- How do you calculate Fractional excretion of sodium (FeNa) and Fractional excretion of urea (FeUrea)?
The fractional excretion of sodium (FENa) and urea (FEUrea) measures the percent of filtered sodium and urea, respectively, that is excreted in the urine.FENa = ((UrineNa * PlasmaCr) / (PlasmaNa * UrineCr)) * 100
FEUrea = ((UUrea * PCr) / (PUrea * UCr)) * 100
- When would you use FeUrea over FeNa?In pre-renal acute renal failure, there is significant reabsorption of filtered Na, resulting in a low urine sodium and fractional excretion of sodium. When patients are exposed to diuretics, there will be renal wasting of Na despite hypovolemia. In this setting, the FEUrea has been shown to be a more reliable discriminator between the pre-renal state and acute tubular necrosis.
- What values of FeNa and FeUrea suggests prerenal azotemia as a cause of elevated creatinine?An FeNa <1% and FeUrea <35% in the setting of acute renal failure suggest a pre-renal state as the etiology.
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 Dec;62(6):2223-9.