FEUrea calculator: Alternative to FENa for diagnosing prerenal vs. intrinsic AKI in diuretic-treated patients. Uses urine/plasma urea and creatinine to bypass sodium interference.
Fractional Excretion of Urea (FEUrea) Calculation:
FEUrea (%) =
× 100
Urine Urea × Plasma Cr
Plasma Urea × Urine Cr
Key Components:
• Urine Urea: Concentration in urine (mg/dL)
• Plasma Cr: Serum creatinine (mg/dL)
• Plasma Urea: Blood urea nitrogen (mg/dL)
• Urine Cr: Creatinine in urine (mg/dL)
• Normal Range: 35-60% in healthy adults
• Critical Value: <35% in prerenal azotemia
• Plasma Cr: Serum creatinine (mg/dL)
• Plasma Urea: Blood urea nitrogen (mg/dL)
• Urine Cr: Creatinine in urine (mg/dL)
• Normal Range: 35-60% in healthy adults
• Critical Value: <35% in prerenal azotemia
Clinical Utility:
• Differentiates prerenal vs intrinsic AKI
• More reliable than FENa in diuretic use
• Assesses renal perfusion status
• Monitors response to volume resuscitation
• Evaluates urea handling in tubules
• Useful in heart failure patients
• More reliable than FENa in diuretic use
• Assesses renal perfusion status
• Monitors response to volume resuscitation
• Evaluates urea handling in tubules
• Useful in heart failure patients
Calculation Example:
• Urine Urea: 250 mg/dL
• Plasma Cr: 1.8 mg/dL
• Plasma Urea: 60 mg/dL
• Urine Cr: 45 mg/dL
• FEUrea = (250 × 1.8) / (60 × 45) × 100
• = (450) / (2700) × 100 = 16.7%
• Interpretation: Prerenal azotemia
• Plasma Cr: 1.8 mg/dL
• Plasma Urea: 60 mg/dL
• Urine Cr: 45 mg/dL
• FEUrea = (250 × 1.8) / (60 × 45) × 100
• = (450) / (2700) × 100 = 16.7%
• Interpretation: Prerenal azotemia
Interpretation Guide:
• <35%: Prerenal failure
• >50%: Intrinsic renal failure
• 35-50%: Indeterminate zone
• Diuretic effect: May elevate FEUrea
• Low protein intake: May lower FEUrea
• Elderly: Higher baseline values
• >50%: Intrinsic renal failure
• 35-50%: Indeterminate zone
• Diuretic effect: May elevate FEUrea
• Low protein intake: May lower FEUrea
• Elderly: Higher baseline values
Sampling Protocol:
• Simultaneous blood and urine samples
• Random urine sample acceptable
• Serum: Gold/serum separator tube
• Urine: Clean-catch midstream
• Process within 2 hours
• Avoid contamination
• Random urine sample acceptable
• Serum: Gold/serum separator tube
• Urine: Clean-catch midstream
• Process within 2 hours
• Avoid contamination
Limitations:
• Less accurate in advanced CKD
• Affected by glucocorticoid use
• Unreliable in osmotic diuresis
• Variability in malnutrition
• Interference from GI bleeding
• Not valid post-renal transplant
• Affected by glucocorticoid use
• Unreliable in osmotic diuresis
• Variability in malnutrition
• Interference from GI bleeding
• Not valid post-renal transplant
Critical Clinical Notes:
• Preferred over FENa in diuretic-treated patients
• Always correlate with urine microscopy
• Values <20% → high probability of prerenal state
• Combine with BUN/Cr ratio for best accuracy
• Not reliable in contrast nephropathy
• Preferred over FENa in diuretic-treated patients
• Always correlate with urine microscopy
• Values <20% → high probability of prerenal state
• Combine with BUN/Cr ratio for best accuracy
• Not reliable in contrast nephropathy
🧪 Fractional Excretion of Urea (FEUrea)
📐 Formula:
FEUrea (%) =
Urine Urea × Plasma Cr
Plasma Urea × Urine Cr
× 100
🖊️ Enter the Following Values:
Core Content Summary:
🔹 FEUrea Formula
Fractional Excretion of Urea (FEUrea) Calculation:
FEUrea (%) =
× 100
Urine Urea × Plasma Cr
Plasma Urea × Urine Cr
Units:
- Urea: mg/dL (urine & plasma)
- Creatinine (Cr): mg/dL (urine & plasma)
🔹 Interpretation Table
| FEUrea (%) | Clinical Significance | Common Scenarios |
|---|---|---|
| < 35% | Prerenal AKI | Dehydration, CHF, Sepsis |
| 35–50% | Indeterminate | Transition phase |
| > 50% | Intrinsic Renal AKI | ATN, Glomerulonephritis |
When to Use FEUrea Instead of FENa
- Diuretic Therapy (e.g., furosemide):
- Diuretics ↑ urinary sodium → falsely elevates FENa
- Urea excretion less affected by most diuretics
- Low Sodium Intake (e.g., heart failure diet)
- Contrast Nephropathy
Calculation Example
Patient Values:
- Plasma Urea: 40 mg/dL
- Urine Urea: 320 mg/dL
- Plasma Cr: 1.8 mg/dL
- Urine Cr: 60 mg/dL
Calculation:
- Numerator:
Urine Urea × Plasma Cr= 320 × 1.8 = 576 - Denominator:
Plasma Urea × Urine Cr= 40 × 60 = 2,400 - Ratio:
576 / 2,400= 0.24 - FEUrea:
0.24 × 100= 24%
Interpretation: Prerenal AKI (24% < 35%)
Key Advantages & Limitations
| Pros | Cons |
|---|---|
| Reliable with diuretics | Less specific than FENa |
| Unaffected by sodium intake | Osmotic diuretics ↓ accuracy |
| Useful in hyponatremia | Not validated in CKD Stage 4-5 |
Clinical Pearls
- Sample Timing: Collect urine before fluid resuscitation
- FEUrea+FENa Combo: Increases diagnostic accuracy in complex cases
- Osmotic Diuretics Warning: Mannitol or hyperglycemia skew results
- Pediatric Note: Use same thresholds (>1 month old)
💡 Critical Tip: FEUrea >50% + muddy brown casts = Acute Tubular Necrosis (ATN)
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