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, characterized by any one of the following: a rise in serum creatinine of ≥0.3 mg/dL (26 μmol/L) within 48 hours, OR a ≥50% increase in serum creatinine from baseline within 7 days, OR urine output <0.5 mL/kg/h for ≥6 hours. 1
Diagnostic Criteria Components
The KDIGO criteria require meeting any one of three distinct parameters 1:
Serum creatinine increase of ≥0.3 mg/dL within 48 hours - This small increment is clinically significant because it independently associates with approximately a fourfold increase in hospital mortality 1
Serum creatinine increase of ≥50% from baseline within 7 days - This represents a 1.5-fold rise and captures more gradual deterioration 1
Urine output <0.5 mL/kg/h for ≥6 consecutive hours - This oliguria criterion can identify AKI before creatinine rises, though it has important limitations in specific populations 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 of ≥0.3 mg/dL within 48 hours OR 1.5-1.9 times baseline within 7 days OR urine output <0.5 mL/kg/h for 6-12 hours 1
Stage 2: Creatinine 2.0-2.9 times baseline OR urine output <0.5 mL/kg/h for ≥12 hours 1
Stage 3: Creatinine ≥3.0 times baseline OR increase to ≥4.0 mg/dL (354 μmol/L) OR urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours OR initiation of renal replacement therapy 1
The staging system directly correlates with mortality risk and other adverse clinical outcomes 1
Relationship to Acute Kidney Disease (AKD)
AKI represents the acute phase (≤7 days) of kidney injury, while Acute Kidney Disease (AKD) encompasses kidney abnormalities or decreased GFR lasting 7-90 days after the initiating event 1
AKI is a subset of AKD - AKD can occur with or without preceding AKI and represents ongoing renal pathophysiologic processes 1
AKD persisting beyond 90 days transitions to chronic kidney disease (CKD) 1, 2
Critical Diagnostic Pitfalls to Avoid
Do not rely solely on serum creatinine without assessing urine output - Using only one criterion misses cases identifiable by the other 1
Establish an accurate baseline creatinine - Using imputed values (back-calculating from assumed GFR of 75 mL/min/1.73 m²) overestimates AKI incidence, particularly in populations with high CKD prevalence; known creatinine values are superior 1
Recognize serum creatinine limitations - Creatinine is affected by muscle mass (decreased formation in sarcopenia), volume expansion (dilutional effect), increased tubular secretion, and assay interference from elevated bilirubin 1
Urine output criteria are unreliable in cirrhotic patients with ascites - These patients are frequently oliguric with avid sodium retention yet may maintain relatively normal GFR 1
AKI occurs in both hospital and community settings - Community-acquired AKI is often undetected, so maintain vigilance outside the hospital 1