The correlation between blood urea nitrogen to creatinine ratio and short-term and long-term all-cause mortality in sepsis-associated acute kidney injury patients aged 50 and above: a large retrospective cohort study

Objective To investigate the association between the blood urea nitrogen to creatinine ratio (BUN/Cr) and all-cause mortality in sepsis-associated acute kidney injury (SA-AKI) patients aged ≥ 50 years. Methods This single-center, retrospective cohort study included 764 patients aged ≥ 50 years with SA-AKI hospitalized at Henan Provincial People’s Hospital from January 2020 to August 2024. Patients were grouped into BUN/Cr tertiles: T1 (≤ 16.00), T2 (16.01–22.50), and T3 (> 22.50). The primary outcomes were 28-day, 90-day, and 1-year all-cause mortality. Associations between BUN/Cr and mortality were evaluated using Cox regression and subgroup analyses. Predictive value and dose-response relationships were assessed via receiver operating characteristic (ROC) curves and restricted cubic spline (RCS) models. Results In the overall cohort, the all-cause mortality rates at 28 days, 90 days, and 1 year were 5.0%, 11.4%, and 22.3%, respectively. After adjusting for confounders, each 1-unit increase in BUN/Cr was linked to higher mortality at 28 days (HR = 1.039, 95% CI: 1.014–1.064), 90 days (HR = 1.038, 95% CI: 1.020–1.055), and 1 year (HR = 1.027, 95% CI: 1.013–1.041). A 1-standard deviation increase in BUN/Cr corresponded to a 45.0%, 43.6%, and 29.4% increased risk at each time point (all P < 0.05). Compared with the T1 group, patients in the T3 had significantly higher risks of 90-day (HR = 3.042, 95% CI: 1.742–5.312) and 1-year (HR = 1.971, 95% CI: 1.349–2.879) mortality. Subgroup analyses confirmed consistent associations between BUN/Cr and 90-day and 1-year mortality across various clinical subgroups. ROC curve analysis demonstrated that BUN/Cr had a moderate predictive ability for mortality at 28 days (AUC = 0.636), 90 days (AUC = 0.656), and 1 year (AUC = 0.610). Notably, adding BUN/Cr to the baseline multivariable model significantly improved discrimination for 28-day mortality (P for comparison = 0.012). RCS modeling showed a linear and positive association between BUN/Cr and mortality without significant non-linearity (P-nonlinear > 0.5 for all time points). Conclusion A higher BUN/Cr ratio is significantly linked to increased short- and long-term mortality in SA-AKI patients aged 50 and above, suggesting its potential utility for early risk stratification and clinical decision-making.