Association Between C-Reactive Protein-to-Albumin Ratio and Length of Hospital Stay in Elderly Patients with Acute Appendicitis: A Mediation Analysis of Complicated Appendicitis.

PURPOSE: To investigate the association of the C-reactive protein-to-albumin ratio (CAR) with complicated appendicitis and length of hospital stay (LOS) in elderly patients with acute appendicitis and to explore the mediating role of complicated appendicitis. METHODS: In this single-center retrospective cohort at Bozhou People's Hospital (Dec 2024-Dec 2025), we consecutively enrolled patients aged ≥65 years with preoperative acute appendicitis undergoing emergency laparoscopic appendectomy. The primary exposure was preoperative C-reactive protein-to-albumin ratio (CAR). Covariates were selected via least absolute shrinkage and selection operator (LASSO) regression, with variance inflation factor (VIF) analysis for multicollinearity. The primary outcome was length of hospital stay; the secondary outcome was complicated appendicitis, defined as intraoperative or pathologically confirmed perforation, gangrene, or periappendiceal abscess. Multivariable logistic regression and Gamma log-link generalized linear models assessed CAR's associations with the two outcomes respectively. Counterfactual-based mediation analysis with bootstrap validation evaluated the mediating role of complicated appendicitis. RESULTS: CAR was identified as an independent predictor of complicated appendicitis (OR=1.431, 95% CI: 1.197-1.710, P<0.001). After adjusting for confounders, each one-unit increase in CAR was associated with approximately 4.4% prolongation of LOS (RR=1.044, 95% CI: 1.024-1.064, P<0.001). Mediation analysis revealed that complicated appendicitis exerted a significant partial mediating effect between CAR and LOS, accounting for 57.8% of the total effect. CONCLUSION: Elevated CAR is associated with complicated appendicitis and prolonged LOS in elderly patients with acute appendicitis, with partial mediation by complicated appendicitis. CAR may assist in preoperative risk stratification.