The study aims to evaluate the diagnostic value of urinary indices in differentiating pre-renal and renal acute kidney injury among critically ill patients.

Critically ill patients with acute kidney injury (AKI) will be included. Urinary indices will be obtained immediately after defining AKI, including specific gravity (SG), urine osmolarity (Uosm), urine sodium (UNa), fractional excretion of sodium (FENa), and fractional excretion of urea (FEurea).

The patterns of renal function and hemodynamic status serve as the clinical definition of pre-renal and renal AKI. Hemodynamic status will be evaluated at inclusion and followed during patients' ICU stay. Those who are hemodynamically stable at inclusion are classified as renal AKI. Those who are hemodynamically unstable at inclusion are followed until hemodynamic stabilization. Those with AKI recovery after hemodynamic stabilization are classified as pre-renal AKI. Those without AKI recovery are defined as renal AKI.

Urinary indices are measured again after hemodynamic stability. One of the investigators objectives is to evaluate the diagnostic value of urinary indices using recovery of renal function after hemodynamic stabilization as the definition of pre-renal AKI.

Transesophageal echocardiography (TEE) has been proved to be a reliable method of measuring left renal blood flow during surgical operation. Decreased renal blood flow measured by TEE may serve as an objective definition of pre-renal AKI. The mesurement will be conducted at inclusion if the patient is sedated, intubated, and without TEE contraindication. If the patient is hemodynamically unstable at inclusion, the measurement will be repeated after hemodynamic stabilization. The investigators study also serves the following aims: (1) feasibility of measuring renal blood flow by TEE among critically ill patients; (2) the correspondence between renal blood flow measured by TEE and clinical definition of pre-renal and renal AKI; (3) the diagnostic value of urinary indices in differentiating pre-renal and renal AKI, which is judged by renal blood flow measured by TEE.

Inclusion Criteria:

All patients admitted to Medical ICU of PUMCH with one of the following:

• Increase in serum creatinine by ≥ 26.5 umol/l within 48 hours;
• Increase in serum creatinine to ≥ 1.5 times baseline, which is known or presumed to have occurred within the prior 7 days;
• Urine volume < 0.5ml/kg/h for 1 hour.

Exclusion Criteria:

• Obstructive renal disease
• Renal replacement therapy (RRT) for chronic kidney disease

Patients diagnosed acute kidney injury at ICU admission or during ICU stay