Definition of Acute Kidney Injury (AKI)
Acute Kidney Injury (AKI) is defined according to KDIGO criteria as an increase in serum creatinine by 0.3 mg/dL (26.5 μmol/L) within 48 hours, or an increase to 1.5 times baseline within 7 days, or urine output less than 0.5 mL/kg/h for 6 hours or more. 1
Diagnostic Criteria and Staging
AKI is diagnosed when any of the following criteria are met:
Serum Creatinine Changes:
- Increase by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours
- Increase to ≥1.5 times baseline within the prior 7 days (where baseline is known or presumed)
Urine Output:
- <0.5 mL/kg/h for 6 hours or more
AKI Staging System
AKI is classified into three stages based on severity 1:
| 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 rise of >26 μmol/L OR Any requirement for renal replacement therapy | <0.3 mL/kg/h for 24h OR Anuria for 12h |
Clinical Significance
AKI is a common and potentially devastating syndrome that affects up to 20% of hospitalized patients and 30-60% of critically ill patients 1. It is associated with:
- Increased mortality (even small rises in creatinine are independently associated with approximately fourfold increase in hospital mortality) 1
- Increased morbidity
- Higher healthcare costs
- Risk of progression to chronic kidney disease
Etiological Classification
AKI is typically categorized into three main types 1, 2:
Prerenal AKI (approximately 55-60% of cases):
- Caused by impaired renal perfusion
- Examples: hypotension, hypovolemia, decreased cardiac output, renal artery occlusion
Intrinsic/Renal AKI (approximately 35-40% of cases):
- Direct damage to renal parenchyma
- Examples: acute tubular necrosis, glomerulonephritis, interstitial nephritis, vasculitis, nephrotoxins
Postrenal AKI (approximately 3-5% of cases):
- Caused by obstruction to urine outflow
- Examples: ureteral, bladder, or urethral obstruction
Important Distinctions from Related Conditions
Acute Kidney Disease (AKD): Acute or subacute damage and/or loss of kidney function occurring for a duration between 7 days and 90 days after an AKI initiating event 1
Chronic Kidney Disease (CKD): Abnormalities in kidney structure or function that persist for >90 days 1
Clinical Implications of AKI Definition
Baseline Kidney Function Impact: The percentage change in serum creatinine after AKI is highly dependent on baseline kidney function. With identical injury, patients with normal baseline function show much larger percentage increases in creatinine compared to those with pre-existing CKD 3.
Time Course Considerations: The time to reach diagnostic thresholds varies by baseline function - patients with normal baseline function reach percentage-based thresholds faster than those with CKD 3.
Absolute vs. Percentage Changes: While percentage changes in creatinine vary widely based on baseline function, absolute increases (e.g., 0.3 mg/dL) occur at similar rates across different baseline kidney functions after moderate-to-severe injury 3.
Pitfalls in AKI Diagnosis
- Delayed Recognition: Relying solely on serum creatinine may delay diagnosis, as it typically rises 24-48 hours after injury
- CKD Impact: In patients with CKD, percentage-based criteria may underestimate AKI severity
- Fluid Overload: Dilution of serum creatinine in fluid-overloaded patients may mask AKI
- Muscle Mass Variations: Creatinine production varies with muscle mass, potentially affecting interpretation in elderly, malnourished, or critically ill patients
The KDIGO definition represents the current consensus standard for AKI diagnosis and staging, providing a framework for early recognition, appropriate management, and research standardization 1, 4.