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Fractional Excretion of Urea (FEUrea)

FEUrea Analysis

Urea Extraction Matrix

Serum Profile

Serum Urea (mg/dL)

Serum Creatinine (mg/dL)

Urine Profile

Urine Urea (mg/dL)

Urine Creatinine (mg/dL)

Urea Clearance Model

Enter paired urea and creatinine values to resolve the prerenal fraction.

Guidelines & Evidence

Verified

Last Review: 2026

When to Use

What is FEUrea?

The fractional excretion of urea (FEUrea) is the percentage of filtered urea that is excreted in the urine. It serves as an alternative to FENa for differentiating prerenal azotemia from intrinsic acute tubular necrosis (ATN), particularly in patients who have received diuretics. Loop diuretics (furosemide, bumetanide, torsemide) directly inhibit sodium reabsorption in the thick ascending limb of the loop of Henle, rendering FENa unreliable. Urea reabsorption, however, occurs primarily in the proximal tubule (passive, urea transporters UT-A1, UT-A3) and the collecting duct (regulated by ADH), and is less affected by loop diuretics. FEUrea is calculated using simultaneous serum and urine samples and is expressed as a percentage. Typical thresholds: <35% suggests prerenal, >50% suggests ATN.

Primary Clinical Indications

AKI in patients on diuretics – Primary indication. When FENa is unreliable due to loop diuretics or thiazides, FEUrea provides a more accurate assessment

Heart failure with AKI (cardiorenal syndrome) – Patients on high-dose furosemide often develop AKI; FEUrea helps distinguish prerenal (due to decreased cardiac output) from ATN (due to nephrotoxic agents, sepsis, or prolonged hypoperfusion)

Cirrhosis with AKI (hepatorenal syndrome vs ATN) – HRS is a prerenal state (avid sodium and urea reabsorption); FEUrea <35% supports HRS, while >50% suggests ATN (e.g., from sepsis, nephrotoxins)

CKD with superimposed AKI – Patients with baseline CKD may have elevated FENa (1-3%) even in prerenal states; FEUrea may provide better discrimination (though limited data)

Post-operative AKI – Many post-operative patients receive diuretics for volume management; FEUrea helps differentiate prerenal from intrinsic AKI

When FEUrea may be superior to FENa – In non-oliguric AKI (where FENa may be falsely low), FEUrea may have better sensitivity, though data are limited

When BUN/Cr ratio is elevated (>20:1) – In prerenal states, BUN is reabsorbed disproportionately; FEUrea will be low, consistent with prerenal physiology

Contraindications / Limitations

Requires BUN measurement – Not always available on basic metabolic panel (BMP includes Cr but not BUN). May require ordering a comprehensive metabolic panel (CMP) or separate BUN.

Affected by protein intake – High protein intake increases urea production and may alter FEUrea. Low protein intake (malnutrition, liver disease) decreases urea production, potentially lowering FEUrea even in ATN (false positive for prerenal).

Affected by liver disease – Impaired urea synthesis in cirrhosis reduces BUN, potentially lowering FEUrea (falsely suggesting prerenal).

Affected by steroids – Corticosteroids increase protein catabolism and urea production, potentially elevating BUN and altering FEUrea.

Affected by ADH (vasopressin) – ADH increases urea reabsorption in the collecting duct (via UT-A1, UT-A3), lowering FEUrea (mimicking prerenal). In SIADH, FEUrea may be low even without prerenal state.

Less studied than FENa – Fewer validation studies; thresholds (<35% prerenal, >50% ATN) are based on limited data (n=110 in Carvounis study). Some studies suggest different thresholds (e.g., <35% prerenal, >35-50% indeterminate, >50% ATN).

Not reliable in severe CKD – In advanced CKD (eGFR <30), urea handling is impaired, and thresholds may not apply.

Not useful in acute glomerulonephritis – FEUrea may be low in GN (intact tubules) despite intrinsic renal disease, similar to FENa.

May be normal in early ATN – Like FENa, FEUrea may not become abnormal until tubular injury is established (24-48 hours).

FENa vs FEUrea: Comparison

Feature	FENa (Sodium)	FEUrea (Urea)	Clinical Implication
Primary use	Differentiating prerenal vs ATN (no diuretics, oliguric)	Differentiating prerenal vs ATN (especially with diuretics)	Use FENa as first-line if no diuretics; use FEUrea if diuretics given
Effect of loop diuretics	Markedly increased (falsely suggests ATN)	Minimally affected (urea reabsorption less inhibited)	FEUrea is superior when patient on furosemide, bumetanide, torsemide
Effect of thiazides	Increased (false ATN)	Minimally affected	FEUrea preferred if patient on HCTZ, chlorthalidone
Effect of CKD	Elevated baseline (1-3%), thresholds not applicable	Less affected, but data limited	Both are problematic in CKD; clinical assessment preferred
Effect of protein intake	None (sodium handling independent)	Significant (high protein intake increases urea production, may raise FEUrea)	Interpret FEUrea with caution in patients on high-protein diets or TPN
Effect of liver disease	None	Reduced urea synthesis → lower BUN → lower FEUrea (false prerenal)	FENa may be more reliable than FEUrea in cirrhosis
Effect of ADH (SIADH, post-op)	None (ADH does not directly affect sodium handling)	Increased urea reabsorption → lower FEUrea (false prerenal)	FENa may be more reliable than FEUrea in SIADH or post-operative states with high ADH
Thresholds	<1% prerenal, >2% ATN	<35% prerenal, >50% ATN (gray zone 35-50%)	Wider gray zone for FEUrea (35-50% vs 1-2% for FENa)
Non-oliguric AKI	Less reliable (often low even in ATN)	May be more reliable (limited data)	Consider urine sediment and clinical context
Cost/availability	Routine labs (Na, Cr in urine and serum)	Requires BUN (CMP or BUN test)	FENa can be done from BMP + urine electrolytes; FEUrea requires BUN (CMP or separate test)

Related Scores in Practice

In clinical practice, this assessment is frequently evaluated alongside other validated measures. Depending on the patient's presentation and specific diagnostic requirements, you may also need to utilize the FENa, Bun Creatinine Ratio, Urine Sodium or the Renal Failure Index to formulate a comprehensive care plan.

Last Comprehensive Review: 2026

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