Aetiology of Pleural Effusions in a Large Multicentre Cohort: Variation Between Outpatients and Inpatients.

Introduction: This multi‐centre retrospective cohort study aimed to determine whether the cause of an undiagnosed pleural effusion differed depending on if a patient presented as an outpatient or inpatient. Methods: A total of 1080 adult patients (556 inpatients and 524 outpatients) presenting primarily with an undiagnosed pleural effusion from 1 January 2021 to 31 December 2022 from four UK hospitals were included. Results: We found malignant effusions were more common in outpatients compared to inpatients (48.3% vs. 36.0% p < 0.0001). Infection was common in inpatients but uncommon in outpatients (36.2% vs. 5.0% p < 0.0001). Other causes in all patients included heart and/or renal failure (13.1%) and non‐specific pleuritis (5.6%). No diagnosis was possible in 11.8% of patients referred. Conclusion: Investigative pathways should vary depending on whether patients present as an inpatient or outpatient. [ABSTRACT FROM AUTHOR]