Definition of Acute Kidney Injury (AKI)
Acute kidney injury (AKI) is defined by the Kidney Disease Improving Global Outcomes (KDIGO) criteria as an increase in serum creatinine by ≥0.3 mg/dL (≥26 μmol/L) within 48 hours, or an increase in serum creatinine by ≥50% from baseline within 7 days, or a reduction in urine output to <0.5 mL/kg/h for >6 hours. 1, 2
AKI Staging System
AKI severity is classified into three stages based on serum creatinine changes and urine output:
| Stage | Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 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 either rise of >26 μmol/L or >50% rise from baseline within 7 days OR Any requirement for renal replacement therapy | <0.3 mL/kg/h for 24h or anuria for 12h |
Evolution of AKI Definition
The current KDIGO definition represents an evolution of previous classification systems:
RIFLE criteria (2004): Risk, Injury, Failure, Loss, End-stage renal disease - defined AKI as a rise in creatinine of ≥50% from baseline, a fall in GFR by ≥25%, or decreased urine output 1
AKIN criteria (2007): Modified RIFLE by including a ≥0.3 mg/dL rise in creatinine within 48 hours as stage 1 AKI and removing GFR criteria 1
KDIGO (2012): Combined elements from both RIFLE and AKIN to create the current consensus definition 1
Clinical Significance of AKI Definition
The standardized definition of AKI is crucial for several reasons:
Mortality correlation: Even small rises in creatinine (>0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 1
Early detection: The definition allows for early recognition of kidney dysfunction, which is essential for timely intervention 2
Staging correlation with outcomes: Higher AKI stages correlate with worse clinical outcomes, including mortality, need for renal replacement therapy, and progression to chronic kidney disease 1, 2
Baseline Creatinine Assessment
A key challenge in diagnosing AKI is determining the baseline creatinine level:
- The use of known creatinine values is superior to imputation methods 1
- For patients without known baseline values, back-calculation from an estimated GFR of 75 mL/min/1.73 m² may be used, though this approach may overestimate AKI in populations with high prevalence of CKD risk factors 1
Relationship to Acute Kidney Disease (AKD)
- AKI is part of a broader spectrum called Acute Kidney Disease (AKD), which describes kidney damage lasting between 7 and 90 days 1
- AKD that persists beyond 90 days is considered chronic kidney disease 1
- Recovery from AKD is defined as a reduction in peak AKI stage based on KDIGO criteria 1
Monitoring and Follow-up
Patients with AKI require:
- Daily serum creatinine and electrolyte measurements
- Strict intake and output monitoring
- Daily weight measurements
- Medication dosage adjustments based on current renal function 2
Pitfalls in AKI Diagnosis
CKD patients: The percentage change in serum creatinine after severe AKI is highly dependent on baseline kidney function - the same degree of kidney injury will cause a much smaller percentage increase in creatinine in CKD patients compared to those with normal baseline function 3
Time factor: Time to reach a 50% increase in serum creatinine varies significantly based on baseline kidney function (4 hours with normal function vs. up to 27 hours with stage 4 CKD) 3
Delayed recognition: Failure to recognize specific AKI phenotypes (e.g., hepatorenal syndrome in cirrhotic patients) can lead to inadequate treatment 2
The KDIGO definition has been extensively validated and represents the current standard for diagnosing and staging AKI in clinical practice and research settings.