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/h for 6 consecutive hours. 1, 2, 3

Core Diagnostic Criteria

The diagnosis requires meeting only a single criterion from the following three options 2:

• 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/h for ≥6 consecutive hours 1, 2, 3

The ≥0.3 mg/dL threshold was specifically chosen because even this small increase is independently associated with a fourfold increase in hospital mortality, making early detection critical 1, 2, 3.

AKI Staging System

AKI severity is classified into three stages based on the most severe criterion met (either creatinine or urine output), with higher stages strongly correlating with increased mortality 2, 3:

Stage 1

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

Stage 2

• Creatinine: 2.0-2.9 times baseline 2, 3
• Urine output: <0.5 mL/kg/h 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/h for ≥24 hours OR anuria for ≥12 hours 2, 3

Any patient receiving acute renal replacement therapy is automatically classified as Stage 3 AKI 3.

Temporal Distinctions in the Kidney Injury Continuum

AKI exists within a broader spectrum of kidney disease defined by duration 1, 2:

• AKI: 0-7 days - Abrupt kidney function decrease 1, 2
• Acute Kidney Disease (AKD): 7-90 days - Kidney damage/dysfunction persisting after the initiating event, which can occur with or without preceding AKI 1, 2
• Chronic Kidney Disease (CKD): >90 days - Persistent kidney disease 1, 2

AKD not associated with AKI is nearly 3 times more prevalent than AKI itself and carries significant long-term risk (adjusted hazard ratio of 2.26 for incident CKD, kidney failure, or death) 1.

Critical Pitfalls in Applying the Definition

Baseline Creatinine Determination

• Use known creatinine values rather than imputation methods - back-calculation from an estimated GFR of 75 mL/min/1.73 m² may overestimate AKI incidence in populations with high CKD prevalence 1, 2
• If no known baseline exists, use the lowest creatinine value available during hospitalization 2
• Failure to establish an accurate baseline creatinine leads to misclassification 1

Serum Creatinine Limitations

Serum creatinine has inherent limitations that can cause misclassification 1, 2:

• Decreased creatinine formation in patients with muscle wasting 1, 2
• Volume expansion diluting serum creatinine 1, 2
• Interference with creatinine assays by elevated bilirubin 1, 2
• Increased tubular secretion of creatinine in chronic kidney disease 2

Urine Output Criteria Limitations

• Urine output criteria are unreliable in cirrhotic patients with ascites - these patients are frequently oliguric with avid sodium retention yet may maintain relatively normal glomerular filtration rate 1, 2
• Urine output criteria are also unreliable in patients on diuretic therapy 2
• However, relying solely on serum creatinine without considering urine output may miss cases of AKI 1, 2

Detection Timing

• Detection must occur in real-time based on initial marker changes rather than waiting for retrospective confirmation 2, 3
• The staging system is applied retrospectively when the episode is complete, classifying according to the most severe criterion met 2
• AKI often presents without specific symptoms, making systematic laboratory monitoring essential in at-risk patients 2