Definition of Acute Kidney Injury
Acute kidney injury (AKI) is defined by the Kidney Disease Improving Global Outcomes (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 (26.5 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
Core Diagnostic Criteria
The KDIGO definition represents the current gold standard and requires meeting any single criterion from the following 1, 2:
Serum Creatinine Criteria
- Absolute increase: Rise of ≥0.3 mg/dL (≥26.5 μmol/L) within any 48-hour period 1, 3
- Relative increase: Rise to ≥1.5 times the baseline value within the previous 7 days 1, 2
Urine Output Criteria
Clinical Significance of the Definition
Even small increases in serum creatinine (≥0.3 mg/dL) are independently associated with approximately a fourfold increase in hospital mortality, which is why this threshold was incorporated into the KDIGO criteria 1, 2. This represents a critical shift from older definitions that required higher creatinine thresholds (such as >1.5 mg/dL absolute value), which often missed clinically significant kidney injury 4.
AKI Staging System
The KDIGO criteria include a three-stage severity classification based on the most severe criterion met 1, 2:
Stage 1 (Least Severe)
- Serum creatinine: 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 3, 2
- Urine output: <0.5 mL/kg/h for 6-12 hours 3, 2
Stage 2 (Moderate)
Stage 3 (Most Severe)
- Serum creatinine: ≥3.0 times baseline OR increase to ≥4.0 mg/dL (with acute rise >0.3 mg/dL or >50% from baseline) OR initiation of renal replacement therapy 1, 3
- Urine output: <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 3, 2
Progression through these stages strongly correlates with increased mortality risk, making staging clinically important for prognosis 3, 2.
Important Caveats and Limitations
Baseline Creatinine Determination
Failure to establish an accurate baseline creatinine is a common pitfall that leads to misclassification of AKI 1. When baseline is unknown, the lowest creatinine value during hospitalization or an estimated baseline based on age, sex, and race may be used 1.
Urine Output Criteria Limitations
In certain populations, particularly 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 glomerular filtration rate 4, 3. Diuretic treatment further confounds urine output interpretation 4, 3. In cirrhotic patients, focus primarily on serum creatinine changes rather than urine output 3.
Serum Creatinine Limitations
Serum creatinine has inherent limitations as a marker of kidney function, being affected by 4, 3:
- Decreased creatinine formation from muscle in patients with muscle wasting
- Increased tubular secretion of creatinine
- Volume expansion diluting serum creatinine
- Interference with creatinine assays by elevated bilirubin (hyperbilirubinemia)
These factors cause serum creatinine to significantly overestimate actual kidney function in certain populations, including those with cirrhosis, malnutrition, or critical illness 3.
Detection in Different Settings
Not recognizing that AKI can occur in both hospital and community settings is a common pitfall, with community-acquired AKI often going undetected 1. Relying solely on serum creatinine without considering urine output criteria may also miss cases of AKI 1.
Relationship to Acute Kidney Disease
AKI is a subset of Acute Kidney Disease (AKD), which is defined as kidney abnormalities or decreased GFR persisting for less than 3 months 1. AKD represents the course of disease after AKI among patients in whom renal pathophysiologic processes are ongoing beyond the initial injury 1.