What are the diagnostic criteria for acute kidney injury?

Diagnostic Criteria for Acute Kidney Injury

Acute kidney injury is diagnosed using the KDIGO criteria: an increase in serum creatinine ≥0.3 mg/dL within 48 hours, OR an 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 classification system provides the current standard for AKI diagnosis and staging, combining both creatinine and urine output parameters 1, 2:

Serum Creatinine Criteria

• Any increase of ≥0.3 mg/dL (26.5 μmol/L) within 48 hours 1
• OR increase to ≥1.5 times baseline (presumed to have occurred within prior 7 days) 1

Urine Output Criteria

• Urine volume <0.5 mL/kg/h for 6 consecutive hours 1

Critical point: You need to meet only ONE criterion (either creatinine OR urine output) to diagnose AKI. 1

Staging System

Once AKI is diagnosed, severity is classified into three stages based on the worst criterion met 1:

Stage 1

• Creatinine increase 1.5-1.9 times baseline OR increase ≥0.3 mg/dL 1
• Urine output <0.5 mL/kg/h for 6-12 hours 1

Stage 2

• Creatinine increase 2.0-2.9 times baseline 1
• Urine output <0.5 mL/kg/h for ≥12 hours 1

Stage 3

• Creatinine increase ≥3.0 times baseline OR increase to ≥4.0 mg/dL (with acute rise of ≥0.3 mg/dL) OR initiation of renal replacement therapy 1
• Urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1

The progression through stages strongly correlates with mortality—even small creatinine increases of 0.3 mg/dL independently increase hospital mortality approximately four-fold. 1

Special Populations: Cirrhosis with Ascites

In patients with cirrhosis and ascites, focus exclusively on serum creatinine changes and disregard urine output criteria. 1 This population presents unique diagnostic challenges:

• These patients are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR 3, 1
• Diuretic therapy further confounds urine output interpretation 3, 1
• A creatinine threshold of ≥1.5 mg/dL predicts AKI progression and worse prognosis in cirrhotic patients 3, 1

For cirrhotic patients with Stage 1 AKI, those whose peak creatinine exceeds 1.5 mg/dL (stage 1-B) have significantly higher short-term mortality compared to those remaining below 1.5 mg/dL (stage 1-A) 3

Critical Diagnostic Pitfalls

Limitations of Serum Creatinine

Serum creatinine significantly overestimates actual kidney function in several populations 1:

• Patients with muscle wasting (decreased creatinine production from muscles) 1
• Increased tubular secretion of creatinine 1
• Volume expansion diluting creatinine concentration 1
• Hyperbilirubinemia interfering with colorimetric assays 1

Do not wait for creatinine to reach 1.5 mg/dL before diagnosing AKI—this outdated threshold indicates GFR has already fallen to approximately 30 mL/min. 1 Instead, monitor temporal changes at 48-hour intervals to detect the 0.3 mg/dL threshold 1

Limitations of Urine Output

Beyond cirrhotic patients, urine output criteria are unreliable in 1, 4:

• Patients receiving diuretics (artificially increases urine output) 4
• Patients with pre-existing oliguria from other causes 1

The Fractional Excretion of Sodium (FENa) becomes unreliable after diuretic administration and should not be used in these patients. 4

Practical Diagnostic Approach

Initial Evaluation

1. Obtain baseline creatinine (if unknown, assume occurred within prior 7 days) 1
2. Measure current creatinine and calculate change 1
3. Assess urine output over 6-hour periods (unless patient has cirrhosis/ascites or is on diuretics) 1
4. Perform urinalysis with microscopy to help differentiate etiology 1:
   • Hematuria (>50 RBCs/hpf) suggests glomerular disease 1
   • Proteinuria (>500 mg/day) suggests glomerular disease 1
   • Renal tubular epithelial cell casts suggest acute tubular necrosis 1

Monitoring Strategy

• Serial creatinine measurements every 48 hours for detecting the 0.3 mg/dL threshold 1, 4
• Continuous urine output monitoring in 6-hour blocks (matches nursing shifts and is equivalent to hourly monitoring) 1
• Renal ultrasound to evaluate kidney size and rule out obstruction 1

When to Diagnose

Diagnose AKI immediately when any single criterion is met—do not wait for multiple criteria or higher thresholds. 1 Early detection based on initial marker changes is critical, as even Stage 1 AKI significantly increases mortality risk 1