Fractional Excretion of Sodium (FENa) Calculator

1. What is a Fractional Excretion of Sodium (FENa) Calculator?

Definition: The Fractional Excretion of Sodium (FENa) Calculator measures the percentage of filtered sodium that is excreted in the urine, helping to assess kidney function.

Purpose: It helps clinicians differentiate between causes of acute kidney injury (AKI), such as prerenal azotemia (low FENa) versus acute tubular necrosis (high FENa).

2. How Does the Calculator Work?

The calculator uses the following formula:

FENa:
\( \text{FENa (\%)} = \left( \frac{\text{Urine Sodium} \times \text{Creatinine}}{\text{Sodium} \times \text{Urine Creatinine}} \right) \times 100 \)

Unit Conversions (if needed):

Creatinine (Urine and Serum): 1 µmol/L = 0.0113 mg/dL

Steps:

Input Creatinine (mg/dL or µmol/L), Sodium (mmol/L), Urine Creatinine (mg/dL or µmol/L), and Urine Sodium (mmol/L).
Validate inputs (all values must be greater than zero).
Convert Urine Creatinine and Creatinine to mg/dL if provided in µmol/L.
Calculate FENa using the formula.
Display the result as a percentage, rounded to 2 decimal places.

3. Importance of FENa Calculations

Calculating FENa is important for:

Diagnosing Acute Kidney Injury (AKI): A FENa < 1% suggests prerenal causes (e.g., dehydration), while a FENa > 2% suggests intrinsic renal damage (e.g., acute tubular necrosis).
Guiding Treatment: Helps determine appropriate interventions, such as fluid resuscitation for prerenal AKI or other therapies for intrinsic renal injury.
Monitoring Kidney Function: Useful in critical care settings to assess sodium handling by the kidneys in patients with AKI.

4. Using the Calculator

Examples:

Example 1: Creatinine: 1 mg/dL, Sodium: 140 mmol/L, Urine Creatinine: 50 mg/dL, Urine Sodium: 20 mmol/L
- FENa: \( \left( \frac{20 \times 1}{140 \times 50} \right) \times 100 = \left( \frac{20}{7000} \right) \times 100 = 0.29\% \)
Example 2: Creatinine: 88.4 µmol/L, Sodium: 135 mmol/L, Urine Creatinine: 4420 µmol/L, Urine Sodium: 50 mmol/L
- Convert Creatinine: \( 88.4 \times 0.0113 = 1.00 \text{ mg/dL} \)
- Convert Urine Creatinine: \( 4420 \times 0.0113 = 50.00 \text{ mg/dL} \)
- FENa: \( \left( \frac{50 \times 1}{135 \times 50} \right) \times 100 = \left( \frac{50}{6750} \right) \times 100 = 0.74\% \)

5. Frequently Asked Questions (FAQ)

Q: What does a low FENa indicate?
A: A FENa < 1% typically indicates prerenal causes of AKI, such as dehydration or hypoperfusion, where the kidneys are reabsorbing sodium.

Q: What does a high FENa indicate?
A: A FENa > 2% often suggests intrinsic renal damage, such as acute tubular necrosis, where the kidneys cannot reabsorb sodium effectively.

Q: When should FENa not be used?
A: FENa may be less reliable in patients on diuretics, with chronic kidney disease, or with recent changes in sodium intake—consult a healthcare provider.