Criteria for Acute Kidney Injury (AKI)
Acute kidney injury is defined as any of the following: an increase in serum creatinine by 0.3 mg/dL (26.5 μmol/L) within 48 hours; or an increase in serum creatinine to 1.5 times baseline within the prior 7 days; or urine output less than 0.5 mL/kg/h for 6 hours. 1
Definition and Classification
The current definition of AKI represents a harmonization of previous criteria (RIFLE and AKIN) into the KDIGO (Kidney Disease: Improving Global Outcomes) guidelines, which are now the gold standard for diagnosing AKI. This standardized approach allows for early detection and staging of kidney injury.
Diagnostic Criteria for AKI (KDIGO)
AKI is diagnosed when any ONE of the following criteria is met:
- Increase in serum creatinine by ≥0.3 mg/dL (26.5 μmol/L) within 48 hours
- Increase in serum creatinine to ≥1.5 times baseline, which is known or presumed to have occurred within the prior 7 days
- Urine volume <0.5 mL/kg/h for 6 hours
Staging of AKI Severity
Once AKI is diagnosed, its severity is staged as follows:
| Stage | Serum Creatinine Criteria | Urine Output Criteria |
|---|---|---|
| 1 | Increase 1.5-1.9 times baseline OR ≥0.3 mg/dL (26.5 μmol/L) increase | <0.5 mL/kg/h for 6-12 hours |
| 2 | Increase 2.0-2.9 times baseline | <0.5 mL/kg/h for ≥12 hours |
| 3 | Increase ≥3.0 times 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 | <0.3 mL/kg/h for ≥24 hours OR Anuria for ≥12 hours |
Clinical Implications of AKI Staging
The staging system has important prognostic implications:
- Higher stages correlate with increased mortality risk 2
- Even Stage 1 AKI is associated with a significantly increased risk of death (HR 2.04) 2
- Stage 3 AKI has the highest mortality risk (HR 4.5) 2
Important Considerations in AKI Assessment
Role of Urine Output Criteria
Adding urine output criteria to serum creatinine measurements significantly improves AKI detection:
- Urine output criteria can detect AKI approximately 11 hours earlier than serum creatinine criteria 3
- Using both criteria together can nearly double the detected incidence of AKI compared to using serum creatinine alone 3
- Patients who meet both urine output and serum creatinine criteria have higher mortality (HR 3.56) than those meeting only one criterion 2
Special Populations
Patients with Cirrhosis
Patients with cirrhosis require special consideration when applying AKI criteria due to:
- Decreased muscle mass and creatine production
- Increased renal tubular secretion of creatinine
- Dilution of serum creatinine due to increased volume of distribution
- Interference with creatinine assays by elevated bilirubin 1
Modified KDIGO criteria have been developed for cirrhotic patients by the International Club of Ascites, maintaining the same diagnostic thresholds but with special attention to baseline creatinine determination 1.
Pediatric Patients
For patients under 18 years, an additional criterion for Stage 3 AKI includes:
- Decrease in eGFR to <35 mL/min/1.73 m² 1
Clinical Pitfalls and Caveats
Baseline creatinine determination: When no previous creatinine value is available, the KDIGO guidelines suggest using the admission value as baseline 1. This may underestimate AKI incidence in patients who already have elevated creatinine on admission.
Limitations of serum creatinine: Creatinine is an imperfect marker of kidney function as it:
- Rises only after significant kidney injury has occurred
- Is affected by muscle mass, age, gender, and nutritional status
- May be normal despite significant kidney injury in early stages
Timing of assessment: AKI can develop rapidly, so frequent monitoring of both serum creatinine and urine output is essential in high-risk patients.
Fluid overload: May dilute serum creatinine and mask AKI diagnosis.
Medications affecting creatinine secretion: Drugs like trimethoprim and cimetidine can increase serum creatinine without affecting GFR.
By applying these standardized KDIGO criteria, clinicians can promptly identify AKI, stage its severity, and implement appropriate management strategies to improve patient outcomes.