FEUrea Calculator — Acute Kidney Injury Cause Differentiation

Enter urine and serum urea nitrogen and creatinine to calculate the fractional excretion of urea (FEUrea) and see whether it points toward prerenal azotemia or intrinsic renal injury — useful even when a patient is on diuretics.

Quick Facts

Formula
FEUrea = (UUN x SCr) / (BUN x UCr) x 100
Any consistent concentration unit works because the units cancel out in the ratio.
Prerenal cutoff
FEUrea ≤ 35%
Suggests prerenal azotemia, and remains useful even after diuretic administration (unlike FENa).
Intrinsic renal cutoff
FEUrea > 50%
Suggests intrinsic renal injury such as acute tubular necrosis (ATN); 35-50% is an indeterminate zone.

Your Results

Calculated
FEUrea
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Fractional excretion of urea
Likely category
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Prerenal vs. intrinsic renal
BUN : Creatinine ratio
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Supporting prerenal marker (>20:1)
Suggested next step
-
Educational guidance, not a diagnosis

Ready

Enter the four lab values, then press Calculate.

How to Use the FEUrea Calculator

The fractional excretion of urea (FEUrea) helps distinguish prerenal azotemia (reduced kidney perfusion, e.g. from volume depletion or heart failure) from intrinsic renal injury (e.g. acute tubular necrosis, ATN) in a patient with acute kidney injury. It is calculated as: FEUrea (%) = (Urine urea nitrogen × Serum creatinine) / (Serum urea nitrogen × Urine creatinine) × 100. Unlike the fractional excretion of sodium (FENa), FEUrea stays informative in patients who have already received diuretics, because roughly half of filtered urea is passively reabsorbed with water in the proximal tubule regardless of diuretic effect on sodium handling.

Where the formula comes from

FEUrea measures what fraction of the urea filtered by the kidney actually appears in the urine, corrected for how concentrated the urine is relative to plasma. Using creatinine (which is freely filtered and not reabsorbed) as the reference marker cancels out urine flow rate, leaving a ratio: (urine urea / serum urea) ÷ (urine creatinine / serum creatinine), multiplied by 100 to express it as a percentage. When kidney perfusion drops (prerenal state), the tubules reabsorb more urea and water to conserve volume, so less urea escapes into urine and FEUrea stays low (typically ≤35%). When the tubules themselves are damaged (intrinsic injury such as ATN), that reabsorption capacity is lost, and a larger fraction of filtered urea passes into the urine, pushing FEUrea above 50%. Values from 35-50% fall into an indeterminate zone and need to be interpreted alongside other findings.

Reading the result correctly

Enter the four labs in the same concentration unit (mg/dL is standard in the US); the ratio is unit-independent as long as urine and serum values use matching units. The calculator also reports the BUN:creatinine ratio as a secondary check — a ratio above roughly 20:1 is a classic (though less specific) sign of prerenal azotemia. No single lab ratio is diagnostic on its own: FEUrea should be interpreted together with the clinical history, physical exam, medication list, urine microscopy/sediment findings, and trend over time. This calculator is an educational tool for understanding the calculation, not a substitute for evaluation by a physician or other qualified clinician.

Frequently Asked Questions

What is the formula for fractional excretion of urea (FEUrea)?
FEUrea (%) = (Urine urea nitrogen × Serum creatinine) / (Serum urea nitrogen × Urine creatinine) × 100. All four values should use the same concentration unit (typically mg/dL) since the units cancel out in the ratio.
What FEUrea value suggests prerenal versus intrinsic acute kidney injury?
A FEUrea of 35% or lower suggests prerenal azotemia (such as volume depletion or reduced renal perfusion), while a FEUrea above 50% suggests intrinsic renal injury such as acute tubular necrosis (ATN). Values between 35% and 50% fall in an indeterminate zone and need additional clinical correlation.
Why use FEUrea instead of fractional excretion of sodium (FENa)?
FENa can be unreliable in patients who have recently received diuretics because diuretics increase sodium excretion regardless of the underlying cause of kidney injury. Urea handling is less affected by loop or thiazide diuretics, so FEUrea remains useful for differentiating prerenal azotemia from intrinsic renal injury in diuretic-treated patients.
Is this FEUrea calculator a diagnosis?
No. FEUrea is one supporting data point among history, exam, medication review, urine microscopy, and other labs. This calculator provides an educational estimate only and is not a substitute for evaluation and diagnosis by a licensed clinician.

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