FENa & FEUrea Calculator
Accurate fractional excretion using standard formulas and unit conversions.
Last updated on: August 18, 2025
Input values
What are FENa and FEUrea?
FENa quantifies the percentage of filtered sodium excreted in urine. FEUrea quantifies the percentage of filtered urea nitrogen excreted.
They help differentiate prerenal azotemia from intrinsic acute kidney injury in the appropriate clinical setting.
Formulas and units
FENa (%) = (Urine Na × Serum Cr) / (Serum Na × Urine Cr) × 100. Use Na in mmol/L (or mEq/L). Use creatinine in the same unit for urine and serum.
FEUrea (%) = (Urine urea nitrogen × Serum Cr) / (Serum urea nitrogen × Urine Cr) × 100.
Conversions: Creatinine mg/dL = µmol/L ÷ 88.4. BUN mg/dL = Urea mmol/L × 2.801.
Common cutoffs
- FENa < 1% → favors prerenal.
- FENa > 2% → favors intrinsic (e.g., ATN).
- FEUrea < 35% → favors prerenal (especially if on diuretics).
- FEUrea > 50% → favors intrinsic. 35–50% is indeterminate.
Limitations
- Interpret with clinical context and urine microscopy.
- FENa can be misleading with diuretics, CKD, or sepsis.
- FEUrea less reliable in high catabolic states or GI bleeding.
- Ensure units are consistent; mismatched units invalidate results.
This page has been medically reviewed by Dr. Khoulah Attia – PharmD, Immunology Specialist.