fractional excretion of sodium (FENA)

Introduction

Defined as: excreted Na+/filtered Na+

Indications

• acute renal failure
  • prerenal azotemia
  • acute tubular necrosis
  • obstructive uropathy

Clinical significance

Useful for distinguishing prerenal from acute tubular necrosis or post-renal obstruction as etiology of acute renal failure (ARF):

FENA	renal disease
* < 1%	prerenal vs glomerulophritis*
* > 1%	renal or post-renal#

* diuretic use may result in a high FENA with prerenal azotemia; for these patients, fractional excretion of urea may be helpful

# in early obstructive uropathy, FENA may be low

FENA < 1%

• urinalysis: benign urine -> prerenal azotemia
• urinalysis: dysmorphic erythroctes, RBC casts -> glomerulonephritis

FENA > 1%

• urinalysis: muddy brown granular casts or tubular epithelial casts -> ATN
• urinalysis: RBC, hemoglobinuria, urinary crystals
  • obstructive uropathy

Procedure

Measured as:

(urine Na+/serum Na+) / (urine creatinine/serum creatinine)

More general terms

• urinary indices

Additional terms

• fractional excretion of urea