The fractional excretion of sodium (FENa) measures the percentage of filtered sodium that is excreted in urine and has traditionally been utilized to differentiate between the two most common forms of acute renal failure: prerenal azotemia and acute tubular necrosis (ATN).

The formula is as listed below

Fractional Excretion of Na (FENa) in % = [ (urine Na) / (serum Na) ] × 100 / [ (urine Cr) / (serum Cr)]

Where Cr in mg/dL; Na in mEq/L.

A FENa <1% is suggestive of pre-renal azotemia and acute glomerulonephritis, while a FENa >1% is seen in most cases of acute tubular necrosis. A FENa <1% may not necessarily be a finding in volume depletion, as a volume overloaded state with a decreased effective circulating volume may also present similarly e.g. congestive heart failure, cirrhosis. Also, the excessive use of diuretics in a euvolemic subject may lead to a prerenal state, often with increased urinary Na and thus increased FENa. In such scenarios, fractional excretion of urea has been utilized in the differential diagnosis of acute renal failure. In the setting of acute renal failure, a low (<1%) fractional excretion of sodium suggests a pre-renal etiology, although other conditions may also present this way.

References:

1. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int . 2002;62(6):2223-9.
2. Espinel CH.The FENa test. Use in the differential diagnosis of acute renal failure. JAMA 1976;236(6):579-81.
3. Singri N, Ahya SN, Levin ML. Acute Renal Failure. JAMA . 2003;289(6):747-751.
4. Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med . 1978;89(1):47-50.