Abstract Background Patients suffering from acute kidney injury (AKI) were associated with impaired sodium and potassium homeostasis. We aimed to investigate how admission serum sodium and potassium independently and jointly modified adverse clinical outcomes among AKI patients. Methods Patient data were extracted from the Multiparameter Intelligent Monitoring in Intensive Care Database III. Participants were categorized into three groups according to admission serum sodium and potassium, and the cut-off values were determined using smooth curve fitting. The primary outcome was 90-day mortality in the intensive care unit (ICU). Cox proportional hazards models were used to evaluate the prognostic effects of admission serum sodium and potassium levels. Results We included 13,621 ICU patients with AKI (mean age: 65.3 years; males: 55.4%). The middle category of admission serum sodium and potassium levels were 136.0–144.9 mmol/L and 3.7–4.7 mmol/L through fitting smooth curve. In multivariable Cox models, compared with the middle category, patients with hyponatremia or hypernatremia were associated with excess mortality and the HRs and its 95%CIs were 1.38 (1.27, 1.50) and 1.56 (1.36, 1.79), and patients with either hypokalemia or hyperkalemia were associated with excess mortality and the hazard ratios (HRs) and its 95% confidential intervals (95% CIs) were 1.12 (1.02, 1.24) and 1.25 (1.14, 1.36), respectively. Significant interactions were observed between admission serum sodium and potassium levels (P interaction = 0.001), with a higher serum potassium level associated with increased risk of 90-day mortality among patients with hyponatremia, whereas the effects of higher sodium level on prognostic effects of potassium were subtle. Conclusions Admission serum sodium and potassium were associated with survival in a U-shaped pattern among patients with AKI, and hyperkalemia predict a worse clinical outcome among patients with hyponatremia.
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