Early Serum Phosphate and Potassium Levels Are Associated with 28‑Day Mortality and Functional Recovery in Mechanically Ventilated ICU Patients: A Retrospective Cohort Study

To investigate the association between early serum electrolyte levels and short-term outcomes in mechanically ventilated ICU patients, and to identify clinically useful electrolyte thresholds. This retrospective cohort study included 152 mechanically ventilated patients. Serum phosphate, potassium, sodium, and corrected calcium were measured during the first three ICU days. Baseline clinical scores (SOFA, APACHE II, NUTRIC) were recorded. The primary endpoint was 28‑day survival. Logistic regression and ROC curve analyses were performed. Among 28‑day survivors, functional outcome was assessed using the Barthel Index at three months. Of 152 patients, 105 survived and 47 died within 28 days. Higher day-1 serum phosphate was independently associated with lower odds of survival (OR = 0.11, 95% CI: 0.02–0.65; P = 0.015), whereas relatively higher day-1 serum potassium within the observed physiological range was associated with greater odds of survival (OR = 3.41, 95% CI: 1.60–7.30; P < 0.001). Higher SOFA score was also associated with lower survival (OR = 0.53, 95% CI: 0.38–0.74; P < 0.001). Exploratory ROC-derived cut-off values were 1.35 mmol/L for phosphate (AUC = 0.725) and 4.33 mmol/L for potassium (AUC = 0.707), indicating limited individual discriminatory ability. Among survivors, potassium > 4.33 mmol/L was associated with a 4.44-point higher unadjusted three-month Barthel Index score ( P = 0.016), although its clinical significance was uncertain. Early phosphate and potassium concentrations were associated with 28-day survival in this cohort. Potassium finding reflects concentrations in the higher-normal range and should not be interpreted as evidence that hyperkalemia or therapeutic potassium elevation is beneficial. The ROC-derived cut-off values were exploratory and should not be used as standalone prognostic thresholds or treatment targets. The Barthel Index findings were unadjusted and hypothesis-generating. Larger prospective multicenter studies with comprehensive adjustment and external validation are required.