What is Acute Kidney Injury (AKI) and what are its stages?

Definition and Staging of Acute Kidney Injury (AKI)

Definition of AKI

Acute Kidney Injury is defined by the KDIGO criteria as an abrupt decrease in kidney function occurring over 7 days or less, characterized by any one of the following: 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

• AKI represents a clinical syndrome with rapid decline in glomerular filtration rate and accumulation of metabolic waste products 3
• The diagnosis requires meeting any one of the three criteria (creatinine rise at 48 hours, creatinine rise at 7 days, or urine output criterion) 1
• Even small increases in creatinine (≥0.3 mg/dL) are independently associated with approximately a fourfold increase in hospital mortality 1, 2

AKI Staging System

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

• Serum creatinine: Increase of 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 1, 2
• Urine output: <0.5 mL/kg/h for 6-12 hours 1, 2

Stage 2 AKI

• Serum creatinine: Increase of 2.0-2.9 times baseline 1, 2
• Urine output: <0.5 mL/kg/h for ≥12 hours 1, 2

Stage 3 AKI

• Serum creatinine: Increase ≥3.0 times baseline OR increase to ≥4.0 mg/dL (when the rise is >0.3 mg/dL or >50% within specified timeframes) OR initiation of renal replacement therapy 1, 2
• Urine output: <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1, 2

Key Clinical Considerations

Staging Methodology

• Staging is performed retrospectively when the episode is complete, classifying according to the most severe criterion that was met 1
• Patients are assigned to the highest stage they reach based on either creatinine or urine output criteria 1

Important Caveats for Urine Output Criteria

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 1
• Urine output criteria should not be relied upon alone in patients receiving diuretics 1
• In cirrhotic patients, a serum creatinine threshold of ≥1.5 mg/dL predicts AKI progression and worse prognosis 1

Temporal Distinctions

Persistent AKI is characterized by continuance of AKI beyond 48 hours from onset, while complete reversal within 48 hours characterizes rapid reversal of AKI. 4

• A minimum of 48 hours of sustained reversal is necessary to separate two distinct AKI episodes 4
• Persistent AKI frequently becomes Acute Kidney Disease (AKD), defined as AKI stage 1 or greater persisting ≥7 days after exposure 4
• AKD describes acute or subacute kidney damage lasting between 7 and 90 days after an AKI initiating event 4
• AKD that persists beyond 90 days is considered chronic kidney disease 4

Limitations of Diagnostic Criteria

• Serum creatinine significantly overestimates actual kidney function in patients with muscle wasting, increased tubular secretion, volume expansion, or hyperbilirubinemia 1
• Hyperbilirubinemia can result in inaccurate measurement of serum creatinine by colorimetric methods 1
• Do not wait for creatinine to reach 1.5 mg/dL before diagnosing AKI, as this threshold often indicates GFR has already fallen to ~30 mL/min 1