FEUrea (fractional excretion of urea)
Prerenal vs tubular AKI when the patient has received diuretics
Sources last checked: 2026-10-13
FEUrea (fractional excretion of urea)
Prerenal vs tubular AKI when the patient has received diuretics
Derived in 102 episodes of acute renal failure in hospitalised adults. Less reliable in sepsis, older age, osmotic diuresis and high-protein feeding; urea and creatinine must each be in the same unit in plasma and urine.
Interpretation
| ≤ 35 % — prerenal pattern | Carvounis 2002: FEUrea ≤ 35 % separated prerenal azotaemia (with or without diuretics) from ATN better than FENa. |
|---|---|
| > 35 % — not typical of prerenal AKI | Mean FEUrea in the ATN group was 58.6 %; values above 35 % argue against a purely prerenal cause. |
What is the FEUrea (fractional excretion of urea)?
The fractional excretion of urea (FEUrea) is the share of filtered urea excreted in urine. Because urea is handled mainly in the proximal tubule by passive mechanisms, diuretics affect it less than sodium. Carvounis and colleagues (2002) found that FEUrea ≤ 35 % separated prerenal azotaemia — including patients on diuretics — from acute tubular necrosis better than FENa.
When to use it
- Acute kidney injury in a patient who has received loop or thiazide diuretics
- Discordant FENa and clinical picture
- Spot plasma and urine samples taken together
When not to use it
- Osmotic diuresis (hyperglycaemia, mannitol) or very high protein intake
- As a stand-alone test without clinical assessment and urine sediment
How it is calculated
FEUrea (%) = (urine urea × plasma creatinine) / (plasma urea × urine creatinine) × 100
Clinical pearls and limitations
- Derived in 102 episodes of acute renal failure in hospitalised adults. Less reliable in sepsis, older age, osmotic diuresis and high-protein feeding; urea and creatinine must each be in the same unit in plasma and urine.
- In Carvounis’ study, 89 % of prerenal patients on diuretics had FEUrea < 35 % while only 48 % had FENa < 1 %.
- Later studies found weaker performance in sepsis and in older patients.
- Plasma and urine urea must share a unit, and so must plasma and urine creatinine — the units cancel out.
Frequently asked questions
What FEUrea suggests prerenal AKI?
35 % or lower. The ATN group in the derivation study averaged about 59 %.
Why use FEUrea instead of FENa?
Diuretics raise sodium excretion and push FENa up even in prerenal states; urea excretion is much less affected.
Can I use BUN instead of urea?
Yes, as long as plasma and urine are both BUN (or both urea) in the same unit — the ratio is unchanged.
Evidence and guidelines
- Primary source: 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(6):2223-2229.
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA 1976;236(6):579-581.
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