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Fractional Excretion of Sodium Calculator

Fractional Excretion of Sodium (FENa)

Understanding Fractional Excretion of Sodium

The Fractional Excretion of Sodium (FENa) is a valuable diagnostic tool used to help determine the cause of acute kidney injury (AKI). It measures the percentage of filtered sodium that is excreted in the urine. This calculation helps differentiate between pre-renal and intrinsic renal causes of acute kidney injury.

What is FENa?

FENa is calculated using the following formula:

FENa = [(Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine)] × 100

Interpretation of Results

FENa < 1%

  • Strongly suggests pre-renal acute kidney injury (seen in 90% of pre-renal cases)
  • Common causes:
    • Volume depletion
    • Decreased cardiac output
    • Decreased effective circulating volume
  • Note: Early acute glomerulonephritis may also show low FENa values

FENa > 1%

  • Strongly suggests acute tubular necrosis (seen in 96-100% of oliguric ATN cases)
  • Common causes:
    • Acute tubular necrosis
    • Established obstructive uropathy
    • Interstitial nephritis

Important Diagnostic Ranges

Additional urinary indices that support pre-renal AKI:

  • Urine osmolality > 500 mOsm/kg H₂O
  • Urine sodium < 20 mEq/L
  • Urine/plasma urea nitrogen ratio > 8
  • Urine/plasma creatinine ratio > 40

Additional urinary indices that support ATN:

  • Urine osmolality < 350 mOsm/kg H₂O
  • Urine sodium > 40 mEq/L
  • Urine/plasma urea nitrogen ratio < 3
  • Urine/plasma creatinine ratio < 20

Clinical Significance

FENa helps clinicians:

  • Differentiate between pre-renal and intrinsic renal causes of AKI
  • Guide appropriate treatment decisions
  • Monitor response to interventions

Important Considerations

The test may be less reliable in:

  • Patients taking diuretics
  • Patients with chronic kidney disease
  • Certain endocrine disorders
  • Contrast nephropathy

When to Calculate FENa

Consider calculating FENa when:

  • Evaluating acute kidney injury
  • Assessing volume status
  • Monitoring response to fluid therapy
  • Investigating unexplained kidney dysfunction

Limitations

FENa has several limitations:

  • Not reliable in patients on diuretics
  • May be affected by certain medications
  • Results can be influenced by underlying conditions
  • Should not be used in isolation for diagnosis

Tips for Accurate Results

  1. Collect specimens properly
  2. Ensure timing of collections is appropriate
  3. Consider patient's medication history
  4. Interpret results in clinical context

Clinical Applications

FENa is particularly useful in:

  • Emergency departments
  • Critical care settings
  • Nephrology practice
  • Internal medicine

Factors Affecting FENa

Several factors can influence FENa results:

  • Medications (especially diuretics)
  • Volume status
  • Underlying kidney disease
  • Hormonal influences
  • Time of specimen collection

Start calculating now to better understand your patient's kidney function!

References:

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
  2. Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50.