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In the setting of acute renal failure, a Fractional Excretion of Sodium (FeNa) <1% suggests a prerenal state, while a value >1% is suggestive of ATN.
These rules are not absolute, however. FeNa can also be low in acute tubular necrosis accompanied by a chronic pre-renal state (such as 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 injury which limits sodium reabsorption. In patients who are on diuretics, the Fractional Excretion of Urea may be a more useful test.
Interpreting the fractional excretion of sodium. Am J Med 1984 Oct;77(4):699-702.