Acute Kidney Injury Definition and Staging

Definition and Diagnosis

The Kidney Disease: Improving Global Outcomes (KDIGO) criteria define Acute Kidney Injury (AKI) as an abrupt decrease in kidney function occurring over 7 days or less, characterized by an increase in serum creatinine ≥0.3 mg/dL within 48 hours, or an increase in serum creatinine to ≥1.5 times baseline within the previous 7 days, or urine output <0.5 mL/kg/h for 6 consecutive hours 1, 2
Even small increases in creatinine (≥0.3 mg/dL) are independently associated with approximately a fourfold increase in hospital mortality 1, 2

AKI severity is classified into three stages based on the most severe criterion met, with progression through stages strongly correlating with increased mortality 1, 2
Stage 1 AKI is characterized by an increase of 1.5-1.9 times baseline serum creatinine, or an increase ≥0.3 mg/dL, or urine output <0.5 mL/kg/h for 6-12 hours 1, 2
Stage 2 AKI is characterized by an increase of 2.0-2.9 times baseline serum creatinine, or urine output <0.5 mL/kg/h for ≥12 hours 1, 2
Stage 3 AKI is characterized by an increase ≥3.0 times baseline serum creatinine, or increase to ≥4.0 mg/dL, or initiation of renal replacement therapy, or urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours 1, 2

In patients with cirrhosis and ascites, focus exclusively on serum creatinine changes rather than urine output, as these patients are frequently oliguric with avid sodium retention yet maintain relatively normal GFR 1
Diuretic therapy further confounds urine output interpretation, and urine output criteria should not be relied upon alone in patients receiving diuretics 1
Persistent AKI is characterized by continuance of AKI beyond 48 hours from onset, while complete reversal within 48 hours characterizes rapid reversal of AKI 3
Acute Kidney Disease (AKD) describes acute or subacute kidney damage lasting between 7 and 90 days after an AKI initiating event 3, 4, 5
AKD that persists beyond 90 days is considered chronic kidney disease 4, 5