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

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 when ANY ONE of the following occurs: serum creatinine rise ≥0.3 mg/dL (26 μmol/L) within 48 hours, OR serum creatinine increase ≥50% (1.5 times baseline) within 7 days, OR urine output <0.5 mL/kg/hour for 6 consecutive hours. 1, 2, 3

Diagnostic Criteria

You need only ONE criterion to diagnose AKI:

• Serum creatinine increase ≥0.3 mg/dL within 48 hours 1, 2, 3
• Serum creatinine increase to ≥1.5 times baseline within 7 days 1, 2, 3
• Urine output <0.5 mL/kg/hour for ≥6 consecutive hours 1, 2, 3

The ≥0.3 mg/dL threshold is clinically significant because even this small increase independently predicts a fourfold increase in hospital mortality, which justifies this sensitive diagnostic threshold. 1, 2, 3

AKI Staging System

Stage AKI based on the most severe criterion met (either creatinine or urine output):

Stage 1:

• Creatinine: 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 2, 3
• Urine output: <0.5 mL/kg/hour for 6-12 hours 2, 3

Stage 2:

• Creatinine: 2.0-2.9 times baseline 2, 3
• Urine output: <0.5 mL/kg/hour for ≥12 hours 2, 3

Stage 3:

• Creatinine: ≥3.0 times baseline OR increase to ≥4.0 mg/dL (with acute rise >0.3 mg/dL or >50%) OR initiation of renal replacement therapy 2, 3
• Urine output: <0.3 mL/kg/hour for ≥24 hours OR anuria for ≥12 hours 2, 3

Higher stages directly correlate with increased mortality risk. 1, 3 Any patient receiving acute renal replacement therapy is automatically classified as Stage 3. 2

Temporal Framework: AKI vs. AKD vs. CKD

AKI exists within a continuum of kidney disease:

• AKI: 0-7 days - Abrupt kidney function decrease 1, 3
• Acute Kidney Disease (AKD): 7-90 days - Kidney damage/dysfunction persisting after the initiating event 1, 3
• Chronic Kidney Disease (CKD): >90 days - Persistent kidney disease 1, 3

AKI can be further subdivided into rapid reversal (<48 hours) versus persistent AKI (>48 hours). 3 AKD captures patients in the subacute recovery or progression phase who still have ongoing kidney dysfunction but don't meet the acute AKI criteria. 1

Critical Pitfalls to Avoid

Baseline creatinine determination:

• Use known creatinine values rather than imputation methods (back-calculating from estimated GFR of 75 mL/min/1.73 m² overestimates AKI incidence in populations with high CKD prevalence) 1, 3
• If no baseline available, use the lowest creatinine value during hospitalization 3

Serum creatinine limitations:

• Muscle wasting decreases creatinine formation, potentially masking AKI 1, 3
• Volume expansion dilutes serum creatinine 1, 3
• Elevated bilirubin interferes with creatinine assays 1, 3
• Increased tubular secretion of creatinine in CKD can underestimate severity 3

Urine output criteria limitations:

• Unreliable in cirrhotic patients with ascites (frequently oliguric with avid sodium retention despite normal GFR) 1, 3
• Unreliable in patients on diuretic therapy 3
• Despite limitations, urine output criteria can identify AKI cases missed by creatinine alone 3

Timing of detection:

• Detection must occur in real-time based on initial marker changes, not retrospectively 2, 3
• Relying solely on serum creatinine without considering urine output may miss cases 1