KDIGO Stages of Acute Kidney Injury
The KDIGO classification defines three stages of AKI based on serum creatinine changes and/or urine output criteria, with patients staged according to whichever criterion is most severe. 1
AKI Definition
AKI is diagnosed when any one of the following occurs: 1
- Serum creatinine increases by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours
- Serum creatinine increases to ≥1.5 times baseline (known or presumed to have occurred within 7 days)
- Urine output <0.5 mL/kg/h for 6 hours
Staging Criteria
Stage 1
- Serum creatinine: 1.5–1.9 times baseline OR increase ≥0.3 mg/dL (≥26.5 μmol/L) 1
- Urine output: <0.5 mL/kg/h for 6–12 hours 1, 2
Stage 2
Stage 3
- Serum creatinine: ≥3.0 times baseline OR increase to ≥4.0 mg/dL (≥353.6 μmol/L) OR initiation of renal replacement therapy 1
- 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, 2
Key Staging Principles
Patients are staged using the most severe criterion met, whether based on creatinine or urine output. 2, 3 For example, a patient meeting Stage 3 by urine output (<0.3 mL/kg/h for ≥24 hours) but only Stage 1 by creatinine is classified as Stage 3 AKI. 2
Staging occurs over the entire course of AKI, regardless of timing. 1 The differential timing for diagnosis (48 hours for 0.3 mg/dL increase vs. 7 days for 1.5-fold increase) applies only to initial AKI detection, not staging. 1
Clinical Significance and Mortality Risk
Mortality increases incrementally with each advancing KDIGO stage. 2, 4 Even the modest 0.3 mg/dL creatinine increase defining Stage 1 is associated with approximately 4-fold increased in-hospital mortality. 1, 3
Stage 3 AKI carries the highest mortality risk, with patients requiring renal replacement therapy experiencing approximately 4-fold higher mortality compared to lower stages. 2
Important Caveats and Pitfalls
Urine Output Criteria Limitations
In cirrhotic patients with ascites, urine output criteria are unreliable and should not be used. 2, 3 These patients are frequently oliguric with avid sodium retention despite relatively preserved glomerular filtration. 2, 3 Focus exclusively on serum creatinine changes in this population. 2, 3
Patients receiving diuretics have confounded urine output measurements, making these criteria less dependable. 2
Heterogeneity Within Stage 1
Stage 1 may include a heterogeneous group of patients because only one criterion (0.3 mg/dL absolute increase OR 50% relative increase) needs to be met. 1 A 0.3 mg/dL increase may represent a significantly smaller change than a 50% increase depending on baseline creatinine. 1
Research suggests subdividing Stage 1 into Stage 1a (0.3 mg/dL absolute increase) and Stage 1b (50% relative increase) may provide additional prognostic information, as these subgroups show clinically meaningful differences in length of stay and mortality. 5
Baseline Creatinine Determination
Use the most recent serum creatinine measured within the prior 3 months, selecting the value closest to hospital admission. 3 If no prior measurement exists, admission creatinine serves as baseline. 3
Do not back-calculate baseline creatinine using MDRD equations in patients with cirrhosis, as this approach is excluded from consensus recommendations. 3
Post-AKI Follow-Up
Evaluate patients 3 months after AKI for resolution, new-onset chronic kidney disease, or worsening of pre-existing CKD. 1, 4 Patients without CKD should be considered at increased risk and managed according to KDOQI guidelines for at-risk populations. 1