What is the normal fractional excretion of sodium (FeNa)?

Normal Fractional Excretion of Sodium (FENa)

The normal fractional excretion of sodium (FENa) in healthy individuals is between 0.5% and 1.5%. 1

Understanding FENa

Fractional excretion of sodium (FENa) is a calculated value that represents the percentage of filtered sodium that is excreted in the urine. It serves as an important diagnostic tool in assessing kidney function and differentiating causes of kidney injury.

Normal Values by Population

• Healthy individuals: 0.5-1.5% 1
• Normal subjects with varied salt intake: Usually less than 1%, but may increase with higher salt intake 2

Clinical Significance

FENa is particularly useful in evaluating acute kidney injury and differentiating between:

• Prerenal causes (FENa typically <1%):

  • Volume depletion
  • Decreased effective circulating volume

• Intrinsic renal causes (FENa typically >1% or >3%):

  • Acute tubular necrosis
  • Glomerulonephritis
  • Interstitial nephritis

Calculation Formula

FENa is calculated using the following formula:

FENa = [(Urine sodium × Serum creatinine) / (Serum sodium × Urine creatinine)] × 100

Important Clinical Considerations

Limitations and Exceptions

FENa must be interpreted carefully as several conditions can produce misleading results:

• Low FENa (<1%) despite intrinsic renal injury may occur in:

  • Hepatic or cardiac failure 2
  • Acute glomerulonephritis
  • Contrast nephrotoxicity
  • Burns
  • Acute obstruction
  • Renal transplant rejection
  • Some cases of non-oliguric acute renal failure 3
  • Severe liver dysfunction (sodium-avid state) 3

• High FENa (>1%) despite volume depletion may occur in:

  • Patients receiving diuretics
  • Some patients with chronic renal insufficiency 2

Timing Considerations

The timing of FENa measurement can significantly affect results:

• FENa may be low early in the course of acute kidney injury even when intrinsic renal damage exists 4
• Repeat measurements may show conversion from low to high FENa as acute tubular necrosis evolves 4

Alternative Measurements

When FENa interpretation is complicated (e.g., in patients on diuretics), alternative measurements may be helpful:

• Fractional excretion of urea (FEUrea):

  • Formula: [(Urine urea × Serum creatinine) / (Serum urea × Urine creatinine)] × 100
  • Value <28.16% indicates high probability of pre-renal kidney insufficiency 5
  • Sensitivity of 75% and specificity of 83% for identifying pre-renal causes 5

• Urinary sodium/potassium ratio:

  • A random "spot" urine sodium concentration greater than potassium concentration correlates with 24-hour sodium excretion >78 mmol/day with approximately 90% accuracy 1

Best Practice Recommendations

1. Always interpret FENa in the context of the complete clinical picture
2. Consider repeat measurements if initial FENa results don't align with clinical presentation
3. Use alternative indices like FEUrea when FENa interpretation may be compromised
4. Remember that sodium-avid states (heart failure, cirrhosis, nephrotic syndrome) can produce falsely low FENa values

FENa remains a valuable tool in kidney assessment when interpreted appropriately within the clinical context.