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The fractional excretion of sodium (FENa) has traditionally been used to differentiate between the two most common forms of acute renal failure: "prerenal" azotemia and acute tubular necrosis (ATN). A FENa <1% is suggestive of a prerenal cause (e.g. volume depletion), while a FENa > 2% is suggestive of ATN. A prerenal cause is not necessarily volume depletion, as a volume overloaded state with a decreased effective circulating volume may also present this way (e.g. congestive heart failure, cirrhosis) and require different treatment. Furthermore, other conditions including acute glomerulonephritis, rhabdomyolysis, contrast-induced acute renal failure may also present with a low FENa. In contrast, volume depletion with associated nasogastric tube suctioning or vomiting may present with a high FENa, because the consequent bicarbonaturia may lead to sodium loss as the negatively charged bicarbonate prevents reabsorption of the positively charged sodium.
Since the FENa is reliant on the phenomenon that sodium reabsorption is enhanced in the setting of volume depletion, active use of diuretics may elevate the FENa even when volume depletion is present. The fractional excretion of urea or fractional excretion of urea nitrogen (FEUN) has been proposed as an alternative tool as urea excretion is not thought to be promoted by diuretics yet is still retained in volume depleted states. A FEUN <. 35% is suggestive of a prerenal etiology of acute renal failure, while a FEUN > 50% suggests ATN.
fraction excretion of x = (urine x/serum x)/(urine creatinine/serum creatinine))
Multiply by 100 for percentage.
- Carvounis CP, Nisar S, and Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int 62: 2223-9, 2002.