Fractional Excretion of Urea (FEUrea)

Calculates the fractional excretion of urea for renal evaluation, particularly useful when diuretics are present.

Nephrology Internal MedicineEvidence-BasedUpdated 2026-08-06

Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.

Clinical Notes

The fractional excretion of urea (FEUrea) expresses the proportion of filtered urea that is excreted rather than reabsorbed: (urine urea ÷ plasma urea) ÷ (urine creatinine ÷ plasma creatinine) × 100. In the setting of intact renal tubular responses, a low value reflects appropriate renal conservation of urea and supports prerenal physiology. FEUrea is used as an adjunct in the assessment of acute kidney injury, most often in patients who have received diuretics. Because diuretics can raise urinary sodium and make FENa unreliable, and because urea reabsorption is generally less affected by loop and thiazide diuretics than sodium handling, FEUrea may retain greater utility in this situation. It remains less widely validated than FENa and is best used as a complement rather than a replacement. Urea handling is influenced by several confounders, including hepatic urea production (liver disease and malnutrition), catabolic states, corticosteroids, and dietary protein intake. FEUrea must never be interpreted alone; it is one component of the AKI evaluation alongside history, volume status, urine microscopy, and other laboratory data, and the result does not by itself establish the cause of kidney injury or constitute a diagnosis.

Calculator Information

Specialty

Internal Medicine

📋 Reference Ranges

Normal Range

< 35% in prerenal azotemia

Reference ranges for this calculator
Classification	Range
Prerenal azotemia	<35.1
Indeterminate	35–50
Intrinsic renal injury (ATN)	≥50

What This Calculator Measures

Estimates the fractional excretion of urea to help distinguish prerenal azotemia from intrinsic renal injury, particularly useful when diuretics make FENa unreliable.

How to Use

Use when the patient is receiving diuretics or FENa is otherwise unreliable.
Obtain paired urine urea, plasma urea, urine creatinine, and plasma creatinine.
Enter all four values.
Review the FEUrea and its suggested category.

How the Calculation Works

Fractional excretion of urea = (urine urea x plasma creatinine) / (plasma urea x urine creatinine) x 100, expressed as a percentage.

Variables Explained

Urine Urea

Urine urea in mg/dL, used in the numerator.

Plasma Urea

Plasma urea in mg/dL, used in the denominator.

Urine Creatinine

Urine creatinine in mg/dL, used in the denominator.

Plasma Creatinine

Plasma creatinine in mg/dL, used in the numerator.

Interpretation Guide

FEUrea < 35% suggests prerenal azotemia, 35–50% is indeterminate, and > 50% suggests intrinsic renal injury such as ATN. Because urea reabsorption is less affected by diuretics than sodium, FEUrea retains diagnostic utility in patients on diuretics.

Clinical Significance

Because diuretics blunt the diagnostic utility of FENa, FEUrea provides a useful complementary test for the prerenal-versus-intrinsic distinction in patients on diuretics.

When It Is Useful

AKI evaluation when diuretics have been given
Complementing FENa in the prerenal-versus-intrinsic distinction
When the BUN/Cr ratio is inconclusive

When It Is Not Useful

As a replacement for FENa when no diuretics are involved
As a standalone diagnostic test
In severe liver disease, where urea production is reduced

Common Mistakes

Using FEUrea in place of FENa when no diuretics have been given, even though FENa is better validated in that setting.
Interpreting the result in severe liver disease, where reduced urea production distorts both plasma and urine urea.
Overlooking corticosteroid use, a catabolic state, or protein intake, all of which alter urea handling and shift the value.

Limitations

Less widely validated than FENa.
Urea handling is influenced by protein intake, catabolic states, and corticosteroids.
Reduced hepatic urea production affects both serum and urine urea.
Thresholds are less well established in some populations.

Worked Example

A 68-year-old woman with heart failure on furosemide has a urine urea of 300 mg/dL, plasma urea 25 mg/dL, urine creatinine 100 mg/dL, and plasma creatinine 1.5 mg/dL.

Inputs:

urineUrea: 300

plasmaUrea: 25

urineCr: 100

plasmaCr: 1.5

Expected Result: FEUrea = (300/25) ÷ (100/1.5) × 100 ≈ 18%. This value below 35% suggests prerenal azotemia.

Evidence & References

Source:: Nephrology
Content updated:: 2026-08

Pépin MN, et al. Clin Invest Med. 2007;30(5):E163-167.

References

Pépin MN, et al. Reassessment of the fractional excretion of urea for the differential diagnosis of acute renal failure. Clin Invest Med. 2007;30(5):E163-167. (Validation Study)

Frequently Asked Questions

When should I use FEUrea instead of FENa?

Use FEUrea when the patient has received diuretics, which can increase urinary sodium and make FENa unreliable.

What does FEUrea > 50% mean?

A FEUrea > 50% suggests intrinsic renal injury such as acute tubular necrosis.

Disclaimer: This calculator is intended for educational and clinical decision support purposes only. FEUrea is a complementary test and must be interpreted alongside FENa, urine studies, and clinical context.