Stages of Acute Kidney Injury (AKI)
Acute Kidney Injury (AKI) is classified into three stages based on serum creatinine elevation and/or urine output reduction according to the Kidney Disease Improving Global Outcomes (KDIGO) criteria. 1
Diagnostic Criteria for AKI
AKI is defined by any of the following:
- 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, known or presumed to have occurred within the prior 7 days 1, 3
- Urine volume reduction to <0.5 mL/kg/h for 6 hours or more 1
AKI Staging Criteria
Stage 1
- Serum creatinine: Increase of 1.5-1.9 times baseline OR increase by ≥0.3 mg/dL (26.5 μmol/L) 1, 2
- Urine output: <0.5 mL/kg/h for 6-12 hours 1
Stage 2
Stage 3
- Serum creatinine: Increase of ≥3.0 times baseline OR increase to ≥4.0 mg/dL (353.6 μmol/L) OR initiation of renal replacement therapy 1, 2
- In patients <18 years: Decrease in eGFR to <35 mL/min/1.73 m² 1
- Urine output: <0.3 mL/kg/h for ≥24 hours OR anuria for ≥12 hours 1
Clinical Implications of AKI Staging
- AKI severity correlates strongly with clinical outcomes - higher stages are associated with increased mortality, longer hospital stays, and greater need for renal replacement therapy 4, 5
- Patients meeting both serum creatinine and urine output criteria for the same AKI stage have significantly worse outcomes than those meeting only one criterion 5, 6
- The progression of AKI through stages (e.g., from stage 1 to 2) is associated with increased mortality in a stage-dependent manner 3, 4
Important Considerations
- Some experts have proposed subdividing Stage 1 AKI into Stage 1a (increase in serum creatinine by ≥0.3 mg/dL) and Stage 1b (50% increase in serum creatinine) due to significantly different outcomes between these groups 4
- Even small increases in serum creatinine (≥0.3 mg/dL) are independently associated with approximately fourfold increase in hospital mortality 1, 3
- Urine output criteria may be problematic in certain populations, such as patients with cirrhosis who may be oliguric despite normal GFR or have increased urine output due to diuretic treatment 1, 3
- The duration of AKI is a significant predictor of long-term outcomes regardless of severity 5
Post-AKI Follow-up
- Patients should be evaluated 3 months after AKI for resolution, new onset, or worsening of pre-existing chronic kidney disease (CKD) 1
- Patients who develop CKD after AKI should be managed according to CKD guidelines 1
- Patients without CKD after AKI should still be considered at increased risk for CKD and monitored accordingly 1, 7
Common Pitfalls in AKI Staging
- Failure to establish an accurate baseline serum creatinine can lead to incorrect staging 1, 3
- Relying solely on serum creatinine without considering urine output may delay AKI diagnosis 8, 5
- Not recognizing that continuous urine output monitoring can detect AKI significantly earlier than serum creatinine criteria (up to 33.6 hours earlier in some studies) 8
- Overlooking that the same stage of AKI may represent different severities of kidney injury and carry different prognoses depending on which criteria are met 6