FEUrea

Why use it

Accurate in patients on diuretics (where FENa is unreliable). Differentiates prerenal from intrinsic AKI. Simple spot urine and blood test calculation.

About

The Fractional Excretion of Urea (FEUrea) differentiates prerenal azotemia from intrinsic AKI, and unlike FENa, it remains accurate in patients receiving diuretics. Urea reabsorption is not significantly affected by loop diuretics, making FEUrea the preferred test when FENa is unreliable.

When to use

• AKI workup in patients receiving diuretics
• When FENa results are unreliable due to diuretic use
• Oliguria workup in diuretic-treated patients

Formula

Pearls & pitfalls

• FEUrea < 35% suggests prerenal azotemia even in patients on diuretics.
• FEUrea > 50% suggests intrinsic renal injury (ATN).
• Unlike FENa, diuretics do NOT significantly affect urea handling.
• BUN and urine urea are both measured in mg/dL for this calculation.
• Can use alongside FENa for additional confidence when diuretics have not been given.

Next steps

FEUrea < 35%: Prerenal — trial of IV fluids. FEUrea > 50%: Intrinsic AKI — avoid excessive fluids. FEUrea 35-50%: Indeterminate — clinical correlation needed.

Evidence

Validated by Carvounis et al. (Kidney Int, 2002) in 102 patients with AKI. FEUrea < 35% had sensitivity 85% and specificity 92% for prerenal AKI in patients on diuretics.

References

1. Carvounis CP et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223-2229. — link