Inclusion Criteria:  Patients within 7 days after cardiac surgery Acute kidney injury (AKI) described as Kidney Disease: Improving Global Outcomes (KDIGO) stage 2 (urine output <0.5mL/kg/h for ≥12h or 2-fold increase in serum creatinine compared with baseline)  Exclusion Criteria:  previous renal replacement therapy AKI secondary to obstructive nephropathy previous kidney transplantation