Fractional Excretion of Sodium (FENa) Calculator
Calculate the Fractional Excretion of Sodium to differentiate between prerenal azotemia and intrinsic renal disease in patients with acute kidney injury.
FENa (%) = [(UNa × SCr) / (SNa × UCr)] × 100
UNa: Urine sodium
SNa: Serum sodium
UCr: Urine creatinine
SCr: Serum creatinine
What is a FENa Calculator?
A FENa calculator is a specialized medical tool that calculates the Fractional Excretion of Sodium, a critical diagnostic parameter used to differentiate between prerenal azotemia and intrinsic renal disease in patients with acute kidney injury (AKI). The FENa calculator provides essential information for determining the underlying cause of kidney dysfunction and guiding appropriate treatment strategies.
The FENa calculator works by analyzing the relationship between sodium and creatinine concentrations in both serum and urine samples. This FENa calculator is particularly valuable in emergency medicine and nephrology, where rapid differentiation between prerenal and intrinsic causes of AKI can significantly impact patient management and outcomes.
Healthcare professionals rely on the FENa calculator for its ability to provide objective, quantitative data that supports clinical decision-making in acute care settings. The Bravo Calculator platform offers this advanced FENa calculator with user-friendly features designed for rapid bedside assessment and clinical workflow integration.
How to Use the FENa Calculator
Collect Serum Sodium
Obtain serum sodium level from recent blood chemistry panel (mEq/L)
Collect Urine Sodium
Obtain urine sodium concentration from spot urine sample (mEq/L)
Input Serum Creatinine
Enter serum creatinine level from the same blood sample (mg/dL)
Input Urine Creatinine
Enter urine creatinine concentration from the same urine sample (mg/dL)
Calculate FENa
Click calculate to get instant FENa percentage and interpretation
Interpret Results
Review the diagnostic interpretation and likely causes
FENa Formula Explained
The FENa Equation
Formula Variables
- UNa: Urine sodium concentration (mEq/L)
- SNa: Serum sodium concentration (mEq/L)
- UCr: Urine creatinine concentration (mg/dL)
- SCr: Serum creatinine concentration (mg/dL)
- Result: Expressed as percentage (%)
Physiological Basis
- Measures sodium handling by kidneys
- Reflects tubular function integrity
- Compares sodium to creatinine clearance
- Accounts for filtration rate changes
- Independent of urine concentration
Real-Life Clinical Example
Case Study: Emergency Department AKI Evaluation
Patient Presentation:
- • 72-year-old male with vomiting and diarrhea
- • Baseline creatinine: 1.0 mg/dL
- • Current creatinine: 2.8 mg/dL
- • Clinical signs of dehydration
Laboratory Values:
- • Serum sodium: 138 mEq/L
- • Urine sodium: 15 mEq/L
- • Serum creatinine: 2.8 mg/dL
- • Urine creatinine: 120 mg/dL
FENa Calculation:
FENa = [(15 × 2.8) / (138 × 120)] × 100
FENa = [42 / 16,560] × 100
FENa = 0.25%
Clinical Decision: FENa of 0.25% indicates prerenal azotemia. The patient received IV fluid resuscitation, and creatinine normalized within 48 hours, confirming the diagnosis.
Key Use Cases for FENa Calculator
Emergency Medicine
- • Rapid AKI evaluation
- • Dehydration assessment
- • Sepsis-related kidney injury
- • Drug-induced nephrotoxicity
Critical Care
- • ICU AKI management
- • Hemodynamic monitoring
- • Fluid resuscitation guidance
- • Dialysis decision support
Nephrology
- • AKI differential diagnosis
- • Tubular function assessment
- • Research applications
- • Teaching and education
Expert Tips and Best Practices
Clinical Best Practices
- •Collect samples before diuretic administration
- •Use fresh urine samples (within 2 hours)
- •Ensure simultaneous blood and urine collection
- •Consider clinical context alongside FENa results
Important Limitations
- •Less reliable in chronic kidney disease
- •Affected by diuretic use within 24-48 hours
- •May be elevated in elderly patients
- •Consider FEUrea if diuretics were used
Frequently Asked Questions
What does a FENa less than 1% indicate?
A FENa <1% typically indicates prerenal azotemia, suggesting that the kidneys are functioning normally but are responding to decreased perfusion by conserving sodium. Common causes include dehydration, heart failure, or hepatorenal syndrome.
When should I use FEUrea instead of FENa?
Use FEUrea when the patient has received diuretics within 24-48 hours, as diuretics can artificially elevate FENa. FEUrea <35% suggests prerenal azotemia, while >50% suggests intrinsic renal disease.
Can FENa be used in chronic kidney disease patients?
FENa is less reliable in patients with chronic kidney disease, as baseline tubular dysfunction may result in higher FENa values even in prerenal states. Clinical context becomes even more important in these patients.
What if FENa is between 1-2%?
FENa values between 1-2% are considered indeterminate and require careful clinical correlation. Consider other factors such as urine microscopy, clinical presentation, and response to fluid challenge to determine the cause of AKI.
How quickly should I collect samples for FENa?
Blood and urine samples should be collected simultaneously or as close together as possible, ideally within 1-2 hours. Use fresh urine samples and avoid delays in processing to ensure accurate results.
What other tests complement FENa in AKI evaluation?
Complement FENa with urine microscopy, urine osmolality, BUN/creatinine ratio, and clinical assessment. Consider imaging studies and nephrology consultation for complex cases or when intrinsic renal disease is suspected.
Facts and Figures
Year FENa was first described
Accuracy in differentiating AKI causes
Cutoff value for prerenal azotemia
Of hospitalized patients develop AKI
Clinical Impact Statistics
- • FENa reduces time to correct AKI diagnosis by 40-60% in emergency settings
- • Early differentiation of AKI causes improves outcomes by 25-35%
- • Appropriate fluid management based on FENa reduces hospital stay by 1-2 days
- • FENa-guided therapy decreases need for dialysis by 15-20% in prerenal AKI
Comparison with Other AKI Assessment Tools
| Test | Sensitivity | Specificity | Limitations |
|---|---|---|---|
| FENa | 85-90% | 85-95% | Affected by diuretics, CKD |
| FEUrea | 80-85% | 80-90% | Less affected by diuretics |
| Urine Osmolality | 70-80% | 70-85% | Affected by age, medications |
| BUN/Cr Ratio | 60-75% | 65-80% | Affected by protein intake, GI bleeding |
Recommendation:
Use the FENa calculator as your primary tool for differentiating prerenal from intrinsic AKI, especially when diuretics have not been used. Combine with clinical assessment and consider FEUrea when diuretics have been administered. Bravo Calc provides both FENa and complementary calculators for comprehensive AKI evaluation.
Why Choose Bravo Calc?
Bravo Calc provides the most accurate and user-friendly FENa calculator available online. Our platform combines clinical precision with intuitive design, making acute kidney injury assessment accessible to healthcare professionals in any setting.
Rapid Results
Instant calculations for emergency situations
Clinical Context
Comprehensive interpretation and guidance
Evidence-Based
Based on validated clinical research