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INTRODUCTION:The kidneys play a crucial role in the regulation of electrolytes and acid-base homeostasis. Urinary Strong Ion Difference (SIDu = NaU + KU-ClU) represents an important aspect of renal acid-base regulation. We evaluated the role of SIDu as a marker of renal dysfunction in critically ill patients. MATERIALS AND METHODS:Patients admitted to the Medical Intensive Care Unit with a diagnosis of AKI for whom concomitant urinary samples available for SIDu calculation were retrospectively reviewed and staged according to KDIGO criteria for 3 days from inclusion. Patients were classified as Recovered (R-AKI) or Persistent-AKI (P-AKI) whether they exited KDIGO criteria within the 3-day observation period or not. A control group with normal renal function and normal serum acid-base and electrolytes was prospectively recruited in order to identify reference SIDu values. RESULTS:One-hundred-and-forty-three patients with a diagnosis of AKI were included: 77 with R-AKI, and 66 with P-AKI. Thirty-six controls were recruited. Patients with P-AKI had more severe renal dysfunction and higher mortality than patients with R-AKI (SCr 2.23(IQR:1.68-3.45) and 1.81(IQR1.5-2.5) mg/dl respectively, p