Fractional Excretion of Sodium

In the setting of acute renal failure, a Fractional Excretion of Sodium (Fe_Na) <1% suggests a prerenal state, while a value >1% is suggestive of ATN.

These rules are not absolute, however. Fe_Na 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. Fe_Na 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.