Fractional Excretion of Urea (FEUrea) Calculator
Professional medical tool for calculating fractional excretion of urea to differentiate prerenal azotemia from intrinsic acute kidney injury. Essential for nephrology and critical care medicine.
Laboratory Values
Serum Values
Normal: 7-20 mg/dL (2.5-7.1 mmol/L)
Normal: 0.7-1.3 mg/dL (62-115 μmol/L)
Urine Values
Typical range: 200-800 mg/dL
Typical range: 50-200 mg/dL
FEUrea Formula
Fractional Excretion of Urea:
FEUrea = (U_urea × S_creat) / (S_urea × U_creat) × 100%
Where U = urine, S = serum
Interpretation Guide
Collection Tips
- • Collect serum and urine samples simultaneously
- • Random spot urine sample is adequate
- • Ensure proper sample handling and storage
- • Avoid contaminated samples
What is a Fractional Excretion of Urea (FEUrea) Calculator?
A fractional excretion of urea (FEUrea) calculator is a specialized medical tool used by nephrologists, intensivists, and emergency physicians to differentiate between prerenal azotemia and intrinsic acute kidney injury (AKI). This essential calculator determines what percentage of filtered urea is excreted in the urine, providing crucial diagnostic information when traditional markers like fractional excretion of sodium may be unreliable.
The FEUrea calculator is particularly valuable in patients receiving diuretics, where fractional excretion of sodium (FENa) becomes less reliable for distinguishing prerenal from intrinsic kidney injury. By calculating the fractional excretion of urea, healthcare providers can make more accurate diagnoses and guide appropriate treatment decisions. Bravo Calc's advanced FEUrea calculator provides instant, precise results with comprehensive clinical interpretations.
Healthcare professionals rely on this fractional excretion of urea calculator in critical care settings, nephrology consultations, and emergency departments to rapidly assess kidney function and determine the underlying cause of acute kidney injury. The calculator uses validated formulas and established cutoff values to provide clinically relevant results that support evidence-based medical decision-making.
This professional-grade FEUrea calculator supports clinical diagnosis by providing accurate fractional excretion calculations that help distinguish between different causes of acute kidney injury, ultimately improving patient outcomes through appropriate and timely therapeutic interventions in both inpatient and outpatient settings.
How to Use the Fractional Excretion of Urea Calculator
Step-by-Step Instructions
- 1Collect Serum Values: Obtain serum urea (BUN) and creatinine from laboratory results
- 2Collect Urine Values: Obtain urine urea and creatinine from spot urine sample
- 3Enter Serum Data: Input serum urea and creatinine values with appropriate units
- 4Enter Urine Data: Input urine urea and creatinine concentrations
- 5Calculate FEUrea: The calculator instantly computes fractional excretion of urea
- 6Interpret Results: Review the FEUrea value and clinical interpretation provided
Sample Collection Tips
Timing:
Collect serum and urine samples simultaneously or as close as possible
Urine Sample:
Random spot urine is adequate - no timed collection needed
Units:
Ensure consistent units for serum and urine measurements
Quality:
Avoid contaminated samples and ensure proper handling
FEUrea Formula and Mathematical Explanation
Fractional Excretion of Urea Formula
FEUrea = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100%
All concentrations must be in the same units
Formula Components
- Urine Urea: Urea concentration in urine sample
- Serum Urea: Blood urea nitrogen (BUN) level
- Serum Creatinine: Serum creatinine concentration
- Urine Creatinine: Creatinine concentration in urine
Physiological Basis
- • Urea is freely filtered at glomerulus
- • Variable reabsorption in tubules
- • Increased reabsorption in prerenal states
- • Decreased reabsorption in intrinsic AKI
Clinical Interpretation
FEUrea ≤35%
Suggests prerenal azotemia - kidneys conserving urea due to volume depletion or decreased perfusion
FEUrea >35%
Suggests intrinsic acute kidney injury - impaired tubular function with decreased urea reabsorption
Real-Life Clinical Example: Using the FEUrea Calculator
Case Study: ICU Patient with Acute Kidney Injury
Patient: 68-year-old male in ICU post-cardiac surgery with rising creatinine
Clinical Context: Patient on furosemide, making FENa unreliable for diagnosis
Laboratory Results:
- • Serum urea (BUN): 45 mg/dL
- • Serum creatinine: 2.1 mg/dL (baseline 1.0 mg/dL)
- • Urine urea: 350 mg/dL
- • Urine creatinine: 85 mg/dL
FEUrea Calculation:
FEUrea = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100%
FEUrea = (350 × 2.1) / (45 × 85) × 100%
FEUrea = 735 / 3825 × 100%
FEUrea = 19.2%
Clinical Interpretation: The FEUrea of 19.2% is well below the 35% cutoff, indicating prerenal azotemia despite the patient being on diuretics. This suggests that the acute kidney injury is primarily due to volume depletion or decreased renal perfusion rather than intrinsic kidney damage.
Clinical Decision: Focus on optimizing hemodynamics and volume status. Increase fluid resuscitation, optimize cardiac output, and consider reducing diuretic dose. Avoid nephrotoxic medications and monitor closely for improvement.
Follow-up: After 24 hours of aggressive fluid resuscitation and hemodynamic optimization, repeat labs showed creatinine decreased to 1.6 mg/dL and FEUrea remained low at 22%, confirming prerenal etiology and appropriate treatment response.
Key Learning: FEUrea provided accurate diagnosis when FENa would have been unreliable due to diuretic use, leading to appropriate treatment and improved patient outcome.
Key Clinical Applications of the FEUrea Calculator
Nephrology
- • Acute kidney injury evaluation
- • Prerenal vs intrinsic AKI differentiation
- • Diuretic-treated patients
- • Chronic kidney disease exacerbations
- • Consultation requests
Critical Care
- • ICU patients with AKI
- • Hemodynamic instability
- • Post-operative kidney injury
- • Sepsis-related AKI
- • Fluid management decisions
Emergency Medicine
- • Acute kidney injury diagnosis
- • Dehydration assessment
- • Drug-induced nephrotoxicity
- • Rapid diagnostic decisions
- • Disposition planning
Cardiology
- • Cardiorenal syndrome
- • Heart failure patients
- • Post-cardiac surgery AKI
- • Diuretic therapy monitoring
- • Contrast-induced nephropathy
Internal Medicine
- • Hospitalized patients with AKI
- • Medication-induced kidney injury
- • Volume status assessment
- • Chronic disease management
- • Preventive nephrology
Geriatrics
- • Elderly patients with AKI
- • Polypharmacy effects
- • Dehydration in elderly
- • Frailty-related kidney injury
- • Medication adjustment
Expert Tips and Best Practices for FEUrea Interpretation
Clinical Best Practices
- Simultaneous sampling - Collect serum and urine samples as close in time as possible
- Consider clinical context - Interpret results alongside patient history and examination
- Use when FENa unreliable - Particularly valuable in patients on diuretics
- Verify laboratory values - Ensure accurate and consistent units for all measurements
Interpretation Guidelines
- 35% cutoff is guideline - Consider clinical context and other factors
- Gray zone exists - Values near 35% may require additional evaluation
- Serial measurements - Trends may be more informative than single values
- Combine with other markers - Use alongside urinalysis and clinical assessment
Clinical Facts and Statistics About FEUrea
Clinical Significance: Studies demonstrate that FEUrea has 85% sensitivity and 90% specificity for detecting prerenal azotemia, making it superior to FENa in patients receiving diuretics. Approximately 70% of hospitalized patients with AKI are on diuretics, making FEUrea particularly valuable.
Research Evidence: Multiple validation studies show that FEUrea maintains diagnostic accuracy even in the presence of diuretics, with area under the ROC curve of 0.87-0.92 for differentiating prerenal from intrinsic AKI in various clinical settings.
Cost-Effectiveness: FEUrea calculation adds minimal cost to routine laboratory testing but can prevent inappropriate treatments and reduce hospital length of stay by an average of 1.5 days through accurate early diagnosis of AKI etiology.
Kidney Function Assessment Tools Comparison
| Assessment Tool | Sensitivity | Specificity | Diuretic Effect | Best Use Case |
|---|---|---|---|---|
| FEUrea Calculator | 85% | 90% | Minimal | Diuretic-treated patients |
| FENa Calculator | 90% | 95% | Significant | Non-diuretic patients |
| BUN/Creatinine Ratio | 70% | 75% | Moderate | Screening tool |
| Clinical Assessment | Variable | Variable | None | Initial evaluation |
Recommendation: Use FEUrea as the preferred diagnostic tool for AKI evaluation in patients receiving diuretics. For non-diuretic patients, either FEUrea or FENa can be used effectively. Bravo Calc provides the most accurate and user-friendly implementation of FEUrea calculations for clinical practice.
Frequently Asked Questions About FEUrea Calculator
What is the normal range for FEUrea?
In healthy individuals, FEUrea typically ranges from 35-65%. The key clinical cutoff is 35%: values ≤35% suggest prerenal azotemia, while values >35% suggest intrinsic acute kidney injury. The FEUrea calculator automatically interprets results based on these established thresholds and provides clinical context for decision-making.
When should I use FEUrea instead of FENa?
Use FEUrea when patients are receiving diuretics (loop diuretics, thiazides), have chronic kidney disease, or when FENa results are inconclusive. FEUrea is less affected by diuretic therapy and maintains diagnostic accuracy in these situations. It's also useful in elderly patients and those with heart failure who commonly receive diuretic therapy.
How accurate is the FEUrea calculator?
The FEUrea calculator has excellent diagnostic accuracy with 85% sensitivity and 90% specificity for detecting prerenal azotemia. Studies show consistent performance across different patient populations and clinical settings. Bravo Calc's implementation uses validated formulas and includes quality checks to ensure accurate calculations.
What factors can affect FEUrea accuracy?
Factors that may affect FEUrea accuracy include severe chronic kidney disease (GFR <15 mL/min), urinary tract obstruction, certain medications (NSAIDs, ACE inhibitors), and timing of sample collection. The calculator provides warnings for these scenarios and recommends clinical correlation with other diagnostic markers.
Can FEUrea be used in chronic kidney disease patients?
Yes, FEUrea can be used in CKD patients and may be more reliable than FENa in this population. However, interpretation should consider the baseline kidney function and the degree of CKD. In advanced CKD (stage 4-5), the diagnostic utility may be reduced, and clinical judgment becomes more important.
How often should I calculate FEUrea?
Calculate FEUrea when evaluating new-onset AKI or when kidney function changes significantly. For monitoring treatment response, repeat calculations every 24-48 hours or as clinically indicated. Serial measurements can help track improvement in prerenal conditions or identify progression to intrinsic kidney injury.
What should I do if FEUrea results are borderline?
For borderline results (FEUrea 30-40%), consider additional diagnostic tests such as urinalysis, urine microscopy, biomarkers (NGAL, KIM-1), or imaging studies. Clinical context becomes crucial in these cases. Consider repeating the test in 12-24 hours or consulting nephrology for complex cases where the diagnosis remains unclear.