What are the diagnostic criteria for acute kidney injury?

Diagnostic Criteria for Acute Kidney Injury

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 falls below 0.5 mL/kg/h for 6 consecutive hours. 1

Core Diagnostic Criteria (KDIGO)

The diagnosis relies on detecting acute changes in kidney function through two primary markers:

Serum Creatinine Criteria

• An absolute increase of ≥0.3 mg/dL occurring within any 48-hour period indicates AKI 1, 2
• A relative increase to ≥1.5 times the baseline value (50% increase) within the preceding 7 days confirms AKI 1, 3
• Even small creatinine elevations of ≥0.3 mg/dL independently associate with approximately four-fold increased hospital mortality 1

Urine Output Criteria

• Urine volume <0.5 mL/kg/h sustained for 6 or more consecutive hours meets diagnostic threshold 1, 4
• This criterion requires accurate body weight measurement and careful urine collection 2

Staging System

Once AKI is diagnosed, severity stratification guides prognosis and management:

Stage 1 (Mild)

• Creatinine: 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 (Moderate)

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

Stage 3 (Severe)

• Creatinine: ≥3.0 times baseline OR ≥4.0 mg/dL with acute rise >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
• Progression through stages strongly correlates with mortality risk 1

Critical Diagnostic Nuances

When Either Criterion Suffices

• AKI is diagnosed when either the creatinine criterion or the urine output criterion is met—you do not need both 1
• However, patients fulfilling both criteria have significantly higher mortality (HR 3.56) compared to those meeting only one criterion 5

Special Population Considerations

In cirrhotic patients with ascites:

• Urine output criteria are unreliable and should not be used 6, 1
• These patients are frequently oliguric with avid sodium retention yet maintain relatively normal GFR 6
• Diuretic therapy further confounds urine output interpretation 6
• Focus exclusively on serum creatinine changes in this population 1
• A threshold of creatinine ≥1.5 mg/dL predicts AKI progression and worse prognosis specifically in cirrhosis 1

Limitations of Serum Creatinine

• Creatinine overestimates actual kidney function in patients with muscle wasting, increased tubular creatinine secretion, volume expansion from ascites, or hyperbilirubinemia 1
• Hyperbilirubinemia interferes with colorimetric creatinine assays, producing falsely elevated values 1
• Despite these limitations, creatinine remains the primary diagnostic marker 3

Practical Diagnostic Approach

Establish Baseline

• Identify the most recent baseline creatinine value within the past 7 days 2
• If no recent baseline exists, presume the baseline from the lowest value available or estimate from clinical context 3

Monitor Temporal Changes

• Check creatinine at 48-hour intervals to detect the 0.3 mg/dL threshold 1
• Review 7-day trends to identify 50% increases from baseline 1
• Staging is performed retrospectively when the episode is complete, classifying by the most severe criterion met 1

Supporting Diagnostic Tests

• Urinalysis with microscopy helps differentiate etiology: hematuria (>50 RBCs/hpf) suggests glomerular disease; renal tubular epithelial cell casts indicate acute tubular necrosis 1
• Renal ultrasound should be performed in most patients, particularly older men, to exclude obstruction 4, 7
• Fractional excretion of sodium helps distinguish prerenal from intrinsic renal causes 4

Common Pitfalls to Avoid

• Do not wait for creatinine to reach 1.5 mg/dL before diagnosing AKI—this outdated threshold often indicates GFR has already fallen to ~30 mL/min 6
• Do not rely on urine output alone in patients receiving diuretics or those with cirrhosis and ascites 6, 1
• Do not dismiss small creatinine increases—even a 0.3 mg/dL rise carries significant mortality risk 1
• Do not use fixed creatinine thresholds without considering dynamic changes from baseline 6