What is the definition of acute kidney injury (AKI)?

Acute Kidney Injury Definition

Acute kidney injury (AKI) 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 a ≥50% increase in serum creatinine from baseline within 7 days, OR urine output <0.5 mL/kg/h for 6 consecutive hours. 1

Core Diagnostic Criteria

The diagnosis requires meeting any single criterion from the following 1, 2:

• Serum creatinine increase ≥0.3 mg/dL within 48 hours 1, 3
• Serum creatinine increase to ≥1.5 times baseline within 7 days 1, 4
• Urine output <0.5 mL/kg/h for ≥6 consecutive hours 1, 4

This definition represents a paradigm shift from the older term "acute renal failure" to capture earlier, less severe kidney injury that still carries significant prognostic implications 5.

Clinical Significance of the Definition

Even the smallest threshold (≥0.3 mg/dL creatinine rise) independently predicts approximately a fourfold increase in hospital mortality, which is why this sensitive cutoff was incorporated into the KDIGO criteria 1, 4. The definition intentionally captures a spectrum from mild to severe injury, recognizing that early detection enables intervention before irreversible damage occurs 1.

AKI Staging System

AKI severity is classified into three stages based on the most severe criterion met (either creatinine or urine output) 4:

Stage 1

• Creatinine: 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 3, 4
• Urine output: <0.5 mL/kg/h for 6-12 hours 3, 4

Stage 2

• Creatinine: 2.0-2.9 times baseline 3, 4
• Urine output: <0.5 mL/kg/h for ≥12 hours 3, 4

Stage 3

• 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 3, 4
• Urine output: <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 3, 4

Progression through higher stages strongly correlates with increased mortality 3, 4.

Temporal Distinctions in the AKI Continuum

The ADQI consensus framework distinguishes AKI from related entities based on duration 6, 1:

• AKI: Abrupt kidney function decrease occurring over ≤7 days 6, 1
• Rapid reversal of AKI: Complete reversal within 48 hours of onset 6
• Persistent AKI: Continuation of AKI beyond 48 hours from onset 6
• Acute Kidney Disease (AKD): Kidney damage/dysfunction persisting 7-90 days after the initiating event 6, 1
• Chronic Kidney Disease (CKD): Kidney disease persisting >90 days 6, 1

A minimum of 48 hours of sustained reversal is necessary to consider two AKI episodes as distinct events 6.

Critical Pitfalls in Applying the Definition

Baseline Creatinine Issues

Using known creatinine values is superior to imputation methods when establishing baseline 1. Back-calculating from an assumed GFR of 75 mL/min/1.73 m² may overestimate AKI incidence, particularly in populations with high CKD prevalence 1. Without a known baseline, use the lowest creatinine value available during the hospitalization 1.

Limitations of Creatinine Criteria

Serum creatinine has inherent limitations that can lead to misclassification 1, 3:

• Muscle wasting reduces creatinine production, underestimating kidney injury 1, 3
• Volume expansion dilutes serum creatinine, masking true severity 1, 3
• Hyperbilirubinemia interferes with colorimetric creatinine assays 1, 3
• Increased tubular secretion of creatinine in chronic kidney disease 1

The percentage change in creatinine after severe AKI varies dramatically with baseline kidney function: a 90% reduction in creatinine clearance produces a 246% creatinine rise with normal baseline function but only 47% rise in stage 4 CKD 7. Absolute creatinine changes (like the 0.3 mg/dL threshold) are more consistent across different baseline kidney function levels 7.

Urine Output Criteria Limitations

Do not rely solely on serum creatinine without considering urine output criteria, as this misses cases of AKI 1. However, urine output criteria are unreliable in specific populations 1, 3:

• Cirrhotic patients with ascites are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR 1, 3
• Diuretic therapy confounds urine output interpretation 3

In cirrhotic patients, focus primarily on serum creatinine changes rather than urine output 3.

Practical Application

Detection must occur in real-time based on initial marker changes rather than waiting for retrospective confirmation 3. The staging system is applied retrospectively when the episode is complete, classifying according to the most severe criterion met 3. AKI often presents without specific symptoms, making systematic laboratory monitoring essential in at-risk patients 4.