Definition of Acute Kidney Injury (AKI)
Acute Kidney Injury (AKI) is defined according to the Kidney Disease Improving Global Outcomes (KDIGO) criteria as an abrupt decrease in kidney function occurring over 7 days or less, characterized by either 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 6 hours or more. 1
Diagnostic Criteria for AKI
AKI is diagnosed and staged based on two key parameters:
Serum Creatinine Criteria:
- Stage 1: Rise of >26 μmol/L (0.3 mg/dL) within 48 hours OR 50-99% increase from baseline within 7 days 1
- Stage 2: 100-199% increase from baseline within 7 days 1
- Stage 3: ≥200% increase from baseline within 7 days OR serum creatinine ≥354 μmol/L with either rise of >26 μmol/L within 48 hours or >50% increase from baseline within 7 days OR any requirement for renal replacement therapy 1
Urine Output Criteria:
- Stage 1: <0.5 mL/kg/h for more than 6 hours 1
- Stage 2: <0.5 mL/kg/h for more than 12 hours 1
- Stage 3: <0.3 mL/kg/h for 24 hours or anuria for 12 hours 1
Evolution of AKI Definition
The current definition evolved through several iterations:
- RIFLE Classification (2004): Risk, Injury, Failure, Loss, End-stage renal disease - first standardized definition that recognized the importance of small changes in kidney function 1
- AKIN Criteria (2007): Modified RIFLE by adding the criterion of ≥0.3 mg/dL rise in creatinine within 48 hours and shortening the timeframe from 7 days to 48 hours 1
- KDIGO Guidelines (2012): Combined elements of both RIFLE and AKIN to create the current definition 1
Clinical Significance of AKI Definition
- Small increases in serum creatinine (>0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 1
- AKI is part of a continuum that can lead to acute kidney disease (AKD) and chronic kidney disease (CKD) 1, 2
- Persistent AKI is characterized by continuance of AKI criteria beyond 48 hours from onset 1
- Complete reversal of AKI within 48 hours is considered rapid reversal and has better outcomes 1
AKI in the Spectrum of Kidney Disease
- Acute Kidney Injury (AKI): Abrupt decrease in kidney function occurring over ≤7 days 1, 2
- Acute Kidney Disease (AKD): Kidney dysfunction present for 7 days to 90 days after an AKI-initiating event 1
- Chronic Kidney Disease (CKD): Kidney dysfunction persisting for >90 days 1, 2
Clinical Implications
- Early identification of AKI using standardized criteria allows for prompt intervention and potentially improved outcomes 1
- The staging system correlates with mortality risk and other clinical outcomes 1
- AKI survivors are at increased risk of developing CKD and end-stage renal disease 3
- When persistent AKI is diagnosed, additional monitoring should be considered to evaluate hemodynamic status, kidney perfusion, and complications such as fluid overload, acidosis, and hyperkalemia 1
Common Pitfalls in AKI Diagnosis
- Relying solely on serum creatinine without considering urine output criteria may miss cases of AKI 1
- Failure to establish an accurate baseline creatinine can lead to misclassification 1
- In patients with low muscle mass (including children), small absolute changes in serum creatinine may represent significant kidney injury but might not meet standard criteria 1
- Standard equations to estimate GFR are not accurate for assessment of renal function in persistent AKI 1