methods(REEMS) guidance. Methods: This study aims to evaluate a novel intervention involving a joint response by police officers and mental health practitioners. A realist and health economic evaluation design will be employed, incorporating multiple data collection methods. Participants will include children and young people aged 5-18, their families, carers, and practitioners. Data will be collected through surveys, interviews, and routine service records, with recruitment via five distinct pathways including direct response and online participation. Results: The evaluation will explore how, why, and for whom the joint response car works, identifying mechanisms and contextual factors that influence outcomes. A cost-consequence analysis will assess the financial implications of the intervention compared to usual care. Discussion: Findings will inform best practice guidance for emergency mental health care and support national implementation of joint response models. The study will also contribute to understanding service integration and stakeholder engagement in crisis care.