Definition of Acute Kidney Injury (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: a rise in serum creatinine ≥0.3 mg/dL (≥26 μmol/L) within 48 hours, OR an increase in serum creatinine to ≥1.5 times baseline within 7 days, OR urine output <0.5 mL/kg/h for 6 consecutive hours. 1, 2
Diagnostic Criteria Components
The diagnosis requires meeting any single criterion from the following 1, 3:
- Serum creatinine increase ≥0.3 mg/dL within 48 hours 1, 2
- Serum creatinine increase to ≥1.5 times baseline within 7 days 1, 2
- Urine output <0.5 mL/kg/h for ≥6 consecutive hours 1, 2
AKI Staging System
The KDIGO classification stages AKI severity from 1 to 3 based on the most severe criterion met 2:
Stage 1
- Serum creatinine increase 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 4, 2
- Urine output <0.5 mL/kg/h for 6-12 hours 4, 2
Stage 2
Stage 3
- Serum creatinine increase ≥3.0 times baseline OR increase to ≥4.0 mg/dL (with acute rise >0.3 mg/dL or >50%) OR initiation of renal replacement therapy 4, 2
- Urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 4, 2
Clinical Significance of the Definition
Even the smallest threshold increase in serum creatinine (≥0.3 mg/dL) is independently associated with approximately a fourfold increase in hospital mortality, which is why this sensitive threshold was incorporated into the KDIGO criteria 1, 2. The staging system directly correlates with mortality risk, with progression through stages strongly associated with increased mortality 1, 4.
Relationship to Acute Kidney Disease (AKD)
AKI exists within a broader continuum of kidney injury 1:
- Acute Kidney Disease (AKD) describes acute or subacute kidney damage and/or loss of function for a duration between 7 and 90 days after exposure to an AKI initiating event 5
- AKD that persists beyond 90 days is considered chronic kidney disease 5
- AKI represents the acute phase (≤7 days), while AKD captures the subacute recovery or progression phase 5, 1
Critical Pitfalls in Applying the Definition
Baseline Creatinine Assessment
Using known creatinine values is superior to imputation methods when establishing baseline 5. Back-calculation from an estimated GFR of 75 mL/min/1.73 m² may overestimate AKI incidence in populations with high CKD prevalence 5.
Urine Output Criteria Limitations
In patients with cirrhosis and ascites, urine output criteria are unreliable because these patients are frequently oliguric with avid sodium retention yet may maintain relatively normal GFR 1, 4. In this population, focus primarily on serum creatinine changes rather than urine output 4.
Serum Creatinine Limitations
Serum creatinine has inherent limitations as a marker, being affected by 1, 4:
- Decreased creatinine formation in patients with muscle wasting 1
- Increased tubular secretion of creatinine 1
- Volume expansion diluting serum creatinine 1
- Interference with creatinine assays by elevated bilirubin 1, 4
Diagnostic Approach
Do not rely solely on serum creatinine without considering urine output criteria, as this may miss cases of AKI 1. Conversely, recognize when urine output criteria should be de-emphasized (e.g., cirrhotic patients, those on diuretics) 4.
Practical Application
The definition allows for real-time detection of AKI based on initial changes in markers, enabling prompt intervention 4. Staging is performed retrospectively when the episode is complete, classifying according to the most severe criterion met during the entire episode 4.