Fractional Excretion of Urea (FEUrea)

In patients with acute kidney injury (AKI), it is important to differentiate between states that are considered, “prerenal”, i.e., experience improvement in AKI with increase in effective circulating volume, and intrinsic kidney diseases such as acute tubular injury. Many tools are used to make this distinction including scores that calculate fractional excretion of solutes such as sodium and urea. In patients on diuretic therapy, sodium excretion and thus fractional excretion of sodium (FENa) can be high even in “prerenal” states where this is expected to be low. To overcome this limitation, fractional excretion of urea (FEUrea) is often measured to differentiate prerenal causes of AKI from intrinsic kidney diseases. The evidence for better accuracy of FEUrea is mixed and higher quality studies show low accuracy of this test for the indication of identifying “prerenal” states. Both FENa and FEUrea are considered adjunct tools for identifying a “pre-renal” state and overall clinical picture must be considered rather than relying on single test results. Since cardiorenal and hepatorenal syndromes also have low effective circulating volume, these states have very similar FENa and FEUrea as a “prerenal” state from true volume depletion. Urinary tract obstruction must be excluded using imaging studies when appropriate.