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

Definition of Acute Kidney Injury (AKI)

Acute kidney injury (AKI) is defined according to the Kidney Disease Improving Global Outcomes (KDIGO) criteria as any of the following: a rise in serum creatinine of ≥26 μmol/l (0.3 mg/dl) within 48 hours, or a 50% or greater increase in serum creatinine from baseline within 7 days, or a reduction in urine output to less than 0.5 ml/kg/h for more than 6 hours. 1

Diagnostic Criteria and Staging

AKI is staged according to severity:

Stage 1

• Increase in serum creatinine by ≥26 μmol/l (0.3 mg/dl) within 48 hours, OR
• 50-99% increase in serum creatinine from baseline within 7 days, OR
• Urine output <0.5 ml/kg/h for >6 hours 1, 2

Stage 2

• 100-199% increase in serum creatinine from baseline within 7 days, OR
• Urine output <0.5 ml/kg/h for >12 hours 1, 2

Stage 3

• ≥200% increase in serum creatinine from baseline within 7 days, OR
• Serum creatinine ≥354 μmol/l (4.0 mg/dl) with either:
  • Rise of >26 μmol/l (0.3 mg/dl) within 48 hours, OR

  • 50% rise from baseline within 7 days, OR

• Any requirement for renal replacement therapy, OR
• Urine output <0.3 ml/kg/h for 24 hours or anuria for 12 hours 1, 2

Evolution of AKI Definition

The current KDIGO definition represents an evolution of previous classification systems:

1. RIFLE Classification (2004): Risk, Injury, Failure, Loss, End-stage renal disease

   • First standardized definition based on creatinine rise, GFR decrease, and urine output 1

2. AKIN Classification (2007): Modified RIFLE by:

   • Including smaller creatinine rises (≥0.3 mg/dl within 48h)
   • Removing GFR criteria
   • Dropping Loss and End-stage categories 1

3. KDIGO Classification (2012): Combined elements of RIFLE and AKIN

   • Currently the most widely accepted definition 1, 2

Clinical Significance

AKI is not merely a laboratory finding but represents a clinically significant syndrome:

• Small rises in creatinine (>0.3 mg/dl) are independently associated with approximately fourfold increase in hospital mortality 1
• AKI is part of a spectrum of acute kidney diseases and disorders (AKD) that can lead to chronic kidney disease 3
• Early detection is crucial as AKI can be lethal and may require kidney replacement therapy 3, 4

Common Pitfalls in AKI Definition and Diagnosis

1. Baseline creatinine uncertainty: When no baseline is available, the rise must be presumed to have occurred within 7 days based on clinical history 1

2. CKD vs. AKI confusion: In patients with CKD, the same absolute rise in creatinine may be classified differently than in patients without CKD 1

3. Urine output criteria underutilization: Many clinicians rely solely on creatinine criteria, missing cases that would be diagnosed by urine output criteria 1

4. Delayed recognition: Relying only on creatinine can delay diagnosis as it's a late marker of kidney injury 3, 5

5. Failure to recognize multifactorial causes: AKI often has multiple contributing factors rather than a single cause 5

The KDIGO definition has revolutionized both research and clinical practice in AKI, providing a standardized framework for diagnosis, staging, and management. However, it's important to note that the definition continues to evolve based on emerging evidence rather than consensus alone 1.