Criteria for Diagnosing Acute Kidney Injury (AKI)
According to the Kidney Disease Improving Global Outcomes (KDIGO) guidelines, AKI is diagnosed when any ONE of the following criteria is met: (1) increase in serum creatinine by ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours; OR (2) increase in serum creatinine to ≥1.5 times baseline within the prior 7 days; OR (3) urine volume <0.5 mL/kg/h for 6 hours. 1
Detailed Diagnostic Criteria
Serum Creatinine Criteria:
- An absolute increase in serum creatinine of ≥0.3 mg/dL (≥26.5 μmol/L) within 48 hours
- A relative increase in serum creatinine of ≥50% (1.5 times baseline) within 7 days
- Any requirement for renal replacement therapy
Urine Output Criteria:
- Urine output <0.5 mL/kg/h for more than 6 hours
AKI Staging System
Once AKI is diagnosed, it should be staged for severity according to the following KDIGO criteria:
| Stage | Serum Creatinine | Urine Output |
|---|---|---|
| 1 | Rise of >26 mmol/L (0.3 mg/dL) within 48h OR 50-99% rise from baseline within 7 days | <0.5 mL/kg/h for >6h |
| 2 | 100-199% rise from baseline within 7 days | <0.5 mL/kg/h for >12h |
| 3 | >200% rise from baseline within 7 days OR serum creatinine ≥354 μmol/L (4.0 mg/dL) with either rise of >26 mmol/L (0.3 mg/dL) within 48h or >50% rise within 7 days OR any requirement for renal replacement therapy | <0.3 mL/kg/h for 24h OR anuria for 12h |
Important Considerations in AKI Diagnosis
Baseline Serum Creatinine
- When determining baseline creatinine, use the most recent value within the previous 3 months when available 1
- If multiple values exist within this timeframe, use the value closest to the time of admission
- If no previous value is available, use the admission creatinine as the baseline 1
Limitations of Creatinine-Based Diagnosis
- Serum creatinine is affected by hydration status and may be diluted in volume-expanded patients 1
- Creatinine production falls during AKI due to reduced hepatic creatine synthesis
- Muscle injury can increase creatinine release independent of kidney function
- Interlaboratory variation in creatinine measurement can affect diagnosis, particularly for AKI stage 1 1
Special Populations
- In patients with cirrhosis, the urine output criteria may be less reliable as these patients are frequently oliguric with avid sodium retention despite relatively normal GFR 1
- Patients with chronic kidney disease who develop AKI may have different baseline values and progression patterns
Clinical Implications
- AKI affects approximately 10-15% of hospitalized patients and up to 50% of ICU patients 2
- Even small increases in creatinine (≥0.3 mg/dL) are independently associated with a fourfold increase in hospital mortality 1
- Early recognition using standardized criteria allows for timely intervention and improved outcomes
- AKI staging correlates with clinical outcomes including mortality, need for renal replacement therapy, and length of hospital stay 3
Common Pitfalls to Avoid
- Failing to establish an accurate baseline creatinine
- Not considering the time frame of creatinine rise (48 hours vs 7 days)
- Overlooking oliguria as a diagnostic criterion when serum creatinine hasn't yet increased
- Misinterpreting fluctuations in creatinine due to hydration status rather than true kidney injury
- Waiting for creatinine to rise significantly before diagnosing AKI, delaying intervention
By applying these KDIGO criteria systematically, clinicians can diagnose AKI early and accurately, allowing for prompt intervention to potentially improve patient outcomes.