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 is present: serum creatinine rise ≥0.3 mg/dL (≥26.5 µmol/L) within 48 hours, OR serum creatinine increase to ≥1.5 times baseline within 7 days, OR urine output <0.5 mL/kg/h for ≥6 consecutive hours. 1

Core Diagnostic Criteria

The KDIGO definition uses three independent criteria—meeting any single criterion is sufficient for diagnosis: 1, 2

• Absolute creatinine criterion: Rise of ≥0.3 mg/dL (≥26.5 µmol/L) within any 48-hour window 1, 2
• Relative creatinine criterion: Increase to ≥1.5× baseline (≥50% rise) within 7 days 1, 2
• Urine output criterion: <0.5 mL/kg/h sustained for ≥6 consecutive hours 1, 2

KDIGO Staging System

AKI severity is staged retrospectively based on the most severe criterion met during the episode: 1, 2

Stage	Creatinine Criterion	Urine Output Criterion
Stage 1	1.5–1.9× baseline OR ≥0.3 mg/dL rise	<0.5 mL/kg/h for 6–12 hours
Stage 2	2.0–2.9× baseline	<0.5 mL/kg/h for ≥12 hours
Stage 3	≥3.0× baseline OR ≥4.0 mg/dL with acute rise ≥0.3 mg/dL OR initiation of dialysis	<0.3 mL/kg/h for ≥24 hours OR anuria ≥12 hours

Clinical Significance of the 0.3 mg/dL Threshold

Even small creatinine increases of ≥0.3 mg/dL are independently associated with approximately a four-fold increase in hospital mortality, which is why this absolute threshold was incorporated into the KDIGO criteria. 1 This modest rise represents clinically important kidney injury across all baseline renal function levels. 1

The absolute 0.3 mg/dL criterion is particularly crucial in patients with advanced chronic kidney disease, where percentage-based criteria systematically under-diagnose AKI. 3 Mathematical modeling demonstrates that a 90% reduction in creatinine clearance produces only a 47% creatinine rise in stage 4 CKD compared to 246% in individuals with normal baseline function. 4 An absolute increase of 0.3 mg/dL reflects a nearly identical acute decline in GFR across all baseline kidney-function levels. 3

Establishing Baseline Creatinine

Use the most recent serum creatinine measured within the prior 3 months, selecting the value closest to hospital admission. 3 This approach is superior to imputation methods. 1

• If no prior measurement exists, the admission creatinine serves as baseline 3
• Do NOT back-calculate baseline creatinine using MDRD equations in patients with cirrhosis, as this approach is explicitly excluded from consensus recommendations 3
• When no baseline exists in other populations, back-calculation assuming a GFR of 75 mL/min/1.73 m² may overestimate AKI incidence in populations with high CKD prevalence 1

Relationship to Acute Kidney Disease (AKD)

AKI exists within a broader temporal continuum: 1

• 0–7 days of kidney dysfunction = AKI
• 7–90 days = Acute Kidney Disease (AKD) (includes persistent AKI plus subacute injury not meeting AKI thresholds)
• >90 days = Chronic Kidney Disease (CKD)

AKI is a subset of AKD; AKD can occur with or without preceding AKI. 1 Patients with AKD have markedly elevated mortality (47% vs 19% for CKD alone) and an odds ratio of 16.8 for incident CKD progression. 1

Special Population: Cirrhosis

In patients with decompensated cirrhosis, the International Club of Ascites (ICA) criteria should be applied: 5

• Use serum creatinine changes alone—do not rely on urine output criteria, as these patients are frequently oliguric with avid sodium retention yet may maintain relatively normal GFR 5, 1
• Baseline creatinine underestimates true GFR in cirrhosis due to reduced muscle mass, making the absolute 0.3 mg/dL rise particularly important 5, 3
• The traditional fixed threshold of ≥1.5 mg/dL is problematic because it often signifies GFR has already fallen to ~30 mL/min 5

Common Pitfalls

• Do not dismiss modest absolute creatinine rises in CKD patients merely because the percentage change is small—the KDIGO absolute criterion captures clinically relevant AKI across all baseline renal functions 3
• Relying solely on serum creatinine without considering urine output may miss cases of AKI in non-cirrhotic patients 1
• Failure to establish an accurate baseline creatinine leads to misclassification 1
• Massive fluid resuscitation can dilute serum creatinine, potentially masking significant GFR reduction; adjust creatinine for volume accumulation when fluid gain exceeds 5–10% of baseline weight 3
• Urine collection is often inaccurate in clinical practice and influenced by diuretic use 1