What is the definition of Acute Kidney Injury (AKI)?

Definition of Acute Kidney Injury (AKI)

Acute Kidney Injury (AKI) is defined by 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 using the following criteria:

• Serum creatinine criteria:

  • Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L) within 48 hours, OR
  • Increase in serum creatinine to ≥1.5 times baseline within the prior 7 days 1, 2

• Urine output criteria:

  • Urine volume <0.5 mL/kg/h for 6 hours or more 1

AKI Staging System

AKI is staged according to severity:

• Stage 1:

  • Serum creatinine increase of >26 μmol/L (0.3 mg/dL) within 48 hours or 50-99% increase from baseline within 7 days, OR
  • Urine output <0.5 mL/kg/h for 6-12 hours 1

• Stage 2:

  • Serum creatinine increase of 100-199% from baseline within 7 days, OR
  • Urine output <0.5 mL/kg/h for ≥12 hours 1

• Stage 3:

  • Serum creatinine increase of ≥200% from baseline within 7 days, OR
  • Serum creatinine ≥354 μmol/L (4.0 mg/dL) with either rise of >26 μmol/L within 48 hours or >50% increase from baseline within 7 days, OR
  • Initiation of renal replacement therapy, OR
  • Urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours 1

Clinical Significance of AKI

• Even small increases in serum creatinine (>0.3 mg/dL) are independently associated with approximately a fourfold increase in hospital mortality 1
• AKI is part of a continuum that can progress to acute kidney disease (AKD) and chronic kidney disease (CKD) 1, 3
• AKI staging correlates with mortality risk and other adverse clinical outcomes 1

AKI in Relation to Acute Kidney Disease (AKD)

• AKI is a subset of AKD, which is defined as kidney abnormalities or decreased GFR for less than 3 months 2
• AKD can occur with or without AKI, consistent with the 2012 AKI guideline 2
• The term "acute kidney disease and disorders" (AKD) describes abnormalities of kidney function and/or structure with implications for health that persist beyond AKI 2
• AKD represents the course of disease after AKI among patients in whom renal pathophysiologic processes are ongoing 2

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 of AKI 1
• Not recognizing that AKI can occur in both hospital and community settings, with community-acquired AKI often going undetected 2
• Overlooking the multifactorial nature of AKI, which can have prerenal, intrarenal, and postrenal causes 4

Clinical Implications

• Early identification using standardized criteria allows for prompt intervention and potentially improved outcomes 1
• AKI increases the risk of developing CKD and cardiovascular morbidity 3, 5
• AKI is associated with increased length of hospital stay, healthcare costs, and in-hospital mortality 6
• Prevention and early detection are essential given the poor prognosis of AKI in critically ill patients 3