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Use this calculator to calculate the fractional excretion of sodium.
About this calculator
In the setting of acute renal failure, a Fractional Excretion of Sodium (FENa) of < 1% suggests pre-renal disease, while > 1% is suggestive of acute tubular necrosis (ATN). These rules are not absolute, however.
FENa can be also be low in acute tubular necrosis accompanied by a chronic pre-renal state (such as congestive heart failure or cirrhosis), contrast nephropathy, pigment nephropathy, and even more rarely in acute glomerulonephritis or interstitial nephritis.
FENa can be > 1% despite hypovolemia in the patient on diuretics or those with chronic tubular dysfunction which limits sodium reabsorption. In patients who are on diuretics, the Fractional Excretion of Urea may be a more useful test._
Steiner RW. Interpreting the fractional excretion of sodium. Am J Med 77(4):699-702. (1984)
QxMD Nephrology contributing author: Daniel Schwartz, MD, FRCP(C)