Stages of Acute Kidney Injury
Acute Kidney Injury is classified into three stages based on KDIGO criteria, using either serum creatinine increases or urine output decreases, with staging determined by whichever criterion is most severe. 1
AKI Definition
AKI is diagnosed when any one of the following criteria is met: 1, 2
- Serum creatinine increase ≥0.3 mg/dL (26.5 μmol/L) within 48 hours
- Serum creatinine increase to ≥1.5 times baseline within 7 days
- Urine output <0.5 mL/kg/h for 6 consecutive hours
Staging Criteria
Stage 1 AKI
Serum Creatinine Criteria: 1, 3, 2
- Increase 1.5-1.9 times baseline, OR
- Increase ≥0.3 mg/dL (26.5 μmol/L)
Urine Output Criteria: 1, 3, 2
- <0.5 mL/kg/h for 6-12 hours
Stage 2 AKI
Serum Creatinine Criteria: 1, 3, 2
- Increase 2.0-2.9 times baseline
Urine Output Criteria: 1, 3, 2
- <0.5 mL/kg/h for ≥12 hours
Stage 3 AKI
Serum Creatinine Criteria: 1, 3, 2
- Increase ≥3.0 times baseline, OR
- Increase to ≥4.0 mg/dL (353.6 μmol/L) with an acute increase of at least 0.3 mg/dL, OR
- Initiation of renal replacement therapy, OR
- In patients <18 years: decrease in eGFR to <35 mL/min/1.73 m²
Urine Output Criteria: 1, 3, 2
- <0.3 mL/kg/h for ≥24 hours, OR
- Anuria for ≥12 hours
Critical Clinical Considerations
Staging is retrospective and based on the most severe criterion met during the entire AKI episode, regardless of timing. 1, 3 Patients meeting both serum creatinine and urine output criteria have significantly worse outcomes than those meeting only one criterion. 4
Even small creatinine increases (≥0.3 mg/dL) independently predict approximately fourfold increased hospital mortality. 3, 2 Progression through AKI stages strongly correlates with increased mortality and need for renal replacement therapy. 3, 2, 4
Important Caveats for Specific Populations
In cirrhotic patients with ascites, rely exclusively on serum creatinine changes and ignore urine output criteria. 2, 5 These patients are frequently oliguric with avid sodium retention yet maintain relatively normal GFR, and diuretic therapy further confounds interpretation. 2
The 0.3 mg/dL absolute increase criterion may capture a heterogeneous group of patients at Stage 1, as this represents a smaller relative change in patients with higher baseline creatinine. 1 However, absolute changes in creatinine (rather than percentage changes) are nearly identical across the spectrum of baseline kidney function after severe AKI. 6
Patients with chronic kidney disease may not demonstrate the same percentage increases in creatinine as those with normal baseline function, even with equivalent reductions in kidney function. 6 After a 90% reduction in creatinine clearance, the 24-hour creatinine rise is 246% with normal baseline but only 47% in stage 4 CKD. 6