AKI Diagnostic Criteria
Acute kidney injury is diagnosed when any one of the following KDIGO criteria is met: serum creatinine increase ≥0.3 mg/dL 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, 2
Core Diagnostic Thresholds
The KDIGO criteria represent the harmonized standard that unified previous RIFLE and AKIN definitions 1, 3. The three diagnostic pathways are:
- Absolute creatinine rise: ≥0.3 mg/dL increase within 48 hours 1, 2
- Relative creatinine rise: ≥1.5 times (50% increase) from baseline within 7 days 1, 2
- Oliguria: Urine output <0.5 mL/kg/h for ≥6 hours 1, 2
Meeting any single criterion is sufficient for AKI diagnosis—you do not need multiple criteria 2. Notably, even the small 0.3 mg/dL creatinine increase independently associates with approximately four-fold increased hospital mortality 1, 3.
Severity Staging
AKI severity is staged retrospectively once the episode is complete, using whichever criterion (creatinine or urine output) indicates the most severe stage 2:
Stage 1:
- Creatinine 1.5-1.9 times baseline OR ≥0.3 mg/dL increase 1, 2
- Urine output <0.5 mL/kg/h for 6-12 hours 1, 2
Stage 2:
Stage 3:
- Creatinine ≥3.0 times baseline OR ≥4.0 mg/dL (with acute rise ≥0.3 mg/dL) OR initiation of renal replacement therapy 1, 2
- Urine output <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1, 2
Progression through stages strongly correlates with increased mortality 2, 3.
Critical Caveats for Specific Populations
Cirrhotic Patients
In cirrhosis, rely primarily on serum creatinine changes and abandon urine output criteria. 1, 2 Patients with cirrhosis and ascites are frequently oliguric with avid sodium retention despite maintaining relatively normal GFR, and diuretics further confound interpretation 1, 2. Urine collection is often inaccurate in clinical practice for these patients 1.
The older cirrhosis-specific criteria (50% creatinine increase to final value ≥1.5 mg/dL) have been replaced by KDIGO criteria, though the ≥1.5 mg/dL threshold still predicts AKI progression and worse prognosis in this population 1, 2.
Limitations of Creatinine
Serum creatinine significantly overestimates actual kidney function in patients with muscle wasting, increased tubular secretion, volume expansion, or hyperbilirubinemia 2. Hyperbilirubinemia specifically causes inaccurate creatinine measurement by colorimetric methods 2. Creatinine reflects functional changes only after >50% of renal mass is impaired due to the kidney's functional reserve 4.
Baseline Creatinine Determination
When baseline creatinine is unknown, the KDIGO guideline presumes it occurred within the prior 7 days for the relative increase criterion 1. In practice, use the most recent available creatinine value, or if unavailable, back-calculate from an assumed GFR of 75 mL/min/1.73m² 1.
Monitoring Recommendations
- At-risk patients: Test with serial creatinine and urine output measurements, with frequency individualized based on risk level 1
- Diagnosed AKI patients: Monitor creatinine and urine output to track stage progression or regression 1
- Post-AKI: Evaluate at 3 months for resolution, new-onset CKD, or worsening of pre-existing CKD 1
Emerging Concepts
Novel biomarkers (NGAL, KIM-1, TIMP-2, IGFBP7) can detect tubular damage before functional decline, identifying "subclinical AKI" where structural injury exists without meeting creatinine/urine output criteria 2, 4. However, these are not yet incorporated into standard diagnostic criteria and remain primarily research tools 5.