What are the criteria for acute kidney injury?

Acute Kidney Injury Diagnostic Criteria

Acute kidney injury is diagnosed when serum creatinine increases by ≥0.3 mg/dL within 48 hours, OR increases to ≥1.5 times baseline within 7 days, OR urine output decreases to <0.5 mL/kg/h for 6 consecutive hours. 1, 2

Core Diagnostic Criteria (KDIGO)

AKI is defined by meeting any one of the following criteria 1, 3:

• Serum creatinine increase ≥0.3 mg/dL within 48 hours, OR
• Serum creatinine increase ≥1.5 times (≥50%) from baseline within 7 days, OR
• Urine output <0.5 mL/kg/h for ≥6 hours

The diagnosis requires only one criterion to be met—you do not need both creatinine and urine output changes 1.

Staging System

Stage 1 1, 2:

• Serum 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

Stage 2 1, 2:

• Serum creatinine 2.0-2.9 times baseline, OR
• Urine output <0.5 mL/kg/h for ≥12 hours

Stage 3 1, 2:

• Serum creatinine ≥3.0 times baseline, OR
• Serum creatinine ≥4.0 mg/dL with acute increase of ≥0.3 mg/dL, OR
• Initiation of renal replacement therapy, OR
• Urine output <0.3 mL/kg/h for ≥24 hours, OR
• Anuria for ≥12 hours

Progression through stages strongly correlates with increased mortality—even small increases in creatinine (≥0.3 mg/dL) independently increase hospital mortality approximately four-fold 1, 4.

Critical Clinical Caveats

Limitations of Serum Creatinine

Serum creatinine significantly overestimates actual kidney function in certain populations 5, 2:

• Muscle wasting decreases creatinine formation from creatine 5, 2
• Increased tubular secretion of creatinine occurs in kidney disease 5
• Volume expansion (especially with ascites) dilutes serum creatinine 5
• Hyperbilirubinemia interferes with colorimetric creatinine assays 5

Special Population: Cirrhosis

In patients with cirrhosis and ascites, urine output criteria are unreliable because these patients are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR 5. Diuretic treatment further confounds urine output interpretation 5.

For cirrhotic patients, focus primarily on serum creatinine changes rather than urine output 5. However, note that a serum creatinine threshold of ≥1.5 mg/dL (133 µmol/L) predicts AKI progression and worse prognosis in this population 5.

Initial Workup

When AKI is identified, immediately obtain 2, 6:

• Serial serum creatinine measurements to track progression and confirm staging 2
• Urinalysis with microscopy to narrow differential diagnosis:
  • RBC casts suggest glomerulonephritis 2
  • WBC casts suggest interstitial nephritis or pyelonephritis 2
  • Muddy brown casts suggest acute tubular necrosis 2
• Renal ultrasound to rule out obstruction (especially in older men) and assess kidney size 2, 6
• Complete blood count and fractional excretion of sodium 6

Prognostic Implications

Stage classification should be based on the most severe criterion met during the episode 1. Patients meeting both oliguria and azotemia criteria have worse disease severity and outcomes than those meeting only one criterion 7. The KDIGO criteria effectively predict hospital mortality, need for renal replacement therapy, and prolonged hospital stay 4.