Study Protocol: A retrospective observational analysis of patients treated for hospital-acquired pneumonia.

Data will be obtained from the Health Data Research UK acute care hub, 'PIONEER' Cases meeting coding criteria or a clinical surveillance definition of HAP over a 5-year period will be extracted. Demographic, clinical, and microbiological variables will be analysed initially descriptively, and subsequently, with multiple logistic regression analysis to investigate factors affecting microbiological diagnosis. Key outcome variables are in-hospital, 30-day and 1 year mortality, as well as all-cause readmissions within 1 year. Secondary outcomes include nosocomial infections, such as <i>C. difficile</i>. Kaplan-Meier curves and a Cox proportional hazards regression model will be used to investigate outcomes and compare subgroups. A key comparison is between those in whom a putative pathogen is identified and those treated entirely empirically. For this purpose, we will also compare outcomes using an inverse probability of treatment weighting analysis. Additionally, we will explore identifying consolidation on chest imaging reports using natural language processing to allow consideration of the relative impact this may have on mortality and readmission rates.