Acute Kidney Injury Definition
Acute Kidney Injury (AKI) is defined by the KDIGO criteria as an abrupt decrease in kidney function occurring over 7 days or less, diagnosed by any one of three criteria: serum creatinine rise >0.3 mg/dL (26 μmol/L) within 48 hours, OR serum creatinine increase ≥50% from baseline within 7 days, OR urine output <0.5 mL/kg/h for 6 hours or more. 1
Core Diagnostic Criteria
The KDIGO criteria, adopted by the American College of Physicians and other major medical societies, require only ONE of the following to diagnose AKI: 1
- Serum creatinine increase >0.3 mg/dL (26 μmol/L) within 48 hours 1
- Serum creatinine increase ≥50% from baseline within 7 days 1
- Urine output <0.5 mL/kg/h for ≥6 hours 1
Clinical Significance of the Definition
The 0.3 mg/dL threshold was specifically chosen because even this small increase is independently associated with approximately a fourfold increase in hospital mortality, making early detection critical for patient outcomes. 1
AKI exists on a continuum that can progress to acute kidney disease (AKD, lasting 7-90 days) and ultimately chronic kidney disease (CKD, >90 days), emphasizing the importance of early identification and intervention. 1
AKI Staging System
The KDIGO guidelines stratify AKI severity into three stages based on the degree of creatinine elevation and urine output: 1
- Stage 1: Creatinine rise >0.3 mg/dL within 48 hours OR 1.5-1.9× baseline within 7 days OR urine output <0.5 mL/kg/h for 6-12 hours 1
- Stage 2: Creatinine 2.0-2.9× baseline OR urine output <0.5 mL/kg/h for ≥12 hours 1
- Stage 3: Creatinine ≥3.0× baseline OR creatinine ≥4.0 mg/dL (354 μmol/L) with acute rise >0.3 mg/dL OR urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1
This staging system directly correlates with mortality risk and other adverse clinical outcomes, as validated by the American College of Cardiology and American Heart Association. 1
Critical Diagnostic Pitfalls to Avoid
Relying solely on serum creatinine without assessing urine output will miss a substantial proportion of AKI cases, as some patients meet only the urine output criteria. 1
Failure to establish an accurate baseline creatinine leads to misclassification—using known prior creatinine values is superior to imputation methods (such as back-calculating from an assumed GFR of 75 mL/min/1.73 m²), which can overestimate AKI incidence in populations with high CKD prevalence. 1
Special Population Considerations
In patients with cirrhosis and ascites, urine output criteria are unreliable because these patients are frequently oliguric with avid sodium retention yet may maintain relatively normal glomerular filtration rate. 1
Serum creatinine has inherent limitations in certain populations, being affected by decreased creatinine formation from muscle wasting, increased tubular secretion, volume expansion causing dilution, and interference with creatinine assays by elevated bilirubin. 1
Broader Context
AKI represents a sudden loss of excretory kidney function and is part of the broader spectrum of acute kidney diseases and disorders (AKD), where persistent kidney dysfunction can lead to irreversible loss of nephrons and progression to CKD. 2