Acute Kidney Injury (AKI) Definition
Acute Kidney Injury is defined by the KDIGO criteria as an abrupt decrease in kidney function occurring within 7 days or less, diagnosed when any of the following occur: serum creatinine rises ≥0.3 mg/dL (≥26 μmol/L) within 48 hours, OR serum creatinine increases ≥50% (1.5 times baseline) within 7 days, OR urine output falls below 0.5 mL/kg/h for 6 consecutive hours. 1, 2, 3
Core Diagnostic Criteria
The diagnosis requires meeting any one of three criteria 2, 3:
- Creatinine rise ≥0.3 mg/dL (≥26 μmol/L) within 48 hours - This small increment is clinically significant, independently associated with approximately fourfold increased hospital mortality 1, 2
- Creatinine increase ≥50% from baseline within 7 days - The rise must be known or presumed to have occurred within this timeframe 1
- Urine output <0.5 mL/kg/h for ≥6 hours - This criterion applies broadly but has important limitations in specific populations 1, 3
AKI Staging System
Stage 1: Creatinine 1.5-1.9 times baseline OR increase ≥0.3 mg/dL, OR urine output <0.5 mL/kg/h for 6-12 hours 1, 3
Stage 2: Creatinine 2.0-2.9 times baseline, OR urine output <0.5 mL/kg/h for ≥12 hours 1, 3
Stage 3: Creatinine ≥3.0 times baseline OR creatinine ≥354 μmol/l (≥4.0 mg/dL) with acute rise >0.3 mg/dL or >50% from baseline, OR initiation of renal replacement therapy, OR urine output <0.3 mL/kg/h for ≥24 hours or anuria for ≥12 hours 1, 3
The staging system directly correlates with mortality risk—higher stages predict worse outcomes 3. Patients receiving renal replacement therapy are automatically classified as Stage 3 3.
Application in Special Populations
Patients with Pre-existing CKD
In patients with chronic kidney disease, the KDIGO criteria apply identically, but interpretation requires careful attention to baseline creatinine 1. A rise to ≥354 μmol/l (≥4.0 mg/dL) in a CKD patient, when accompanied by acute rise >0.3 mg/dL or >50% within specified timeframes, classifies as Stage 3 AKI, whereas the same absolute rise in a patient without CKD may only be Stage 1 1.
Older Adults, Diabetes, and Hypertension
The diagnostic criteria remain unchanged regardless of age, diabetes, or hypertension status 1, 2. However, establishing accurate baseline creatinine is critical—using known creatinine values is superior to imputation methods, as back-calculation from an estimated GFR of 75 mL/min/1.73 m² may overestimate AKI incidence in populations with high CKD prevalence 2.
Critical Pitfalls to Avoid
Baseline creatinine errors: Failure to establish accurate baseline creatinine leads to misclassification 2. Always use the most recent known value rather than imputed estimates when available.
Ignoring urine output criteria: Relying solely on serum creatinine without considering urine output misses cases of AKI 2. However, urine output criteria are unreliable in patients with cirrhosis and ascites, who are frequently oliguric with avid sodium retention yet may maintain relatively normal GFR 2.
Serum creatinine limitations: Creatinine is affected by decreased muscle mass (muscle wasting), increased tubular secretion, volume expansion causing dilution, and interference from elevated bilirubin 2. These factors can mask true kidney injury severity.
Community-acquired AKI: AKI occurring outside hospital settings often goes undetected because it doesn't meet dramatic criteria, yet carries significant long-term health implications 2.
Relationship to Acute Kidney Disease (AKD)
AKI exists within a continuum of kidney injury 2, 3. Acute Kidney Disease (AKD) is defined as kidney abnormalities or damage lasting 7-90 days after an AKI initiating event 2. AKI represents the acute phase (≤7 days), while AKD captures the subacute recovery or progression phase 2.
Importantly, AKD can occur without preceding AKI—patients whose creatinine rises slowly over 2 weeks (not meeting the 48-hour or 7-day AKI criteria) still have AKD and require intervention 2. AKD not associated with AKI is nearly 3 times more prevalent than AKI itself, with patients having an adjusted hazard ratio of 2.26 for incident CKD, kidney failure, or death 2.
After 90 days, persistent kidney dysfunction transitions to chronic kidney disease 2, 3.