The Fractional Excretion of Sodium (FENa) Calculator is a crucial tool in clinical nephrology and internal medicine, designed to assess renal tubular function and help differentiate between prerenal azotemia and acute tubular necrosis (ATN).
How to Use the FENa Calculator
Using the Fractional Excretion of Sodium Calculator is simple and takes only a few seconds. Follow these steps:
- Enter the plasma sodium (PNa) in mEq/L.
This is the sodium concentration in the blood plasma. - Enter the plasma creatinine (PCr) in mg/dL.
This indicates kidney filtration efficiency. - Enter the urine sodium (UNa) in mEq/L.
This is the sodium concentration in the urine. - Enter the urine creatinine (UCr) in mg/dL.
This measures how concentrated the urine is. - Click “Calculate FENa.”
The calculator will instantly display the fractional excretion of sodium in percentage.
FENa Formula (Plain Text)
The Fractional Excretion of Sodium (FENa) is calculated using the following formula:
FENa (%) = [(Urine Sodium × Plasma Creatinine) ÷ (Plasma Sodium × Urine Creatinine)] × 100
Where:
- Urine Sodium (UNa) = Sodium concentration in urine (mEq/L)
- Plasma Sodium (PNa) = Sodium concentration in plasma (mEq/L)
- Urine Creatinine (UCr) = Creatinine concentration in urine (mg/dL)
- Plasma Creatinine (PCr) = Creatinine concentration in plasma (mg/dL)
Example Calculation
Let’s use an example to understand the calculation:
Given:
- Urine Sodium (UNa) = 35 mEq/L
- Plasma Sodium (PNa) = 140 mEq/L
- Urine Creatinine (UCr) = 100 mg/dL
- Plasma Creatinine (PCr) = 2.0 mg/dL
Applying the formula:
FENa (%) = [(35 × 2.0) ÷ (140 × 100)] × 100
FENa (%) = (70 ÷ 14,000) × 100
FENa (%) = 0.005 × 100
FENa = 0.5%
Result Interpretation:
A FENa of 0.5% indicates prerenal azotemia, meaning the kidneys are conserving sodium effectively due to decreased blood flow or volume depletion.
Interpretation of FENa Values
| FENa Value (%) | Interpretation | Possible Condition |
|---|---|---|
| < 1% | Sodium retention due to low perfusion | Prerenal azotemia (e.g., dehydration, heart failure) |
| 1–2% | Borderline | Indeterminate or mixed cause |
| > 2% | Sodium wasting due to tubular injury | Intrinsic renal failure (e.g., acute tubular necrosis) |
Clinical Significance
The Fractional Excretion of Sodium helps clinicians determine whether acute kidney injury (AKI) is caused by prerenal (low blood flow to kidneys) or intrinsic renal (damage to kidney tissue) mechanisms.
- Prerenal Azotemia:
The kidneys respond to reduced perfusion by conserving sodium, leading to a FENa < 1%. - Acute Tubular Necrosis (ATN):
In ATN, tubular damage prevents sodium reabsorption, resulting in FENa > 2%.
This differentiation guides appropriate treatment — whether to restore perfusion (in prerenal cases) or manage tubular injury (in intrinsic renal failure).
Factors Affecting FENa Accuracy
While FENa is a useful tool, certain conditions can affect its accuracy:
- Diuretic use:
Increases urinary sodium, leading to falsely elevated FENa.
(In such cases, use Fractional Excretion of Urea instead.) - Chronic kidney disease (CKD):
May result in higher baseline FENa values. - Non-oliguric renal failure:
May give misleading results due to altered sodium handling. - Recent fluid therapy:
Can dilute or concentrate sodium levels, affecting results.
Alternative Diagnostic Measures
When FENa results are unreliable (especially in patients on diuretics), the Fractional Excretion of Urea (FEUrea) is preferred:
Formula (Plain Text):
FEUrea (%) = [(Urine Urea × Plasma Creatinine) ÷ (Plasma Urea × Urine Creatinine)] × 100
A FEUrea < 35% also suggests prerenal azotemia.
Clinical Example
Case 1 – Prerenal AKI
- UNa = 20 mEq/L
- PNa = 138 mEq/L
- UCr = 80 mg/dL
- PCr = 2.0 mg/dL
FENa = [(20 × 2) ÷ (138 × 80)] × 100 = 0.36%
→ Indicates Prerenal Azotemia
Case 2 – Intrinsic Renal Failure
- UNa = 60 mEq/L
- PNa = 140 mEq/L
- UCr = 50 mg/dL
- PCr = 2.0 mg/dL
FENa = [(60 × 2) ÷ (140 × 50)] × 100 = 1.71%
→ Suggests Acute Tubular Necrosis (ATN)
Why Use the FENa Calculator
- Fast & Accurate: Computes complex equations instantly.
- Clinical Reliability: Helps confirm differential diagnoses in AKI.
- Educational Value: Ideal for students and healthcare professionals learning renal physiology.
- Evidence-Based: Based on established nephrology research and validated formulas.
Important Considerations
- FENa is most accurate in oliguric patients with AKI.
- Avoid using FENa immediately after diuretic administration.
- Always interpret results in clinical context with patient history and lab findings.
- It should complement, not replace, other diagnostic tests like urinalysis or renal imaging.
Additional Insights
- FENa is particularly valuable in intensive care or emergency settings, where rapid assessment of kidney function is crucial.
- It reflects the tubular reabsorption of sodium and, therefore, indirectly shows how well the kidneys respond to circulatory changes.
- Combining FENa with urine osmolality can strengthen diagnostic accuracy.
20 Frequently Asked Questions (FAQs)
1. What does FENa stand for?
FENa stands for Fractional Excretion of Sodium, a measure of how much filtered sodium is excreted in the urine.
2. What is the purpose of the FENa Calculator?
It helps determine whether acute kidney injury is prerenal or intrinsic (renal tubular) in origin.
3. What is the formula for FENa?
FENa (%) = [(Urine Na × Plasma Cr) ÷ (Plasma Na × Urine Cr)] × 100
4. What units are required?
All concentrations are measured in mEq/L for sodium and mg/dL for creatinine.
5. What is a normal FENa value?
In healthy individuals, FENa typically ranges from 0.5% to 1%.
6. What does a FENa less than 1% mean?
It usually indicates prerenal azotemia, suggesting the kidneys are conserving sodium.
7. What does a FENa greater than 2% mean?
It suggests acute tubular necrosis (ATN) or intrinsic renal failure.
8. Can diuretics affect FENa results?
Yes, they can falsely elevate urinary sodium, making FENa unreliable.
9. What should I use instead of FENa if the patient is on diuretics?
Use the Fractional Excretion of Urea (FEUrea) test.
10. What is prerenal azotemia?
A condition where decreased blood flow to the kidneys reduces filtration, leading to sodium conservation.
11. What is acute tubular necrosis?
A type of intrinsic renal failure caused by damage to kidney tubule cells.
12. Is FENa useful in chronic kidney disease (CKD)?
It’s less reliable in CKD because sodium handling is often altered chronically.
13. What are typical FENa values in healthy kidneys?
Usually less than 1%.
14. How quickly can I get results using the calculator?
Instantly — results are shown as soon as you input your lab values.
15. What if I input wrong units?
Ensure correct units (mEq/L and mg/dL) for accurate results.
16. Is FENa applicable in all AKI patients?
Best used in oliguric AKI without diuretic interference.
17. Can the calculator be used for pediatric patients?
Yes, but results should be interpreted cautiously with pediatric reference ranges.
18. How often should FENa be measured?
Only when evaluating the cause of AKI — not for routine monitoring.
19. What does a borderline FENa (1–2%) mean?
It can represent mixed prerenal and intrinsic causes of renal injury.
20. Is the FENa Calculator free?
Yes, it’s completely free and available online for clinicians, students, and researchers.
Conclusion
The Fractional Excretion of Sodium (FENa) Calculator is a valuable diagnostic aid for clinicians managing acute kidney injury. By quickly analyzing the relationship between plasma and urine sodium and creatinine levels, this tool distinguishes between prerenal and intrinsic renal causes with remarkable precision.