Nomogram based on the C-reactive protein-to-albumin ratio (CAR) for predicting 28-day outcomes in middle-aged and older patients with sepsis

Sepsis is associated with high short-term mortality among middle-aged and older critically ill patients. The C-reactive protein-to-albumin ratio (CAR) reflects both inflammation and nutritional/synthetic reserves, with prognostic significance. However, predictive models for ICU sepsis patients aged ≥ 45 years remain limited. This retrospective study included ICU patients with sepsis aged ≥ 45 years admitted to the Fifth Affiliated Hospital of Anhui Medical University between January 2022 and December 2024. Candidate predictors were screened using LASSO regression with tenfold cross-validation and the lambda.1se criterion. Variables with non-zero LASSO coefficients were then entered into a full post-LASSO multivariable logistic regression model, and a nomogram was constructed based on the final predictors. Model discrimination was assessed using the area under the receiver operating characteristic curve (AUC). Calibration was evaluated using calibration plots, the Brier score, and the Hosmer–Lemeshow goodness-of-fit test. Internal validation was performed using 1,000 bootstrap resamples, and clinical net benefit was assessed by decision curve analysis. A total of 148 patients were included, of whom 50 died within 28 days. Mechanical ventilation (OR = 3.46, 95% CI 1.07–11.25), CAR (OR = 1.14, 95% CI 1.02–1.28), blood lactate (OR = 1.20, 95% CI 1.03–1.40), and APACHE II score (OR = 1.09, 95% CI 1.02–1.17) were incorporated into the final nomogram. The model showed acceptable discrimination, with an apparent AUC of 0.847 (95% CI 0.785–0.910), and bootstrap internal validation yielded an optimism-corrected C-index of 0.832. The model also showed acceptable calibration, with a Brier score of 0.147 and no significant lack of fit in the Hosmer–Lemeshow test (P = 0.948). Decision curve analysis suggested potential clinical net benefit across a range of threshold probabilities. We developed and internally validated a preliminary nomogram incorporating mechanical ventilation, CAR, blood lactate, and APACHE II score for predicting and stratifying 28-day mortality risk in ICU sepsis patients aged ≥ 45 years.