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, diagnosed when ANY ONE of the following three criteria is met: serum creatinine rise ≥0.3 mg/dL (26 μmol/L) within 48 hours, OR serum creatinine increase ≥50% (1.5 times baseline) within 7 days, OR urine output <0.5 mL/kg/hour for 6 consecutive hours. 1, 2, 3
Core Diagnostic Criteria
The diagnosis requires meeting only a single criterion from the following three options:
Serum creatinine criterion #1: Absolute increase ≥0.3 mg/dL (26 μmol/L) within any 48-hour period 1, 2, 3
Serum creatinine criterion #2: Relative increase to ≥1.5 times baseline (≥50% rise) occurring within 7 days 1, 2, 3
Urine output criterion: Reduction to <0.5 mL/kg/hour for 6 or more consecutive hours 1, 2, 3
Clinical Significance of the Definition
The ≥0.3 mg/dL threshold was specifically chosen because even this small increase independently predicts a fourfold increase in hospital mortality, making early detection critical for patient outcomes 1, 2, 3. This sensitive threshold allows intervention before severe kidney dysfunction develops 2.
AKI Staging System
Stage 1 (least severe):
- Creatinine: 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 2, 3
- Urine output: <0.5 mL/kg/hour for 6-12 hours 2, 3
Stage 2 (moderate):
Stage 3 (most severe):
- Creatinine: ≥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 2, 3
- Urine output: <0.3 mL/kg/hour for ≥24 hours OR anuria for ≥12 hours 2, 3
The staging is determined by whichever criterion (creatinine or urine output) indicates the most severe stage, and higher stages directly correlate with increased mortality risk 2, 3.
Temporal Distinctions in the Kidney Injury Continuum
AKI exists within a broader spectrum of kidney dysfunction defined by duration:
- AKI: Abrupt kidney function decrease occurring over ≤7 days 2
- Rapid reversal of AKI: Complete reversal within 48 hours of onset 2
- Persistent AKI: Continuation beyond 48 hours from onset 2
- Acute Kidney Disease (AKD): Kidney damage/dysfunction persisting 7-90 days after the initiating event 1, 2, 3
- Chronic Kidney Disease (CKD): Kidney disease persisting >90 days 1, 2, 3
Critical Pitfalls in Applying the Definition
Baseline creatinine determination: Use known creatinine values rather than imputation methods, as back-calculation from an estimated GFR of 75 mL/min/1.73 m² overestimates AKI incidence in populations with high CKD prevalence 1, 2. If no baseline is available, use the lowest creatinine value during hospitalization 2.
Serum creatinine limitations: Creatinine is affected by muscle wasting (decreased production), volume expansion (dilutional effect), hyperbilirubinemia (assay interference), and increased tubular secretion in CKD, all of which can lead to misclassification 1, 2.
Percentage changes vary by baseline kidney function: After a 90% reduction in creatinine clearance, the percentage rise in creatinine at 24 hours is 246% with normal baseline function but only 47% in stage 4 CKD, while absolute increases remain nearly identical (1.8-2.0 mg/dL) across all baseline kidney function levels 4. This is why the definition includes both absolute (0.3 mg/dL) and relative (50%) criteria 2.
Urine output criteria limitations: Do not rely on urine output criteria in cirrhotic patients with ascites (who are frequently oliguric despite normal GFR due to avid sodium retention) or patients on diuretic therapy 1, 2. However, urine output criteria can identify AKI cases missed by creatinine alone in other populations 2.
Timing of detection: Detection must occur in real-time based on initial marker changes rather than waiting for retrospective confirmation, though staging is applied retrospectively when the episode is complete 2, 3.