How to Determine if a Patient Has Acute Kidney Injury
Diagnose AKI when serum creatinine rises ≥0.3 mg/dL within 48 hours OR increases to ≥1.5 times baseline within 7 days OR urine output falls below 0.5 mL/kg/h for 6 consecutive hours—meeting any single criterion is sufficient. 1
Primary Diagnostic Criteria
The KDIGO criteria define AKI using three independent parameters; you need only ONE to make the diagnosis: 1
- Absolute creatinine change: Rise of ≥0.3 mg/dL (≥26.5 µmol/L) within any 48-hour window 1
- Relative creatinine change: Rise to ≥1.5 times the baseline value within the prior 7 days 1
- Urine output: <0.5 mL/kg/h sustained for ≥6 consecutive hours 1
Even the modest 0.3 mg/dL rise carries approximately 4-fold increased in-hospital mortality, making this threshold clinically critical despite appearing small. 2, 3
Establishing Baseline Creatinine
Use the most recent serum creatinine from the prior 3 months, selecting the value closest to hospital admission. 4, 2 If no prior value exists, use the admission creatinine as baseline. 4, 2
Do NOT back-calculate baseline using MDRD equations in cirrhotic patients—this approach is specifically excluded from consensus recommendations for this population. 4, 2 For non-cirrhotic patients without any prior creatinine, you may back-calculate assuming a GFR of 75 mL/min/1.73 m² using MDRD. 3
Staging AKI Severity
Once AKI is diagnosed, stage by the most severe criterion met (either creatinine or urine output): 1
| Stage | Creatinine Criterion | Urine Output Criterion |
|---|---|---|
| Stage 1 | 1.5–1.9× baseline OR ≥0.3 mg/dL rise | <0.5 mL/kg/h for 6–12 hours |
| Stage 2 | 2.0–2.9× baseline | <0.5 mL/kg/h for ≥12 hours |
| Stage 3 | ≥3.0× baseline OR ≥4.0 mg/dL (with acute rise ≥0.3 mg/dL) OR dialysis initiated | <0.3 mL/kg/h for ≥24 hours OR anuria ≥12 hours |
Mortality risk increases progressively with each stage—Stage 3 carries roughly 60-fold increased odds of death compared to no AKI. 5
The BUN-to-Creatinine Ratio: Not Reliable for AKI Diagnosis
The BUN/creatinine ratio does NOT reliably distinguish prerenal from intrinsic AKI and should not guide your diagnosis. 6 A large study of 1,103 ED patients with AKI found no statistical difference in mean BUN/creatinine ratio between prerenal (90.55) and intrinsic (91.29) groups, with an area under the ROC curve of 0.5—equivalent to a coin flip. 6
Instead, use these indices to differentiate etiology AFTER diagnosing AKI: 7, 8
- Urine sodium (UNa): <20 mEq/L suggests prerenal; >40 mEq/L suggests intrinsic (ATN) 8
- Fractional excretion of sodium (FENa): <1% suggests prerenal; >2% suggests ATN 2, 7
- Fractional excretion of urea (FEUrea): Use when diuretics confound FENa 2, 8
- Renal failure index (RFI): High specificity (>85%) for both prerenal and ATN when used with other parameters 8
- Urine specific gravity (USG): >1.020 suggests prerenal 8
These indices remain valid even in patients on ACE inhibitors, ARBs, or with pre-existing CKD. 8
Critical Caveats for Specific Populations
Cirrhotic Patients with Ascites
Use serum creatinine changes ONLY—ignore urine output criteria entirely. 4, 2 These patients are frequently oliguric with avid sodium retention despite relatively preserved GFR, and diuretics further confound interpretation. 4, 2 A creatinine threshold of ≥1.5 mg/dL predicts AKI progression and worse prognosis in cirrhosis. 2
Patients Receiving Diuretics
Urine output criteria are unreliable—rely on creatinine changes for diagnosis. 2, 9 Loop diuretics do not affect the reliability of urine sodium or RFI for etiology determination. 8
Massive Fluid Resuscitation
Serum creatinine can be diluted by large-volume crystalloid, potentially masking significant GFR reduction. 2 When cumulative fluid gain exceeds 5–10% of baseline body weight, adjust creatinine for volume expansion to avoid underestimating AKI severity. 2
Common Pitfalls to Avoid
Don't wait for creatinine to reach 1.5 mg/dL—this outdated threshold often indicates GFR has already fallen to ~30 mL/min. 2 Monitor 48-hour intervals to catch the 0.3 mg/dL threshold early. 2
Hyperbilirubinemia interferes with creatinine assays—Jaffe methods show false elevation, enzymatic methods show false reduction. 2 Check for laboratory flags indicating icterus. 2
Trimethoprim and cimetidine block tubular creatinine secretion, causing spuriously high values unrelated to true GFR changes. 2 Review medication lists before interpreting creatinine rises. 2
Reduced muscle mass (elderly, malnourished, critically ill) lowers baseline creatinine, blunting the apparent rise during AKI. 2 A small absolute increase may represent substantial kidney injury in these patients. 2
Monitoring and Follow-Up
Monitor serum creatinine and urine output to stage severity once AKI is diagnosed. 1 Re-evaluate all patients at 3 months to assess for resolution, new-onset CKD, or progression of pre-existing kidney disease. 1 Even patients without established CKD who experience AKI are at increased risk and require ongoing monitoring per KDOQI guidelines. 1
Obtain urgent nephrology consultation for Stage 2 or Stage 3 AKI. 2, 9