Diagnostic Criteria for Acute Kidney Injury (AKI)
Acute Kidney Injury is diagnosed when any of the following criteria are met: increase in serum creatinine by ≥0.3 mg/dL within 48 hours; 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 Criteria for AKI Diagnosis
The Kidney Disease Improving Global Outcomes (KDIGO) criteria represent the most current consensus for diagnosing AKI. These criteria harmonize previous definitions (RIFLE and AKIN) and include:
Serum creatinine criteria:
Urine output criteria:
AKI Staging
Once AKI is diagnosed, it should be staged for severity:
| Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 1 | Increase 1.5-1.9 times baseline OR ≥0.3 mg/dL 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 OR Initiation of renal replacement therapy | <0.3 mL/kg/h for ≥24h OR Anuria for ≥12h |
Baseline Creatinine Determination
For accurate diagnosis, establishing the baseline creatinine is critical:
- Use a serum creatinine value obtained within the previous 3 months as baseline 1
- When multiple values are available, use the value closest to the current admission
- If no previous value is available, the admission creatinine may be used as baseline, though this may underestimate AKI incidence 1
Clinical Implications of AKI Staging
The staging of AKI has important prognostic implications:
- Higher AKI stages correlate with worse outcomes including mortality, need for renal replacement therapy, and progression to chronic kidney disease 1
- Even Stage 1 AKI (small increases in serum creatinine) is independently associated with increased mortality 2
- In critically ill patients, AKI diagnosed by both creatinine and urine output criteria carries a higher mortality risk than AKI diagnosed by only one criterion 4
Special Considerations
Urine Output Criteria
- Adding urine output criteria can significantly increase AKI detection rates (from approximately 20% to 38% in critically ill patients) 5
- Urine output criteria typically detect AKI earlier (median 13 hours after admission) compared to serum creatinine criteria (median 24 hours) 5
Patients with Cirrhosis
- The International Club of Ascites has modified the KDIGO criteria for patients with cirrhosis:
- Removed urine output criteria due to limitations in cirrhotic patients who may be oliguric with avid sodium retention yet maintain relatively normal GFR 2
- Extended the baseline creatinine timeframe to 3 months 1
- Serum creatinine in cirrhotic patients may underestimate kidney dysfunction due to decreased creatinine formation, increased tubular secretion, dilution from ascites, and interference with assays by elevated bilirubin 2
AKI Stage 1 Substaging in Cirrhosis
- Some experts propose substaging Stage 1 AKI in cirrhosis:
- Stage 1A: serum creatinine <1.5 mg/dL
- Stage 1B: serum creatinine ≥1.5 mg/dL
- This substaging has prognostic value as patients with Stage 1B fare significantly worse 2
Common Pitfalls in AKI Diagnosis
Failing to recognize AKI early: Even small increases in serum creatinine (0.3 mg/dL) are associated with increased mortality 2
Overlooking urine output criteria: Using only serum creatinine may delay AKI diagnosis by approximately 11 hours 5
Misinterpreting creatinine in special populations: Serum creatinine may underestimate kidney dysfunction in patients with cirrhosis, sarcopenia, or malnutrition 2
Using incorrect baseline creatinine: Using admission creatinine as baseline when prior values are available may lead to underdiagnosis of AKI
Not recognizing the prognostic significance of AKI staging: All stages of AKI are associated with increased mortality, with higher stages carrying progressively worse prognosis 4
By understanding and correctly applying these diagnostic criteria, clinicians can identify AKI early, initiate appropriate management, and potentially improve outcomes for patients with this serious condition.