What is FEUrea and when is it used?

Fractional excretion of urea (FEUrea) = (Urine Urea × Serum Cr) / (Serum Urea × Urine Cr) × 100. It is used to differentiate prerenal azotemia from intrinsic renal disease, particularly when FENa is unreliable due to diuretic use.

How do you interpret FEUrea?

FEUrea 50% suggests intrinsic renal disease (impaired tubular function). Values between 35–50% are indeterminate. Unlike FENa, FEUrea remains reliable in patients receiving diuretics.

Why is FEUrea better than FENa when patients are on diuretics?

Diuretics (especially loop and thiazide diuretics) directly increase urinary sodium excretion, causing FENa to rise above 1% even in prerenal states. Urea reabsorption is not directly affected by most diuretics, so FEUrea maintains its discriminative ability between prerenal and intrinsic renal disease.

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Fractional Excretion of Urea (FEUrea)

Differentiate prerenal from intrinsic AKI — particularly useful in patients on diuretics (unlike FENa).

Inputs

Urine Urea (mg/dL)

Serum BUN (mg/dL)

Urine Creatinine (mg/dL)

Serum Creatinine (mg/dL)

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Enter all values to calculate FEUrea.

FEUrea < 35%Suggests prerenal azotemia

FEUrea > 50%Suggests intrinsic renal disease (ATN)

35–50%Indeterminate — use clinical context

Formula: FEUrea = (Urine Urea × Serum Cr) / (Serum BUN × Urine Cr) × 100

Disclaimer: For educational purposes only. Not a substitute for clinical judgment.

When to use

FEUrea is especially valuable for differentiating prerenal from intrinsic AKI in patients receiving diuretics, which render FENa unreliable. Diuretics increase urinary sodium excretion, falsely elevating FENa, but do not significantly affect urea handling.

Clinical pearls

FEUrea <35% with FENa >1% on diuretics strongly suggests prerenal disease.
FEUrea is not affected by diuretics because urea reabsorption is primarily passive and follows water in the proximal tubule.
Neither FENa nor FEUrea is reliable in CKD, post-obstructive AKI, or contrast nephropathy.

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):2223-2229.
Diskin CJ, et al. Towards an understanding of overt urinary indices. Ren Fail. 2008;30(2):155-166.

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About This Tool

What Is FEUrea?

The fractional excretion of urea (FEUrea) is a calculated measure of the fraction of filtered urea that is excreted in the urine. Like the fractional excretion of sodium (FENa), FEUrea helps differentiate prerenal azotemia from intrinsic acute kidney injury (AKI). However, FEUrea has a critical advantage: it remains reliable in patients receiving diuretics, which increase urinary sodium excretion and render FENa falsely elevated. Since urea reabsorption is primarily passive and follows water in the proximal tubule and collecting duct, it is not directly affected by most diuretic medications.

Calculation and Interpretation

FEUrea is calculated as: (Urine Urea × Serum Creatinine) / (Serum BUN × Urine Creatinine) × 100. A FEUrea <35% suggests prerenal azotemia, as the kidneys are appropriately reabsorbing urea along with water in a volume-depleted state. A FEUrea >50% suggests intrinsic renal disease (typically acute tubular necrosis), where the damaged tubules can no longer efficiently reabsorb urea. Values between 35–50% are indeterminate and require clinical correlation.

Clinical Context

The landmark study by Carvounis et al. (2002) demonstrated that FEUrea maintained its diagnostic accuracy in patients on diuretics, whereas FENa did not. In their cohort, FEUrea <35% had a sensitivity of 85% and specificity of 92% for prerenal AKI, including in the diuretic subgroup. However, like all urinary indices, FEUrea has limitations: it is unreliable in patients with CKD (where baseline tubular function is already impaired), post-obstructive uropathy, contrast nephropathy, and rhabdomyolysis. It should always be interpreted in the context of the full clinical picture, including volume status, medication history, and renal ultrasound findings.

🔑 Clinical Pearls

FEUrea is the preferred urinary index in patients on diuretics — FENa is unreliable in this setting.
Some labs report urine urea nitrogen (UUN) rather than urine urea — ensure you're using the correct value (urine urea = UUN × 2.14 if converting from nitrogen to urea).
A combined approach (FENa + FEUrea) improves diagnostic accuracy compared to either test alone.
In sepsis-associated AKI, FEUrea may be misleadingly low despite intrinsic renal injury — use with caution.

Key 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):2223-2229.
Perazella MA, Coca SG. Traditional urinary biomarkers in the assessment of hospital-acquired AKI. Clin J Am Soc Nephrol. 2012;7(1):167-174.
Steiner RW. Interpreting the fractional excretion of sodium. Am J Med. 1984;77(4):699-702.

Formula last verified: February 2026