Developing and integrating physician assistants / associates in UK hospital teams: a realist review of lessons from international experiences

BACKGROUND: Physician assistants/associates (PAs) were introduced into NHS secondary care facilities to help address workforce shortages in the UK. However, recent controversy and the government-commissioned Leng Review in England highlighted concerns around role clarity, supervision, and professional boundaries relating to PAs, largely due to inconsistent implementation and local variations. We examined how PA roles are developed and integrated in hospital teams across high-income countries, generating insights relevant to ongoing workforce reforms in the UK, including those recently recommended in England by the Leng Review. METHODS: We conducted a realist review to explain how, why, and under what contexts PA roles are developed and integrated in secondary care. We systematically searched peer-reviewed studies from high-income settings and UK-specific grey literature (Jan 2000-March 2025). We extracted and synthesised data to develop context-mechanism-outcome configurations (CMOCs). We mapped history, regulation, and scope of practice in included countries to support contextual interpretation. We iteratively refined CMOCs to produce a final programme theory. RESULTS: We developed 56 CMOCs from 122 sources across nine high-income settings, which were synthesised into five inter-related themes: (1) organisational drivers, such as service design, workforce shortages, and policy reforms, created opportunities for new workforce models like introducing PA roles; (2) PAs' role and identity formation were shaped through time, supervision, and opportunities for meaningful, appropriately challenging work; (3) negotiation of professional boundaries revealed unclear or overlapping roles creating tensions, whereas well-defined, complementary roles reducing resistance; (4) role perceptions and acceptance from team members and patients depended on perceived value and relative advantages, also shaped by psychologically safe team cultures; and (5) evidence and impact were difficult to measure using standard metrics, which often overlooked PAs' contributions to teamwork and continuity, and role variations and methodological limitations constrained generalisability. CONCLUSIONS: Our findings offer a transferrable framework for understanding workforce innovations and new roles in complex health systems. We provide practical insights for hospital managers and clinical leaders in the NHS, including those in England who are implementing the reforms recommended by the Leng Review. Realist evaluations are needed to refine our programme theory and inform effective workforce changes.