KDIGO Criteria Are the Current Standard for AKI Diagnosis and Staging
The KDIGO (Kidney Disease: Improving Global Outcomes) criteria are the universally accepted standard for diagnosing and staging acute kidney injury, having harmonized and replaced the older RIFLE and AKIN classifications. 1
Why KDIGO Superseded AKIN and RIFLE
The KDIGO classification unified the previously competing RIFLE (Risk, Injury, Failure, Loss, End-stage) and AKIN (Acute Kidney Injury Network) definitions in 2012, creating a single consensus definition that is now widely validated and implemented globally 1. The older RIFLE and AKIN classifications should no longer be used, as KDIGO has harmonized these prior definitions into a superior framework 1.
KDIGO Definition of AKI
AKI is diagnosed when any one of the following criteria is met 1:
- Serum creatinine increase ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours, OR
- Serum creatinine increase to ≥1.5 times baseline (known or presumed to have occurred within the prior 7 days), OR
- Urine output <0.5 mL/kg/hour for 6 hours 1
KDIGO Staging System
Once AKI is diagnosed, severity is staged as follows 1:
Stage 1
- SCr 1.5-1.9× baseline OR ≥0.3 mg/dL increase, OR
- Urine output <0.5 mL/kg/hour for 6-12 hours 1
Stage 2
- SCr 2.0-2.9× baseline, OR
- Urine output <0.5 mL/kg/hour for ≥12 hours 1
Stage 3
- SCr ≥3.0× baseline OR increase to ≥4.0 mg/dL (≥353.6 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/hour for ≥24 hours OR anuria for ≥12 hours 1
Clinical Application and Validation
The KDIGO criteria have been independently validated in multiple studies and are now the standard of care for AKI diagnosis in both critically ill and non-critically ill patients 1, 2. AKI occurs in approximately half of adult critically ill patients admitted to intensive care units, with stepwise increases in mortality associated with increasing KDIGO stages 2.
Important Caveats
Differential Timing for Diagnosis
The KDIGO definition introduces differential timing: 48 hours for the 0.3 mg/dL increase criterion versus 7 days for the 1.5-fold increase criterion 1. This distinction is somewhat arbitrary and patients are staged over the entire course of AKI regardless of timing 1.
Heterogeneity Within Stage 1
Because only one criterion needs to be met, AKI Stage 1 may include a heterogeneous group of patients—a 0.3 mg/dL increase may represent a significantly smaller percentage increase than a 50% increase depending on baseline creatinine 1. Research suggests subdividing Stage 1 into Stage 1a (0.3 mg/dL increase) and Stage 1b (50% increase) may provide additional prognostic information, as these subgroups show markedly different mortality rates (odds ratio 4.3 vs 10.9) 3.
Urine Output Criteria Limitations
Urine output criteria are generally accepted as applicable only in intensive care settings where accurate monitoring is feasible; ascertainment of AKI from serum creatinine changes alone is acceptable in other clinical settings 1.
Baseline Creatinine Issues
AKI may be inappropriately diagnosed in patients with low baseline serum creatinine when using definitions incorporating percentage increases over baseline 4. Patients with acute-on-chronic kidney disease (baseline renal impairment) show different patterns—they more frequently reach the absolute SCr ≥4.0 mg/dL criterion in Stage 3, while those with previously normal function more often fulfill the SCr ≥3.0× baseline criterion 5.
Classification Best Practices
KDIGO classification should specify both cause and stage rather than stage alone (e.g., "AKI stage 3 due to acute tubular necrosis") 1. This approach provides more clinically relevant information for management decisions 1.