Prognostic Value of Admission Serum Chloride and of the Composition of Metabolic Acidosis in Postoperative Intensive Care: A Single-Centre Retrospective Cohort Study

Background/Objectives: Serum chloride is measured in almost every postoperative intensive care unit (ICU) admission. We examined whether it carries prognostic information once operative case-mix and the composition of the metabolic acidosis are accounted for. Methods: In 1136 adults admitted from the operating theatre to a single 20-bed ICU, admission chloride was modelled with restricted cubic splines in multivariable logistic regression, and three acid–base features (lactate > 2, chloride > 108, anion gap > 16 mmol/L) were counted in patients with base excess <−3 mmol/L. The primary outcome was ICU mortality. Results: A total of 95 patients (8.4%) died in the ICU. Chloride showed no association with ICU mortality on the overall test (χ2 = 4.98, 2 df; p = 0.083), and its removal left discrimination unchanged (ΔC +0.003, 95% CI −0.004 to +0.021), although the odds ratio for 115 versus 105 mmol/L was 1.91 (1.08–3.38). ICU mortality rose from 6.6% with one acid–base feature to 58.6% with three (Firth’s penalised odds ratio 10.50 (4.65–23.70) versus no metabolic acidosis), a contrast dominated by the depth of the acidosis; adjusted for that depth, each feature carried an odds ratio near 2.0. The anion gap, uncorrected for albumin, and bicarbonate, but not the strong ion difference, were associated with death. Conclusions: Admission chloride added no detectable prognostic information, though a modest association is not excluded. Coexisting acid–base abnormalities identified high-risk patients and place the signal in the bicarbonate deficit and unmeasured anions, but this is a descriptive marker needing external validation, not an established risk-stratification tool.