Patterns and predictors of high-cost users of the health system: a data linkage protocol to combine a cohort study and randomised controlled trial of adults with a history of homelessness

INTRODUCTION: Homelessness is a global issue with a detrimental impact on health. Individuals who experience homelessness are often labelled as frequent healthcare users; yet it is a small group of individuals who disproportionately use the majority of services. This protocol outlines the approach to combine survey data from a prospective cohort study and randomised controlled trial with administrative healthcare data to characterise patterns and predictors of healthcare utilisation among a group of adults with a history of homelessness. METHODS AND ANALYSIS: This cohort study will apply survey data from the Health and Housing in Transition study and the At Home/Chez Soi study linked with administrative healthcare databases in Ontario, Canada. We will use count models to quantify the associations between baseline predisposing, enabling, and need factors and hospitalisations, emergency department visits and physician visits in the following year. Subsequently, we will identify individuals who are high-cost users of the health system (top 5%) and characterise their patterns of healthcare utilisation. Logistic regression will be applied to develop a set of models to predict who will be high-cost users over the next 5 years based on predisposing, enabling and need factors. Calibration and discrimination will be estimated with bootstrapped optimism (bootstrap performance—test performance) to ensure the model performance is not overestimated. ETHICS AND DISSEMINATION: This study is approved by the St Michael’s Hospital Research Ethics Board and the University of Toronto Research Ethics Board. Findings will be disseminated through publication in peer-reviewed journals, presentations at research conferences and brief reports made available to healthcare professionals and the general public. TRIAL REGISTRATION NUMBER: This is a secondary data analysis of a cohort study and randomized trial. The At Home/Chez Soi study has been registered with the International Standard Randomised Control Trial Number Register and assigned ISRCTN42520374.