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 ≥0.3 mg/dL (≥26 μmol/L) within 48 hours, or a 50% or greater increase in serum creatinine from baseline within 7 days, or a reduction in urine output to <0.5 mL/kg/hour for more than 6 hours. 1
AKI Staging Criteria
AKI is staged according to severity using the following KDIGO criteria:
| Stage | Serum Creatinine | Urine Output |
|---|---|---|
| 1 | Rise of >26 μmol/L (0.3 mg/dL) within 48h OR 50-99% rise from baseline within 7 days | <0.5 mL/kg/h for >6h |
| 2 | 100-199% rise from baseline within 7 days | <0.5 mL/kg/h for >12h |
| 3 | ≥200% rise from baseline within 7 days OR Creatinine ≥354 μmol/L with rise of >26 μmol/L OR Any requirement for renal replacement therapy | <0.3 mL/kg/h for 24h OR anuria for 12h |
Evolution of the AKI Definition
The current KDIGO definition represents an evolution of earlier classification systems:
RIFLE Criteria (2004): Risk, Injury, Failure, Loss, and End-stage renal disease - defined AKI as a rise in creatinine of ≥50% from baseline and/or a fall in GFR by ≥25% 1
AKIN Criteria (2007): Modified RIFLE by including a ≥0.3 mg/dL rise in creatinine within 48 hours as Stage 1 AKI and removed GFR criteria 1
KDIGO (2012): Merged RIFLE and AKIN criteria into the current definition 1
Clinical Significance of AKI Definition
The KDIGO definition has important clinical implications:
Even small increases in serum creatinine (≥0.3 mg/dL) are associated with a fourfold increase in hospital mortality 1
The definition allows for early detection and intervention, which is critical as AKI can lead to:
- Increased mortality
- Progression to chronic kidney disease (CKD)
- Cardiovascular morbidity 2
Important Considerations in AKI Assessment
Baseline Kidney Function Impact
- The percentage change in serum creatinine after AKI is highly dependent on baseline kidney function 3
- With identical injury severity:
- Normal baseline function: 246% rise in creatinine at 24h
- Stage 4 CKD: only 47% rise at 24h
- However, absolute increase remains similar (1.8-2.0 mg/dL) 3
Time Course Considerations
- Time to reach diagnostic thresholds varies by baseline function:
Relationship to Acute Kidney Disease (AKD)
AKI is part of a spectrum that includes Acute Kidney Disease (AKD):
- AKD refers to kidney damage present for <3 months
- AKI is a subset of AKD where the disease process occurs within 7 days 1
- AKD can be staged congruently with AKI stages (1-3) 1
Common Pitfalls in AKI Diagnosis
Failure to establish accurate baseline: Without knowledge of baseline creatinine, AKI may be missed or misclassified
Overlooking urine output criteria: Focusing solely on creatinine changes may delay diagnosis
Misinterpreting creatinine changes in CKD: Percentage-based criteria may underestimate injury severity in CKD patients
Delayed recognition: Creatinine rises lag behind actual kidney injury, potentially delaying intervention
Confounding factors: Muscle mass changes, volume status, and medications can affect creatinine levels independent of kidney function
Clinical Application
The KDIGO definition should be applied in clinical practice to:
- Enable early recognition and intervention
- Guide staging and management decisions
- Standardize communication among healthcare providers
- Facilitate research and quality improvement initiatives
By using this standardized definition, clinicians can more effectively identify, stage, and manage AKI, potentially improving outcomes for patients with this serious condition.