Formula for Calculating Fractional Excretion of Sodium (FeNa)
The formula to calculate fractional excretion of sodium (FeNa) is: [(Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine)] × 100%. 1
Understanding FeNa Calculation
FeNa represents the percentage of filtered sodium that is excreted in the urine. It's calculated using the following components:
- Urine sodium concentration (mmol/L)
- Serum sodium concentration (mmol/L)
- Urine creatinine concentration (mg/dL)
- Serum creatinine concentration (mg/dL)
Clinical Applications of FeNa
Differential Diagnosis of Acute Kidney Injury
- FeNa <1% suggests prerenal causes of acute kidney injury (hypovolemia, hepatorenal syndrome) 1, 2
- FeNa >1% typically indicates intrinsic renal causes such as acute tubular necrosis 1, 3
- In acute tubular necrosis, FeNa is often >3% 3
Sodium Handling Assessment
- FeNa helps assess sodium handling by the kidneys to determine if low urine output is due to volume depletion or intrinsic kidney damage 1
- In cystic fibrosis patients, FeNa can be used to assess the need for sodium supplementation (target range: 0.5-1.5%) 4
- For patients with cirrhosis and ascites, monitoring sodium excretion helps guide diuretic therapy 4
Important Considerations When Interpreting FeNa
Factors Affecting FeNa Values
- Diuretic use can artificially elevate FeNa even in prerenal states 2, 5
- Chronic kidney disease may have altered baseline FeNa values 2
- Sodium intake affects FeNa - higher sodium intake results in higher FeNa values 6
- Glomerular filtration rate influences FeNa - a multiple regression formula based on GFR and sodium excretion can predict over 80% of the variation in FeNa 6
Clinical Context Is Essential
- FeNa must be interpreted in light of the specific clinical setting and other laboratory data 2
- Low FeNa can also be seen in non-volume responsive conditions like acute glomerulonephritis, contrast nephrotoxicity, and early obstruction 2
- Changes in FeNa over time may be more informative than single measurements 7
Alternative Measurements
- Fractional excretion of urea (FEUrea) can be more reliable than FeNa in patients on diuretics 5
- A random "spot" urine sodium/potassium ratio can correlate with 24-hour sodium excretion with approximately 90% accuracy in patients with cirrhosis 4
Remember that while FeNa is a valuable diagnostic tool, it should always be interpreted within the appropriate clinical context and in conjunction with other clinical and laboratory findings.