FEUrea Calculator — Fractional Excretion of Urea
Calculate the fractional excretion of urea (FEUrea) from spot urine and serum values to distinguish prerenal acute kidney injury (AKI) from intrinsic renal injury (ATN). FEUrea is preferred over FENa when the patient is receiving loop or thiazide diuretics.
Urea units (serum and urine)
mg/dL
mg/dL
mg/dL
mg/dL
Intrinsic renal (ATN or other parenchymal injury)
- 1
Urine/serum urea ratio
400 ÷ 15 = 26.6667Units cancel — both must be the same (mg/dL or mmol/L). - 2
Urine/serum creatinine ratio
80 ÷ 2 = 40 - 3
FEUrea
(26.6667 ÷ 40) × 100 = 66.7
How does this calculator work?
FEUrea (%) = (Urine_Urea/Serum_Urea) ÷ (Urine_Cr/Serum_Cr) × 100. A result below 35% suggests prerenal AKI; above 50% suggests intrinsic renal injury (ATN). The test is most useful in patients on diuretics where FENa is unreliable. Values 35–50% are indeterminate.
Formula
How this is calculated
The fractional excretion of urea quantifies what fraction of the freely filtered urea is excreted in the urine, analogous to the fractional excretion of sodium (FENa). In prerenal states — volume depletion, heart failure, hepatorenal syndrome — the kidney reabsorbs water and sodium avidly, concentrating the urine; urea reabsorption also increases, resulting in a low FEUrea (< 35%). In intrinsic renal injury such as acute tubular necrosis (ATN), tubular urea reabsorption is impaired and FEUrea exceeds 50%.
The key advantage over FENa is robustness to diuretic therapy. Loop and thiazide diuretics block tubular sodium reabsorption and can raise FENa above 1% even in genuinely prerenal states, leading to misclassification. Urea reabsorption occurs via a separate pathway and is not directly inhibited by common diuretics, preserving the diagnostic value of FEUrea in patients who are already on them.
Values between 35% and 50% are indeterminate — both prerenal and intrinsic injury can produce results in this range. FEUrea must always be interpreted alongside clinical history, fluid status, urine microscopy, and other laboratory findings. It is not a substitute for clinical judgment.
Frequently asked questions
Diuretics block tubular sodium transport, so FENa can be falsely elevated (>1%) even in prerenal AKI, suggesting intrinsic injury when the kidney is actually responding normally to volume depletion. Urea reabsorption uses a separate pathway (UT-A transporters) unaffected by common diuretics, so FEUrea remains a reliable marker of prerenal physiology.
The Carvounis et al. 2002 study and subsequent reviews suggest FEUrea < 35% indicates prerenal AKI (sensitivity ~85%, specificity ~92%), and > 50% indicates intrinsic renal injury. Values of 35–50% are indeterminate. Like all such thresholds, these are guides, not rules.
Yes — as long as both serum and urine urea are expressed in the same units (both mg/dL or both mmol/L), the units cancel in the ratio and FEUrea is identical. Serum and urine creatinine should also share units (both mg/dL or both μmol/L).
TG we-Calculate Editorial Team. (2026). FEUrea Calculator — Fractional Excretion of Urea [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/feurea-calculator
TG we-Calculate Editorial Team. "FEUrea Calculator — Fractional Excretion of Urea." TG we-Calculate. 2026. https://we-calculate.com/calculator/feurea-calculator.
TG we-Calculate Editorial Team, "FEUrea Calculator — Fractional Excretion of Urea," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/feurea-calculator
@misc{wecalculate_feurea_calculator, title = {FEUrea Calculator — Fractional Excretion of Urea}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/feurea-calculator}}, year = {2026}, note = {TG we-Calculate} }
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