What is Considered AKI
Acute kidney injury (AKI) is diagnosed when ANY ONE of the following KDIGO criteria is met: serum creatinine rises by ≥0.3 mg/dL within 48 hours, OR serum creatinine rises to ≥1.5 times baseline within 7 days, OR urine output falls below 0.5 mL/kg/h for more than 6 consecutive hours. 1
Core Diagnostic Criteria (KDIGO)
The diagnosis requires meeting any single criterion—you do not need all three 1:
- Creatinine criterion #1: Absolute increase of ≥0.3 mg/dL (≥26 μmol/L) within any 48-hour period 1, 2
- Creatinine criterion #2: Rise to ≥1.5× baseline (≥50% increase) within the prior 7 days 1, 2
- Urine output criterion: <0.5 mL/kg/h for >6 consecutive hours 1, 2
Even the modest 0.3 mg/dL threshold is clinically significant—it independently associates with approximately 4-fold increased in-hospital mortality 1, 3.
AKI Staging (KDIGO)
Once AKI is diagnosed, stage it retrospectively using the most severe criterion met during the episode 1:
| Stage | Creatinine Criterion | Urine Output Criterion |
|---|---|---|
| 1 | 1.5–1.9× baseline OR increase ≥0.3 mg/dL | <0.5 mL/kg/h for 6–12 h |
| 2 | 2.0–2.9× baseline | <0.5 mL/kg/h for ≥12 h |
| 3 | ≥3.0× baseline OR ≥4.0 mg/dL with acute rise ≥0.3 mg/dL OR initiation of dialysis | <0.3 mL/kg/h for ≥24 h OR anuria ≥12 h |
Higher stages correlate strongly with increased mortality risk. 1
Establishing Baseline Creatinine
Use the most recent known creatinine value from the medical record within the prior 3 months—this is superior to any imputation method 1, 2. Search aggressively for outpatient labs, emergency-department visits, pre-operative assessments, and prior admissions 2.
- If no prior value exists, admission creatinine serves as baseline 1
- Do NOT back-calculate baseline using MDRD equations (assuming GFR 75 mL/min/1.73 m²) in patients with cirrhosis or populations with high CKD prevalence—this overestimates AKI incidence 1, 2
Special Populations: Cirrhosis & Ascites
In cirrhotic patients with ascites, rely exclusively on serum creatinine changes—ignore urine output criteria. 1, 2
- These patients are frequently oliguric with avid sodium retention yet maintain relatively normal GFR 1, 2
- Diuretic therapy further confounds urine output interpretation 1, 2
- Volume expansion from ascites dilutes serum creatinine, lowering its concentration independent of true kidney function 1
- A creatinine threshold ≥1.5 mg/dL predicts AKI progression and worse prognosis in this population 1
Common Pitfalls & Caveats
Serum creatinine limitations: Affected by muscle mass, age, sex, nutritional status, and hyperbilirubinemia (which interferes with colorimetric assays) 1. Massive fluid resuscitation can dilute creatinine, masking significant GFR reduction 1.
Urine output unreliability: Beyond cirrhosis, urine collection is often inaccurate in clinical practice and influenced by body weight, race, age, gender, and diuretic use 2. Do not rely on urine output alone in patients receiving diuretics 1.
Temporal detection: Monitor creatinine at 48-hour intervals to detect the 0.3 mg/dL threshold in real time 1. Do not wait for creatinine to reach 1.5 mg/dL—this outdated threshold often indicates GFR has already fallen to ~30 mL/min 1.
AKI on CKD: Apply the same KDIGO criteria relative to the patient's established CKD baseline 1. Even small acute rises meeting AKI thresholds confer markedly higher mortality (47% vs 19% for CKD alone) 1.
When to Consult Nephrology
Stage 2 or Stage 3 AKI warrants prompt nephrology involvement. 1
Additional indications include 4:
- Inadequate response to supportive treatment
- AKI without a clear cause
- Pre-existing stage 4 or higher CKD
- Need for renal replacement therapy