When to use FeUrea over FeNa?

When trying to determine the etiology of AKI (pre-renal, renal, post-renal) FeNa is often checked. It would be expected in Pre-renal to be <1%, but this is NOT the case in patients using diuretics. there will be renal wasting of sodium (Na) despite hypovolemia and be high regardless of the pathologic process. In this setting, FeUrea is a more reliable indicator of pre-renal vs. renal. [1]

References

1. Carvounis, CP et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.  Kidney Int. 2002 Dec;62(6):2223-9.