Fractional Excretion of Urea (FEUrea): Preferred over FENa when diuretics have been given — urea handling is not affected by loop diuretics, making it more reliable in this setting.
Plasma Values
Urine Values
Guidelines & Evidence
Clinical Details
Section 1
When to Use
When to Use
AKI differential (Prerenal vs. ATN) in patients who have received loop or thiazide diuretics within 24 hours.
Cases where FENa is unreliable: diuretics, contrast nephropathy, pigment nephropathy, early ATN.
Suspicion of Hepatorenal Syndrome (HRS) — FEUrea is typically very low.
Key Advantage Over FENa
Diuretics block Na reabsorption in the loop of Henle, falsely elevating FENa above 1% even in pure prerenal states. Urea reabsorption is not affected by diuretics, making FEUrea a more reliable marker when diuretic therapy is active.
In hypovolaemic states, the kidney maximally reabsorbs water and urea in the collecting duct under ADH stimulation, minimising urinary urea loss. In ATN, tubular cell damage impairs this reabsorption regardless of volume status, leading to high fractional excretion of urea in the urine.
Section 3
Pearls/Pitfalls
Accuracy Compared to FENa
FEUrea has sensitivity 86%, specificity 98% for prerenal AKI in patients on diuretics (Carvounis 2002).
Significantly superior to FENa in this population — FENa has only 48% sensitivity under diuretic use.
Most clinical guidelines recommend FEUrea as the first-line test if diuretics have been given.
Nephrology consult if Stage 2–3 AKI or meeting RRT criteria.
Complementary Tools
FENa — Fractional Excretion of Sodium
AKI Staging (KDIGO)
Anion Gap + Delta-Delta
Section 5
Evidence Appraisal
Primary Reference
Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.
Carvounis CP et al. • Kidney Int.. 2002;62(6):2223–9. Landmark study demonstrating superiority of FEUrea over FENa in diuretic-treated patients.
Section 6
Literature
Historical Context
FEUrea was proposed as a clinical tool in the early 2000s as a direct response to the recognised limitation of FENa in the modern era of widespread diuretic use. The landmark Carvounis study in 2002 finally provided the evidence base to make FEUrea a clinically actionable bedside calculation.
