Fractional Excretion of Urea
FEUrea is the diuretic-resistant alternative to FENa for AKI differentiation. <35% prerenal, >50% ATN.
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<35% Prerenal >50% ATN 35-50% Indeterminate
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Why: FENa unreliable with diuretics. FEUrea uses urea (less affected by loop diuretics) for AKI workup.
How: Enter urine/serum urea and Cr. FEUrea = (UUrea×SCr)/(SUrea×UCr)×100.
Run the calculator when you are ready.
Prerenal on Furosemide
CHF patient on loop diuretics with low FEUrea
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ATN Despite Diuretics
Post-surgical ATN with recent furosemide
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Cirrhosis on Diuretics
Hepatorenal vs ATN differentiation
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Indeterminate Zone
Mixed or transitional picture
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No Diuretics - FENa Valid
Comparison case where FENa is preferred
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Sepsis-Associated AKI
Septic shock with diuretic resuscitation
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Urine Values
Serum Values
Diuretic Information
Clinical Context
For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.
🏥 Health Facts
Use when on loop diuretics
— Nephrology
Urea less affected than Na by diuretics
— AKI
What is Fractional Excretion of Urea (FEUrea)?
Fractional Excretion of Urea (FEUrea) is a calculated value that measures the percentage of filtered urea that is excreted in the urine. Unlike FENa, urea reabsorption in the proximal tubule is not directly affected by diuretics, making FEUrea the preferred test for differentiating prerenal azotemia from ATN when the patient has received loop or thiazide diuretics.
Diuretic-Resistant
FEUrea maintains accuracy even with recent loop or thiazide diuretic administration.
Key Advantage:
Proximal tubular urea handling unaffected by diuretics
Clear Cutoffs
Well-established thresholds: <35% prerenal, >50% ATN, with high sensitivity.
Sensitivity:
~90% for prerenal when <35%
Complement to FENa
Use FEUrea when FENa would be unreliable; use FENa when no recent diuretics.
Best Practice:
Calculate both when possible for comparison
How FEUrea Calculation Works
Collect Simultaneous Samples
Obtain spot urine and serum samples at the same time for urea and creatinine.
Measure Four Values
Urine urea (or BUN), urine creatinine, serum urea (BUN), serum creatinine.
Apply FEUrea Formula
FEUrea = (Urine Urea × Serum Cr) / (Serum Urea × Urine Cr) × 100
Interpret Result
<35% = prerenal, 35-50% = indeterminate, >50% = ATN
Compare with FENa if Available
Discordance between FENa and FEUrea supports diuretic effect on FENa.
When to Use FEUrea
Recent Diuretic Use
Primary indication - when patient received loop or thiazide diuretics within 24-48 hours
Heart Failure Patients
CHF patients on chronic diuretics where FENa would be falsely elevated
Cirrhosis + Diuretics
Hepatorenal syndrome assessment in patients on spironolactone and furosemide
ICU Patients
Critical care settings where diuretics are commonly administered
FENa Discordant
When FENa seems inconsistent with clinical picture (may be diuretic effect)
Post-Cardiac Surgery
AKI differentiation after cardiac surgery where diuretics are routine
FEUrea Formula & Interpretation
FEUrea Formula
FEUrea (%) = [(Urine Urea × Serum Cr) / (Serum Urea × Urine Cr)] × 100Uses the same structure as FENa but substitutes urea for sodium
Why Urea is Diuretic-Resistant
Urea reabsorption occurs primarily in the proximal tubule and inner medullary collecting duct through passive diffusion and urea transporters (UT-A1, UT-A3). Loop diuretics act on the thick ascending limb (NKCC2), and thiazides on the distal convoluted tubule (NCC) - neither affects urea handling directly.
Interpretation Thresholds
| FEUrea | Category | Interpretation |
|---|---|---|
| <35% | Prerenal Azotemia | Kidney retaining urea appropriately despite diuretics |
| >50% | Intrinsic Renal (ATN) | Tubular injury with impaired urea reabsorption |
| 35-50% | Indeterminate | May represent transition or mixed etiology |
FEUrea vs FENa Comparison
| Parameter | FENa | FEUrea |
|---|---|---|
| Prerenal cutoff | <1% | <35% |
| ATN cutoff | >2% | >50% |
| Diuretic effect | Falsely elevated | Unaffected |
| Preferred when | No diuretics | Recent diuretics |
Frequently Asked Questions
Why choose FEUrea over FENa?
FEUrea should be used when the patient has received diuretics (especially loop or thiazide) within 24-48 hours. Diuretics block sodium reabsorption, falsely elevating FENa, but do not affect urea handling.
Can I use FEUrea without diuretics?
Yes, FEUrea is valid without diuretics too. However, FENa is more commonly used and validated in that setting. Both can be calculated for comparison.
What about potassium-sparing diuretics?
Potassium-sparing diuretics (spironolactone, amiloride) have less effect on urinary sodium than loop/thiazide diuretics. FENa may still be useful, but FEUrea is safer to use regardless.
What if FENa and FEUrea are discordant?
Discordance (e.g., high FENa with low FEUrea) strongly suggests diuretic effect on FENa. Trust FEUrea in this scenario as it reflects true tubular function.
Does diet affect FEUrea?
High protein diet increases urea production but the FEUrea ratio remains interpretable. Very low protein intake (malnutrition) may affect both serum and urine urea proportionally.
Is FEUrea accurate in GI bleeding?
GI bleeding increases urea production (digested blood = protein load), which elevates serum BUN. This may affect FEUrea interpretation, though the ratio may still be useful.
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