Fractional Excretion of Urea (FEUrea) Calculator
Fractional Excretion of Urea (FEUrea) Calculator
Distinguish pre-renal from intrinsic renal injury when diuretics have already been given, using urea instead of sodium clearance.
Fractional Excretion of Urea (FEUrea)
Urea clearance ratioUrine urea 900 mg/dL, serum urea nitrogen 40 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
Formula
- urine urea, serum urea nitrogen (BUN)
- mg/dL, from a single paired sample
- urine creatinine, serum creatinine
- mg/dL
- why urea
- urea reabsorption is proximal, passive and largely unaffected by loop or thiazide diuretics, unlike sodium
Worked example
Urine urea 900 mg/dL, serum urea nitrogen 40 mg/dL, urine creatinine 90 mg/dL, serum creatinine 2.4 mg/dL
900 × 2.4 = 2,160
40 × 90 = 3,600
2,160 ÷ 3,600 × 100 = 60.0%
Above 50% → intrinsic renal pattern
Interpretation
| FEUrea | Pattern | Notes |
|---|---|---|
| < 35% | Pre-renal | Threshold holds even on diuretics |
| 35 – 50% | Indeterminate | Overlap zone |
| > 50% | Intrinsic renal | Impaired proximal tubular reabsorption |
Why FEUrea when FENa fails
The fractional excretion of sodium is unreliable once a patient has received a diuretic, because loop and thiazide agents force sodium excretion directly and swamp any physiological signal. Urea offers a way around this. Most urea reabsorption happens passively in the proximal tubule, driven by the same water reabsorption that concentrates the tubular fluid, and this process is largely untouched by diuretics acting further downstream in the loop of Henle or distal tubule.
That mechanistic independence is what makes FEUrea the preferred index in a patient who is already on furosemide or a thiazide by the time acute kidney injury is being worked up — a common scenario, since diuretics are often started before the differential diagnosis is settled. The thresholds are lower than FENa’s, at 35% and 50% rather than 1% and 2%, reflecting how much more urea is reabsorbed under normal physiology.
FEUrea is not perfect. It performs less well than FENa when diuretics are genuinely absent, and it can be affected by conditions that independently alter urea handling, such as a high protein intake or gastrointestinal bleeding, which raise the urea load presented to the tubule. It should be read as the diuretic-tolerant alternative to FENa, not as a universally superior test.
Frequently asked questions
When should FEUrea be used instead of FENa?
Whenever the patient has already received a loop or thiazide diuretic. Urea reabsorption is largely proximal and passive, so it is far less affected by diuretics than sodium handling is.
What FEUrea suggests a pre-renal cause?
Below about 35%. Between 35% and 50% is indeterminate, and above 50% suggests intrinsic renal injury such as acute tubular necrosis.
Why are the FEUrea cut-offs so much higher than FENa's?
Because under normal physiology roughly half of filtered urea is reabsorbed, compared with over 99% of filtered sodium — the two solutes are handled very differently even in a healthy kidney.
Does anything else affect FEUrea besides renal perfusion?
Yes. A high dietary protein intake or gastrointestinal bleeding raises the urea load and can shift the result, so interpret it alongside the wider clinical picture rather than in isolation.
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References
- 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):2223–9.
- Kaplan AA, Kohn OF. Fractional excretion of urea as a guide to renal dysfunction. Am J Nephrol. 1992;12(1–2):49–54.