Carvounis and colleagues prospectively compared FEUrea with FENa during 102 episodes of AKI [ARF] due to prerenal azotemia or ATN. They divided the patients into 3 groups: those with prerenal azotemia (n=50), those with prerenal azotemia treated with diuretics (n=27) and those with ATN (n=25).
Bottom-line: low FEUrea (less than 35%) is a more sensitive and specific index than FENa in differentiating between ARF due to prerenal azotemia and that due to ATN, especially if diuretics have been administered. There are on-line calculators for FEUrea on the internet.
(ps: Nate Hellman had a nice post on FENa in the Renal Fellow Network that is worth looking at also).