Fractional Excretion of Urea (FeUrea)

Calculates the fractional excretion of urea, particularly useful for differentiating prerenal from intrinsic AKI in patients on diuretic therapy, where FeNa is unreliable. Urea reabsorption occurs primarily in the proximal tubule and is less affected by loop/thiazide diuretics.

mg/dL
Spot urine urea nitrogen (or urine BUN) in mg/dL. If mmol/L urea, multiply by 2.8 for mg/dL BUN equivalent.
mg/dL
Serum BUN in mg/dL.
mg/dL
Spot urine creatinine in mg/dL.
mg/dL
Simultaneous serum creatinine in mg/dL.

References

  1. Finding the cause of acute kidney injury — Cleveland Clinic Journal of Medicine (2012)
  2. Fractional Excretion of Sodium and Urea are Useful Tools in the Evaluation of AKI — Kidney360/PMC (2023)

Note: Kaplan and Kohn (1992) first described utility in diuretic-treated patients. Carvounis et al (2002) showed 89% of prerenal AKI patients on diuretics had FeUrea <35%. When using SI units, ensure both urea and creatinine are in the same mass or molar units before calculation.

Warnings & Limitations

  • Meta-analysis (2024) shows limited overall utility; sensitivity 66%, specificity 75% for intrinsic AKI at >35% cutoff.
  • In ICU patients with sepsis, FeUrea may perform poorly.
  • Not reliable in elderly due to reduced urea reabsorption capability.
  • Less reliable in patients on osmotic diuretics or with high protein catabolism.
  • Always interpret with clinical context, urinalysis, and other labs.