FEUrea (fractional excretion of urea) is a calculation that estimates what fraction of filtered urea nitrogen is excreted in the urine. It is used to differentiate pre-renal AKI (kidney hypoperfusion) from intrinsic renal AKI (acute tubular necrosis) — particularly when the patient is on diuretics, which make the conventional FENa unreliable.

When should I use FEUrea instead of FENa?

Use FEUrea when the patient with AKI is on loop or thiazide diuretics. Diuretics increase urinary sodium excretion and falsely raise FENa above 1%, even in pure pre-renal states. FEUrea is not significantly affected by diuretics and provides a more reliable assessment of tubular function in these patients.

How is FEUrea calculated?

FEUrea (%) = (urine BUN × serum creatinine) / (serum BUN × urine creatinine) × 100. Enter all values from a simultaneous spot urine and blood draw. No timed collection is required.

What are the FEUrea cutoffs for pre-renal vs ATN?

FEUrea <35%: suggests pre-renal AKI (avid urea reabsorption, tubules intact). FEUrea >50%: suggests intrinsic AKI (ATN) with impaired tubular reabsorption. Range 35–50%: indeterminate — interpret with full clinical context. These cutoffs have approximately 80–85% sensitivity for pre-renal in diuretic-treated patients.

Is FEUrea useful in heart failure with AKI?

Yes. Decompensated heart failure patients are often on loop diuretics and frequently develop AKI from renal hypoperfusion (cardiorenal syndrome). FENa is unreliable in this group due to diuretic use. FEUrea <35% supports pre-renal physiology, guiding consideration of diuretic adjustment, fluid management, and hemodynamic optimization.

What is the normal FEUrea range?

In normal individuals without AKI, FEUrea is approximately 40–60%, reflecting normal urea excretion relative to filtration. In pre-renal AKI, tubular reabsorption increases and FEUrea falls below 35%. In ATN, tubular damage impairs reabsorption and FEUrea typically rises above 50%.

How does FENa compare to FEUrea?

FENa <1% is the gold standard for pre-renal AKI diagnosis when no diuretics are present. Its high sensitivity and specificity in non-diuretic patients make it the first-line test. FEUrea is preferred in diuretic-treated patients where FENa is unreliable. Both tests are complementary — using both together in diuretic-treated patients can improve diagnostic accuracy.

What is pre-renal AKI?

Pre-renal AKI is acute kidney injury caused by reduced renal perfusion — from dehydration, heart failure, sepsis, hemorrhage, or any state of decreased effective circulating volume. The tubules remain intact and respond appropriately by reabsorbing sodium and water. It is potentially reversible with restoration of kidney perfusion and distinguishable from ATN by biomarkers like FENa and FEUrea.

Acute tubular necrosis (ATN) is intrinsic kidney injury caused by direct tubular cell damage from ischemia, nephrotoxins, or sepsis. Unlike pre-renal AKI, the tubules are injured and cannot concentrate urine or reabsorb sodium normally. ATN is characterized by impaired tubular function, urinary muddy brown casts, and higher FENa and FEUrea values. Recovery depends on severity.

How reliable is FEUrea?

FEUrea has approximately 80–85% sensitivity for pre-renal AKI in diuretic-treated patients (Carvounis et al., 2002). Its performance can be reduced in patients with high-protein intake, GI bleeding, or advanced CKD. No single biomarker perfectly distinguishes pre-renal from ATN. FEUrea should always be interpreted alongside urinalysis, clinical volume assessment, and response to treatment.

Is FEUrea reliable in CKD?

FEUrea interpretation is more challenging in advanced CKD because baseline tubular reabsorptive function is reduced. Even in a pre-renal state, a CKD patient may not reabsorb urea as efficiently, potentially pushing FEUrea above the 35% threshold. Interpret results with awareness of the patient's baseline eGFR, and use clinical context and response to volume challenge alongside the calculation.

Online Medical Tools — FEUrea Calculator

Printed on 9/2/2026

For informational purposes only. This is not medical advice.

Nephrology

FEUrea Calculator

FEUrea estimates the fraction of filtered urea excreted in urine. It is often used in acute kidney injury when patients are receiving diuretics, because FEUrea is generally less affected by natriuretic therapy than FENa.

Formula: FEUrea (%) = (Urine Urea x Serum Cr) / (Serum Urea x Urine Cr) x 100.

Urine Urea Nitrogen

mg/dL

Serum Urea Nitrogen (BUN)

mg/dL

Urine Creatinine

mg/dL

Serum Creatinine

mg/dL

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How It Works

Collect simultaneous spot urine and blood samples

Obtain a spot urine and a simultaneous blood sample to measure urine urea nitrogen (BUN), serum urea nitrogen (BUN), urine creatinine, and serum creatinine. No timed collection is required — a spot sample is sufficient.

Apply the FEUrea formula

FEUrea (%) = (urine BUN × serum creatinine) / (serum BUN × urine creatinine) × 100. This calculates the fraction of filtered urea that is excreted in urine, expressed as a percentage.

Interpret: FEUrea below 35% suggests pre-renal; above 50% suggests intrinsic AKI

FEUrea <35% indicates avid urea reabsorption consistent with renal hypoperfusion (pre-renal AKI). FEUrea >50% suggests tubular damage (ATN) with impaired reabsorption. The range 35–50% is indeterminate. FEUrea is more reliable than FENa in diuretic-treated patients.

Who Uses the FEUrea Calculator

AKI Workup in Patients on Diuretics

Nephrologists, hospitalists

FEUrea is the preferred alternative to FENa when a patient with AKI is on loop or thiazide diuretics. Diuretics dramatically increase urinary sodium excretion, falsely elevating FENa and suggesting intrinsic AKI even in purely pre-renal states. FEUrea avoids this confound.

Distinguishing Pre-renal vs ATN in Heart Failure

Cardiologists, nephrologists

Decompensated heart failure patients are often on loop diuretics and develop AKI from renal hypoperfusion. FENa is unreliable in this group. FEUrea <35% supports pre-renal (cardiorenal syndrome type 1), while >50% suggests coexisting ATN from renal congestion or ischemia.

AKI in Chronic Diuretic Users

Internal medicine, nephrology

Patients on chronic thiazide or loop diuretics for hypertension, cirrhosis, or edema frequently develop AKI from dehydration or infection. FEUrea provides a diuretic-resistant assessment of tubular function to guide fluid resuscitation decisions.

When FENa is Unreliable

Emergency physicians, intensivists

Beyond diuretics, FENa can be falsely low or high in many conditions (contrast nephropathy, myoglobinuria, early obstruction). In these settings, FEUrea may provide complementary information when combined with clinical context, urinalysis, and renal ultrasound.

Rapid Bedside AKI Triage

Hospitalists, internal medicine residents

FEUrea requires only a spot urine and blood sample, results are available within hours, and the calculation is simple. It provides an objective data point to guide initial AKI management — fluid challenge vs. watchful waiting — in diuretic-treated patients.

Pro Tips

FEUrea preferred over FENa when patient is on diuretics

Loop diuretics (furosemide, bumetanide) and thiazides increase tubular sodium excretion, falsely raising FENa above 1% even in pure pre-renal states. FEUrea is largely unaffected because urea reabsorption is passive and not directly driven by the sodium transporters blocked by diuretics.

Urea is not actively secreted by tubules

Unlike sodium (which is actively reabsorbed), tubular urea transport is largely passive. This means diuretic-induced natriuresis does not directly alter urea reabsorption in the same way. FEUrea therefore better reflects the overall tubular reabsorptive capacity independent of diuretic effect.

FEUrea has limitations with high-protein intake and GI bleeding

High dietary protein intake increases urea production beyond kidney excretion, raising serum BUN. GI bleeding provides a protein load from digested blood, also raising BUN. Corticosteroids are catabolic and increase urea generation. In these settings, high serum BUN relative to urine BUN can make FEUrea appear falsely low (suggesting pre-renal) even in ATN.

FEUrea below 35% has approximately 80% sensitivity for pre-renal in diuretic users

The original Carvounis et al. (2002) study found FEUrea <35% had 85% sensitivity and 92% specificity for pre-renal AKI in diuretic-treated patients. Subsequent studies have generally supported its utility, though performance varies by clinical context and patient population.

Always confirm with clinical context

FEUrea is one data point, not a standalone diagnosis. Combine with: urine output trends, blood pressure and volume status exam, response to fluid challenge, urinalysis microscopy (muddy brown casts = ATN), renal ultrasound (obstruction), and medication review (nephrotoxins).

FEUrea can be above 35% in pre-renal with coexisting CKD

Patients with CKD have impaired tubular reabsorptive capacity at baseline. Even in a pre-renal state, their tubules may not concentrate urine as effectively, leading to a higher FEUrea than expected for pure pre-renal. Interpret in the context of baseline renal function.

FENa below 1% remains gold standard when no diuretics are involved

In patients NOT on diuretics, FENa <1% is a well-validated predictor of pre-renal AKI with high sensitivity and specificity. FEUrea should be used as the primary test only when diuretics are present. When neither drug is given, both FENa and FEUrea can be used together for complementary assessment.

Check urine osmolality for additional confirmation

Urine osmolality >500 mOsm/kg strongly supports pre-renal physiology and tubular concentrating ability. Combined with FEUrea <35%, this provides a more robust picture of pre-renal AKI. In ATN, urine osmolality is typically <350 mOsm/kg due to tubular injury impairing concentrating ability.

BUN:Cr ratio adds complementary diagnostic value

A BUN:creatinine ratio >20 is an additional marker of pre-renal physiology (or upper GI bleeding). Combining the BUN:Cr ratio with FEUrea strengthens the pre-renal diagnosis. However, GI bleeding can elevate BUN:Cr even in ATN, so always verify the clinical story.

Sodium restriction and HF exacerbation can give low FENa even on diuretics

In severe heart failure with intense neurohormonal activation (RAAS, ADH), avid sodium retention can overcome diuretic natriuresis, resulting in low FENa despite diuretic use. This makes FEUrea particularly valuable in this subgroup, though even FEUrea can be borderline in advanced cardiorenal syndrome.

Common Questions About Your Results

This is clinically difficult. Pre-renal and ATN exist on a continuum — prolonged pre-renal leads to ischemic ATN. Early ATN may show borderline FEUrea (35–50%). Look for supporting features: urinalysis with muddy brown casts suggests ATN; bland urinalysis, rapid response to fluid challenge, and high urine osmolality support pre-renal. Serial assessments and response to volume challenge are more informative than any single lab value.

Evidence-Based Methodology

Fractional excretion of urea was proposed as an alternative to FENa for patients on diuretics by Carvounis et al. (Am J Kidney Dis 2002), who showed FEUrea <35% had 85% sensitivity and 92% specificity for pre-renal AKI in diuretic-treated patients. Subsequent studies have validated its utility in decompensated heart failure patients (Steinhausen et al., NDT 2008). FEUrea limitations in high-protein states are reviewed in Diskin (NDT 2010).

Clinical Content Trust

Last reviewed:: April 21, 2026
Guideline version:: General evidence framework v2026.04
Source set version:: Primary-source set v1

How to Interpret Your Result

Lower FEUrea values are more consistent with pre-renal kidney hypoperfusion, while higher values suggest intrinsic renal tubular dysfunction in the proper clinical setting.

When to Use This Tool

Use FEUrea during AKI evaluation when the patient is on loop or thiazide diuretics and FENa may be misleading.

Limitations

Thresholds are not absolute and performance varies by population and timing. FEUrea should be interpreted with full clinical context rather than used as a standalone diagnostic test.

Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider with questions about your health.

Changelog

April 21, 2026 · trust-baseline
Clinical trust metadata enabled for this tool page with structured review/version fields.

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Frequently Asked Questions

Diuretics block sodium transporters in the tubule, directly increasing sodium excretion into urine. This raises FENa regardless of kidney perfusion status. Urea reabsorption is largely passive and occurs by different mechanisms — it follows water reabsorption passively and is not directly controlled by the same transporters. FEUrea is therefore largely unaffected by diuretic-induced natriuresis.

Online Medical Tools — FEUrea Calculator

FEUrea Calculator

How It Works

Collect simultaneous spot urine and blood samples

Apply the FEUrea formula

Interpret: FEUrea below 35% suggests pre-renal; above 50% suggests intrinsic AKI

Who Uses the FEUrea Calculator

AKI Workup in Patients on Diuretics

Distinguishing Pre-renal vs ATN in Heart Failure

AKI in Chronic Diuretic Users

When FENa is Unreliable

Rapid Bedside AKI Triage

Pro Tips

FEUrea preferred over FENa when patient is on diuretics

Urea is not actively secreted by tubules

FEUrea has limitations with high-protein intake and GI bleeding

FEUrea below 35% has approximately 80% sensitivity for pre-renal in diuretic users

Always confirm with clinical context

FEUrea can be above 35% in pre-renal with coexisting CKD

FENa below 1% remains gold standard when no diuretics are involved

Check urine osmolality for additional confirmation

BUN:Cr ratio adds complementary diagnostic value

Sodium restriction and HF exacerbation can give low FENa even on diuretics

Common Questions About Your Results

Evidence-Based Methodology

Clinical Content Trust

How to Interpret Your Result

When to Use This Tool

Limitations

Changelog

Related Tools

FENa Calculator

BUN/Cr Ratio

eGFR Calculator

Further Reading

A Comprehensive Guide to Nephrology: Kidney Assessment Tools and Renal Health

Frequently Asked Questions