FENa = (Urine Na/Plasma Na)/(Urine Cr/Plasma Cr) x 100
FENa <1%: Pre-Renal/Hepatorenal
FENa >1%: Renal (ATN, etc)
FENa is less accurate without oliguria
In presence of met alkalosis, urinary Cl is more accurate indicator of volume status than urinary Na
Some cases of contrast-induced and rhabdo-related ATN may have low FENa
FE Urea
FE Urea = Urine Urea/Plasma Urea)/(Urine Cr/Plasma Cr)
FE Urea <35%: Pre-Renal/Hepatorenal
References
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; 62:2223-2229
Kaplan AA, Kohn OF. Fractional excretion of urea as a guide to renal dysfunction. Am J Nephrol 1992; 12:49-54