MDCalc

Fractional Excretion of Sodium (FENa)

Determines if renal failure is due to prerenal or intrinsic pathology.

  • Do not use in patients taking diuretics or with known chronic kidney disease, urinary tract obstruction, or acute glomerular disease. Use FEUrea in patients on diuretics.
  • Remember that FENa <1% cannot differentiate hepatorenal syndrome (HRS) from other causes of renal disease.

Result:

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Advice
  • No absolute FENa percentage indicates true “prerenal” disease. Always consider the history, clinical context, physical exam, and current medications.
  • Obtaining repeat FENa or urine studies throughout a patient’s hospital course can give more clinical clues.
  • Non-volume-depleted states with low urine sodium (and consequently low FENa) include: acute glomerulonephritis, cardiorenal syndrome, hepatorenal syndrome, contrast-related nephropathy, and rarely, acute obstruction and early acute interstitial nephritis (AIN) or acute tubular necrosis (ATN).
Management
  • In patients with acute kidney and suspected obstructive uropathy, consider bedside ultrasound to evaluate for hydronephrosis, bladder distention, etc.
  • Empiric Foley catheter placement may also be valid in some settings, though it may also be associated with complications and does not rule out ureteral obstructions above the bladder.
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