This study aims to explore the correlation between serum chloride levels and all-cause mortality among patients in the Surgical Intensive Care Unit (SICU).A retrospective cohort study was conducted using data extracted from the Medical Information Mart for Intensive Care IV (MIMIC-IV) database, focusing on patients admitted to surgical/trauma ICUs. Data included demographic information, laboratory results, medical history, vital signs, and prognosis-related variables. Patients were categorized into four groups based on serum chloride levels. Outcome measures included 30-day, 90-day, and 180-day mortality rates post-ICU admission. Kaplan–Meier survival analysis curves, Cox proportional hazards regression analysis, and Restricted Cubic Spline (RCS) models were used to analyze the relationship between serum chloride levels and clinical outcomes.A total of 10,996 patients were included in the study. The 30-day mortality rate was 12.78%, the 90-day mortality rate was 17.14%, and the 180-day mortality rate was 20.32%. Kaplan-Meier survival analysis demonstrated significant differences in survival rates among different serum chloride level groups during the 30-day, 90-day, and 180-day follow-up periods (p < 0.001). Multivariate Cox regression analysis results indicated that elevated serum chloride levels were significantly associated with decreased all-cause mortality rates at 30-days (adjusted hazard ratio [aHR]: 0.96; 95% confidence interval [CI]: 0.94–0.98; P < 0.001), 90-days (aHR: 0.97; 95% CI: 0.96–0.98; P < 0.001), and 180-days (aHR: 0.97; 95% CI: 0.96–0.98; P < 0.001). Moreover, compared with the first quartile of serum chloride levels, the risk of death was significantly lower in the fourth quartile (30-days aHR = 0.65, 90-days aHR = 0.71, 180-days aHR = 0.69, P < 0.001).RCS curves showed an L-shaped relationship between serum chloride levels and all-cause mortality risk for SICU patients at 30-days, 90-days, and 180-days. Although the magnitude of reduction diminished when levels reached 104 mmol/L, an increase in serum chloride levels was associated with a decreased risk of mortality.This study demonstrates a significant nonlinear relationship between serum chloride levels and all-cause mortality rates among SICU patients. This finding contributes to a more precise monitoring of chloride levels in SICU patients.
