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ObjectiveAcute exacerbation of chronic obstructive pulmonary disease (AECOPD) is associated with high rates of intensive care unit admission and elevated mortality. While urine output (UO) has been demonstrated to predict short-term adverse outcomes in critically ill patients, there is a paucity of research on its prognostic value for in-hospital mortality in AECOPD patients. This study aims to examine the association between UO and in-hospital mortality in AECOPD patients, and to assess its potential as a non-invasive prognostic indicator.MethodsA retrospective cohort study included 938 AECOPD patients from the Medical Information Mart for Intensive Care IV database. Patients were grouped by initial 24-h UO: low UO (≤800 mL), moderate UO (800-2500 mL, reference), and high UO (≥2500 mL). Restricted cubic spline regression, receiver operating characteristic curves, Kaplan-Meier (K-M) survival analysis, Cox proportional hazards regression models, subgroup analysis, and mediation analysis evaluated UO's association with mortality.ResultsIn-hospital mortality was 16.2%. UO showed a U-shaped non-linear association with mortality (P for nonlinearity < 0.05), with the lowest risk at 1375-4988 mL/24 h (hazard ratio [H.R.] < 1). UO showed limited predictive ability as a standalone marker (area under the curve = 0.62), suggesting it should be interpreted alongside other clinical parameters. K-M analysis indicated higher survival in the polyuria group versus oliguria (P < 0.05). In unadjusted Cox regression, polyuria was associated with a lower risk of mortality (H.R. = 0.51, 95% CI: 0.30-0.85, P = 0.011), whereas oliguria was associated with a higher risk of 28-day mortality (H.R. = 1.67, 95% CI: 1.22-2.27, P = 0.001). Subgroup analyses were consistent. Glasgow Coma Scale partially mediated 8.68% of the UO-mortality relationship (P = 0.026).ConclusionA U-shaped association exists between initial 24-h UO and short-term mortality in AECOPD. UO is an important but non-independent prognostic marker, functioning as a surrogate for overall disease severity. Its clinical application requires corroboration with other clinical markers.