Outreach services with a health specialism for people rough sleeping in the UK: An intervention optimisation study

Background People who experience homelessness report poorer health outcomes than those who are housed. Outreach with a health specialism is an intervention aiming to remove barriers to healthcare by having health professionals deliver services directly to people in their own environment. Despite widespread use, currently there is limited evaluation evidence for the intervention in the UK. This study describes the intervention’s programme theory and reports the optimisation of components, implementation strategy and context to maximise intervention functioning across the UK health and housing system. It will be followed by a pilot cluster Randomised Controlled Trial with nested process and economic evaluation. Methods We conducted a mixed-method optimisation study in England between June 2024 and January 2025. The optimisation process comprised three research phases: development of candidate programme theory; refinement of programme theory; and confirmation of final programme theory and development of operational delivery plan. Research methods employed across optimisation phases were: rapid evidence review of intervention manual, training materials and evidence-base; three professional (n = 12 individuals) and lived experience (n = 7 individuals) stakeholder workshops; and semi-structured interviews with system stakeholder with experience of the intervention (n = 8). The final optimised intervention was reported according to the Template for Intervention Description and Replication (TIDieR) checklist. Results The programme theory describes intervention mechanisms, components, outcomes and context. We identified potential system inhibitors to intervention functioning in the UK, which include local variations in population profile, limited organisational capacity and availability of follow-on services. Recommendations for intervention optimisation were: integrating lived experienced voices into training; ensuring training aligns with person-centred, responsive, and trauma-informed principles; tailoring outreach; developing supervision and a peer-led community of practice; improving data generation and flow; nurses to attend appointments with clients; and empowering nurses to work with other sectors. Conclusion This is one of the first studies to generate a comprehensive programme theory for outreach with a health specialism and describe how it can be optimally delivered in the UK. A clear theoretical understanding of how and why the should work will support future outcome evaluation and potential scale-up. It also provides a useful methodological example of intervention optimisation. Trial registration: ISRCTN Registry ISRCTN11572394 (Registration 13/12/2024)