Acute Kidney Injury (AKI) Diagnostic Criteria
The diagnosis of Acute Kidney Injury (AKI) is based on the Kidney Disease Improving Global Outcomes (KDIGO) criteria, which includes either an increase in serum creatinine by ≥0.3 mg/dL within 48 hours, an increase in serum creatinine to ≥1.5 times baseline within 7 days, or urine output <0.5 mL/kg/h for 6 hours or more. 1
KDIGO Diagnostic Criteria for AKI
Serum Creatinine Criteria (either):
- Increase in serum creatinine by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours; OR
- Increase in serum creatinine to ≥1.5 times baseline within the prior 7 days 2, 1
Urine Output Criteria:
- Urine output <0.5 mL/kg/h for 6 hours or more 1
AKI Staging
Once AKI is diagnosed, it should be staged according to severity:
| Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 1 | Increase 1.5-1.9 times baseline OR ≥0.3 mg/dL (26.5 μmol/L) increase | <0.5 mL/kg/h for 6-12h |
| 2 | Increase 2.0-2.9 times baseline | <0.5 mL/kg/h for ≥12h |
| 3 | Increase ≥3.0 times baseline OR Increase to ≥4.0 mg/dL (353.6 μmol/L) with an acute increase ≥0.3 mg/dL OR Initiation of renal replacement therapy | <0.3 mL/kg/h for ≥24h OR Anuria for ≥12h |
Baseline Serum Creatinine Determination
Determining the baseline serum creatinine is crucial for AKI diagnosis:
- A serum creatinine value obtained in the previous 3 months, when available, can be used as baseline 2, 1
- When multiple values exist within the previous 3 months, use the value closest to the admission time 2
- In patients without a previous creatinine value, the admission creatinine may be used as baseline 2, 1
Clinical Implications of AKI Criteria
Importance of Both Criteria
- Using both serum creatinine and urine output criteria significantly increases AKI detection compared to using serum creatinine alone 3
- Urine output criteria can detect AKI approximately 11 hours earlier than serum creatinine criteria (median 13h vs 24h) 3
- AKI diagnosed by urine output criteria alone is independently associated with increased mortality 4
Prognostic Value
- Higher AKI stages correlate with worse outcomes, including increased mortality, need for renal replacement therapy, and progression to chronic kidney disease 1
- Even small increases in serum creatinine (Stage 1 AKI) are independently associated with increased mortality 1
Special Considerations
Patients with Cirrhosis
The International Club of Ascites has modified the KDIGO criteria for patients with cirrhosis:
- Removed urine output criteria
- Extended baseline creatinine timeframe to 3 months
- Serum creatinine may underestimate kidney dysfunction due to decreased creatinine formation, increased tubular secretion, and dilution from ascites 2, 1
Common Pitfalls to Avoid
Delayed Recognition: Failing to recognize small increases in serum creatinine (0.3 mg/dL) can lead to missed opportunities for early intervention 1
Ignoring Urine Output: Relying solely on serum creatinine can delay AKI diagnosis by approximately 11 hours and miss cases that would be detected by urine output criteria 3
Misinterpreting Creatinine in Special Populations: Serum creatinine may underestimate kidney dysfunction in patients with cirrhosis, sarcopenia, or malnutrition 1
Inconsistent Definition Application: Using different AKI definitions or criteria inconsistently can affect reported incidence rates and clinical outcomes 5
Inadequate Follow-up: Patients should be evaluated 3 months after an AKI episode to assess for resolution, new onset of chronic kidney disease, or worsening of pre-existing chronic kidney disease 1