Acute Kidney Injury (AKI) Diagnostic Criteria
The diagnosis of AKI is established by an increase in serum creatinine ≥0.3 mg/dL within 48 hours, or an increase ≥50% from baseline within 7 days, or a decrease in urine output to <0.5 mL/kg/h for 6 hours or more, according to the KDIGO criteria. 1, 2, 3
Definition of AKI
AKI is defined by any of the following criteria:
- Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L) within 48 hours 1, 2
- Increase in serum creatinine to ≥1.5 times baseline, which is known or presumed to have occurred within the prior 7 days 1, 4
- Urine output <0.5 mL/kg/h for 6 hours or more 1, 2
AKI Staging System
AKI is staged according to severity:
Stage 1
- Increase in serum creatinine by 1.5-1.9 times baseline; OR
- Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L); OR
- Urine output <0.5 mL/kg/h for 6-12 hours 1, 2, 3
Stage 2
- Increase in serum creatinine by 2.0-2.9 times baseline; OR
- Urine output <0.5 mL/kg/h for ≥12 hours 1, 2, 3
Stage 3
- Increase in serum creatinine by ≥3.0 times baseline; OR
- Increase in serum creatinine to ≥4.0 mg/dL (353.6 μmol/L) with an acute increase of at least 0.5 mg/dL (44 μmol/L); OR
- Initiation of renal replacement therapy; OR
- In patients <18 years, decrease in eGFR to <35 mL/min/1.73 m²; OR
- Urine output <0.3 mL/kg/h for ≥24 hours; OR
- Anuria for ≥12 hours 1, 2, 3
Evolution of AKI Criteria
The current KDIGO criteria represent a harmonization of previous classification systems:
- RIFLE criteria (2004): Defined AKI as ≥50% increase in serum creatinine from baseline within 1-7 days, GFR decrease >25%, or oliguria >6 hours 4
- AKIN criteria (2007): Added absolute increase in serum creatinine of ≥0.3 mg/dL and reduced timeframe to 48 hours 4
- KDIGO criteria (2012): Combined elements from both systems 1, 4
Clinical Implications
- Even small increases in serum creatinine (≥0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 4, 5
- Progression through AKI stages strongly correlates with increased mortality 2, 5
- Patients who meet both serum creatinine and urine output criteria have worse outcomes than those meeting only one criterion 5, 6
- Duration of AKI is a significant predictor of long-term outcomes regardless of severity 5
Special Considerations
- In patients with cirrhosis, urine output criteria may be problematic due to avid sodium retention despite relatively normal GFR 1, 3
- Patients may be oliguric with sodium retention yet maintain normal GFR, or have increased urine output due to diuretic treatment 1
- For cirrhotic patients, kinetic changes in serum creatinine become the primary diagnostic criterion 1
Monitoring and Follow-up
- Patients with AKI should be monitored with serial measurements of serum creatinine and urine output to track progression and stage severity 1, 3
- Patients should be evaluated 3 months after AKI for resolution, new onset, or worsening of pre-existing CKD 1
- Even one episode of AKI increases the risk of cardiovascular disease, chronic kidney disease, and death 7, 8