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: 2026-07-17

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)

How it Works

FEUrea Formula (Carvounis 2002)

FEUrea (%) = (Urine Urea × Serum Creatinine) / (Serum Urea × Urine Creatinine) × 100 Where: • Urine Urea = spot urine urea concentration (mg/dL) • Serum Creatinine (SCr) = serum creatinine (mg/dL) • Serum Urea = blood urea nitrogen, BUN (mg/dL) • Urine Creatinine (UCr) = spot urine creatinine (mg/dL) Example (Prerenal AKI, on furosemide): Urine urea = 400 mg/dL, SCr = 2.0 mg/dL, BUN = 30 mg/dL, UCr = 80 mg/dL FEUrea = (400 × 2.0) / (30 × 80) × 100 = (800) / (2,400) × 100 = 0.333 × 100 = 33.3% Example (ATN, on furosemide): Urine urea = 600 mg/dL, SCr = 3.0 mg/dL, BUN = 50 mg/dL, UCr = 40 mg/dL FEUrea = (600 × 3.0) / (50 × 40) × 100 = (1,800) / (2,000) × 100 = 0.9 × 100 = 90%

Interpretive Thresholds (Carvounis 2002, KDIGO)

FEUrea (%)	Interpretation	Likely Diagnosis	Physiologic Basis	Action
<35%	Prerenal azotemia (avid urea reabsorption)	Volume depletion, decreased cardiac output, hepatorenal syndrome	In prerenal states, reduced renal blood flow and increased ADH enhance urea reabsorption in proximal tubule and collecting duct. FEUrea <35% indicates appropriate conservation.	Volume resuscitation (unless contraindicated). Treat underlying cause. Monitor response (urine output, Cr, FEUrea should rise toward normal).
35-50%	Indeterminate (gray zone)	Early ATN, CKD with prerenal, recovery phase, diuretic effect (mild), mixed disorders	Partial impairment of urea reabsorption but not complete loss. May represent transitional state between prerenal and ATN.	Repeat FEUrea in 4-6 hours. Assess urine sediment (granular casts suggest ATN). Consider FENa if no diuretics. Clinical correlation essential.
>50%	Intrinsic renal (ATN) – or established tubular injury	Acute tubular necrosis (ischemic, toxic), sepsis-associated AKI, post-obstructive AKI (recovery phase), severe tubulointerstitial nephritis	Impaired urea reabsorption due to tubular damage. Loss of proximal tubule urea transporters (UT-A1, UT-A3) and collecting duct responsiveness to ADH.	Avoid volume overload. Remove nephrotoxins. Treat underlying cause. Monitor for complications (hyperkalemia, acidosis, uremia). Nephrology consult if severe or not improving.

Modified Thresholds for Special Populations

Population	Prerenal Threshold	ATN Threshold	Reason for Modification	Alternative
Cirrhosis	<25-30% (lower due to reduced urea synthesis)	>40-45%	Liver disease reduces BUN production, lowering FEUrea baseline. Thresholds may need adjustment; limited data.	FENa (if no diuretics) or clinical assessment. Hepatorenal syndrome (HRS) typically has FEUrea <35% (prerenal).
High protein intake / TPN	<35% (unchanged)	>50% (unchanged, but may be higher due to increased urea production)	High protein increases urea production, potentially raising FEUrea even in prerenal states? Actually, both urine urea and BUN rise proportionally, so FEUrea may be unchanged. But if tubular function impaired, FEUrea may rise more.	Clinical assessment; repeat FEUrea after 4-6 hours if indeterminate.
CKD (eGFR <30)	Limited data; prerenal FEUrea may be <35%	ATN FEUrea may be >50% but not well studied	CKD affects urea handling; thresholds not well validated. Baseline FEUrea may be elevated due to reduced functional nephrons.	FENa also unreliable; use clinical judgment, urine sediment, and trend in Cr.
SIADH	May be falsely low (<35%) even without prerenal state	Not applicable	ADH increases urea reabsorption via UT-A1/UT-A3, lowering FEUrea. A low FEUrea in SIADH does NOT indicate prerenal AKI.	FENa is more reliable in SIADH (typically >1% due to mild natriuresis). Use FENa instead of FEUrea.
Post-obstructive AKI	Variable (early obstruction may have low FEUrea; post-obstruction diuresis has high FEUrea)	>50% common in post-obstructive diuresis	After relief of obstruction, there is a urea and sodium diuresis (impaired reabsorption). High FEUrea does not necessarily indicate ATN.	Clinical context (relief of obstruction, diuresis). Manage fluids carefully (replace urine output).
Neonates/infants	Limited data; <30% may be prerenal	>40% may indicate ATN	Immature tubular function; urea handling differs from adults.	Use clinical assessment; consult pediatric nephrology.

Clinical Pearls

Critical Pearl #1: FEUrea is Most Useful When FENa is Unreliable (Diuretics)

The primary indication for FEUrea is AKI in a patient who has received diuretics. Loop diuretics (furosemide, bumetanide, torsemide) inhibit sodium reabsorption in the thick ascending limb, causing FENa to rise >2% even in prerenal states. FEUrea remains relatively unaffected because urea reabsorption occurs mainly in the proximal tubule (passively, via urea transporters) and collecting duct (regulated by ADH), not in the thick ascending limb. Clinical scenario: A 70-year-old with heart failure on furosemide 80 mg daily presents with AKI (Cr 2.5 from 1.2). Urine output is low (300 mL/day). Urine sodium is 45 mEq/L (elevated). FENa = 2.8% (suggests ATN). But the patient has no other risk factors for ATN (no contrast, no aminoglycosides, stable blood pressure). FEUrea is calculated: urine urea 450 mg/dL, BUN 55 mg/dL, SCr 2.5, UCr 75 mg/dL. FEUrea = (450 × 2.5)/(55 × 75) × 100 = (1125)/(4125) × 100 = 27% (<35%, prerenal). Interpretation: Prerenal AKI due to worsening heart failure (decreased effective arterial volume), despite furosemide-induced natriuresis. Treatment: Optimize heart failure therapy (increase diuretics? Paradoxically, the patient needs better perfusion, not more diuresis. In this case, vasodilators (nitroglycerin, nitroprusside) or inotropes (dobutamine) may improve cardiac output. Discontinue or reduce furosemide if not essential. Key takeaway: In any patient on diuretics with AKI, if FENa is >2% (suggesting ATN), confirm with FEUrea before concluding ATN. A low FEUrea (<35%) suggests prerenal, and fluids or hemodynamic optimization may be appropriate.

Critical Pearl #2: Low FEUrea in Cirrhosis May Be Falsely Low (Hepatorenal Syndrome vs ATN)

In cirrhosis, FEUrea must be interpreted with caution because liver disease reduces urea synthesis. A patient with cirrhosis may have a low BUN (e.g., 10 mg/dL) due to impaired hepatic function. In the FEUrea calculation, the denominator (BUN) is low, which increases FEUrea (makes it look higher). Actually, lower BUN decreases the denominator, making FEUrea larger. Let me recalc: FEUrea = (Uurea × SCr)/(BUN × UCr). If BUN is low (e.g., 10 vs normal 30), the denominator decreases, so FEUrea increases (falsely suggests ATN). Example: Cirrhosis patient with HRS (prerenal state). Urine urea 400, SCr 2.0, BUN 12 (low due to liver disease), UCr 100. FEUrea = (400 × 2)/(12 × 100) × 100 = (800)/(1200) × 100 = 66% (falsely suggests ATN). Solution: Use FENa in cirrhotics if no diuretics (FENa <1% supports HRS). If diuretics are present (common in ascites management), FEUrea may still be helpful but thresholds may need adjustment (some experts use <25% for prerenal). Better approach: clinical assessment, urine sodium (UNa <20 mEq/L supports HRS even with diuretics? Not reliable). Renal consult often needed.

Critical Pearl #3: High FEUrea in SIADH Does NOT Indicate ATN

In SIADH (syndrome of inappropriate antidiuretic hormone), ADH is elevated, which increases urea reabsorption in the collecting duct (via UT-A1, UT-A3). This results in a low FEUrea (often <35%), mimicking a prerenal state. However, SIADH patients are typically euvolemic (or hypervolemic if severe hyponatremia). A low FEUrea in SIADH does NOT indicate prerenal AKI and should not be treated with fluids. Clinical scenario: A 65-year-old with small cell lung cancer presents with hyponatremia (Na 118), BUN 8, Cr 0.8 (normal), urine osmolality 600, urine sodium 60. FEUrea = (urine urea 500 × SCr 0.8)/(BUN 8 × UCr 50) × 100 = (400)/(400) × 100 = 100% (high, suggests ATN). But Cr is normal (0.8), so no AKI! FEUrea is elevated because BUN is very low (8) due to dilution (SIADH causes water retention, low BUN). The formula is not useful when Cr is normal (no AKI). Key point: FEUrea (and FENa) are designed for AKI (rising Cr). In SIADH without AKI, FEUrea may be low or high depending on BUN and hydration. Do NOT use FEUrea to assess prerenal status in patients with normal Cr.

Critical Pearl #4: FEUrea May Be Normal in Early ATN (Like FENa)

Both FENa and FEUrea rely on established tubular injury to become abnormal. In very early ATN (first 12-24 hours after insult), tubular function may be partially preserved, and FEUrea may be <35% (prerenal pattern) despite evolving injury. This is particularly true in non-oliguric ATN (e.g., aminoglycoside toxicity, contrast nephropathy). Clinical implication: A single FEUrea <35% does NOT rule out ATN. If clinical suspicion is high (exposure to nephrotoxin, sepsis, hypotension), repeat FEUrea in 12-24 hours. A rising FEUrea (e.g., from 25% to 60%) indicates progression from prerenal to intrinsic ATN. Additionally, urine sediment (muddy brown granular casts) may appear before FEUrea becomes abnormal. Management: If patient has risk factors for ATN (contrast, aminoglycosides, vancomycin, sepsis) and FEUrea is <35% but oliguric, a fluid challenge may be attempted (if no volume overload). If no response (urine output remains low, Cr continues to rise), suspect early ATN despite low FEUrea. Remove nephrotoxins, optimize hemodynamics, and monitor.

Common Pitfalls in FEUrea Interpretation

Using FEUrea when BUN is not available – FEUrea requires BUN (not just Cr). If only BMP is ordered (Cr but no BUN), cannot calculate FEUrea. Order CMP or separate BUN.

Using FEUrea in patients with normal renal function – FEUrea is only meaningful in AKI (rising Cr). In stable patients (Cr normal), FEUrea may vary widely and does not indicate prerenal or ATN.

Ignoring the effect of protein intake – High protein diets (bodybuilders, patients on TPN) increase urea production. FEUrea thresholds may not apply; use clinical judgment.

Ignoring the effect of liver disease – Cirrhosis reduces BUN production, falsely elevating FEUrea (makes ATN look more likely). Interpret with caution; use FENa if no diuretics.

Ignoring the effect of ADH – In SIADH, FEUrea may be low (mimicking prerenal) even without AKI. Do not misinterpret as prerenal in euvolemic patients.

Using FEUrea in non-oliguric AKI – FEUrea may be low (<35%) in non-oliguric ATN (e.g., aminoglycosides). Do NOT be falsely reassured; check urine sediment and clinical context.

Assuming FEUrea thresholds are as robust as FENa – FEUrea has a wider gray zone (35-50%) and fewer validation studies. An FEUrea of 40% is truly indeterminate; additional tests (urine sediment, repeat FEUrea) are needed.

Using FEUrea without simultaneous serum and urine samples – As with FENa, serum and urine samples must be drawn concurrently (within 1-2 hours). Using a serum Cr from 12 hours prior invalidates the calculation.

Failing to correlate with urine sediment – FEUrea is a functional test; urine sediment provides structural information. Granular casts (ATN) are specific for tubular injury even if FEUrea is low (early ATN). Hyaline casts alone suggest prerenal, supporting low FEUrea.

Over-relying on FEUrea when clinical picture is clear – If a patient has clear evidence of ATN (nephrotoxin exposure, muddy brown casts, oliguria, hyperkalemia, acidosis), FEUrea >50% confirms but <35% does not exclude ATN. Treat ATN regardless of FEUrea.

Calculating FEUrea Step-by-Step (Practical Guide)

Next Steps

Step-by-Step Clinical Action Based on FEUrea

Clinical Scenarios and FEUrea Interpretation

Scenario	FEUrea Result	Likely Diagnosis	Management
Heart failure on furosemide, AKI (Cr rise 1.0→2.0), oliguric, JVP elevated, SBP 110	25% (<35%, prerenal pattern)	Prerenal AKI due to worsening heart failure (decreased cardiac output). FENa would be >2% (diuretic effect, misleading).	Optimize heart failure therapy (vasodilators, inotropes). Reduce or hold furosemide if not essential. Do NOT give fluids (volume overload). Consider ultrafiltration if refractory.
Sepsis, AKI (Cr rise 0.8→2.5), oliguric, MAP 65 on norepinephrine, received furosemide 40 mg 12 hours ago	75% (>50%, ATN pattern)	Sepsis-associated AKI (ATN). FENa would be unreliable due to furosemide, but FEUrea >50% confirms ATN.	Treat sepsis (antibiotics, source control). Avoid nephrotoxins. Maintain MAP >65 (vasopressors). Monitor for need for renal replacement therapy (RRT). Nephrology consult.
Cirrhosis, ascites, AKI (Cr 1.2→2.5), on spironolactone (not loop diuretic), SBP 90, JVP flat	FEUrea 22% (<35%)	Hepatorenal syndrome (HRS) – prerenal pattern. FENa also low (<1%), but FEUrea confirms prerenal.	IV albumin 1 g/kg x2 days, vasoconstrictors (midodrine + octreotide, or terlipressin if available). Avoid large-volume paracentesis without albumin. Nephrology consult.
Post-cardiac surgery, AKI (Cr 1.0→2.2), oliguric, no diuretics	FEUrea 65% (>50%)	Ischemic ATN (post-cardiopulmonary bypass). FENa would also be >2%.	Supportive care (avoid overload, remove nephrotoxins). Monitor for hyperkalemia. Renal replacement therapy if severe.
Contrast-induced AKI (post-cath), Cr 1.0→2.5, non-oliguric, no diuretics	FEUrea 32% (<35%)	Contrast-induced nephropathy often presents with low FEUrea (non-oliguric ATN variant).	Do NOT give fluids (already given peri-procedure). Avoid further nephrotoxins. Most cases resolve spontaneously in 5-7 days. FEUrea may be misleading; rely on clinical context.
Aminoglycoside toxicity (gentamicin), Cr 1.0→2.2, non-oliguric, no diuretics	FEUrea 28% (<35%)	Non-oliguric ATN (aminoglycosides). FEUrea often low despite tubular injury.	Discontinue gentamicin. Avoid other nephrotoxins. Supportive care. FEUrea may normalize as injury resolves? Actually, may stay low. Use urine sediment (granular casts) to confirm ATN.

Sample Clinical Documentation for FEUrea Cases

Example 1: Heart failure, diuretics, AKI (prerenal pattern despite high FENa): "Patient is a 72-year-old male with HFrEF (EF 25%) on furosemide 80 mg BID. Presents with AKI (Cr 2.5, baseline 1.3), oliguria (300 mL/24h), JVP 12 cm, no edema. Labs: BUN 55, Na 135. Urine: UNa 65 mEq/L, UCr 40 mg/dL, urine urea 500 mg/dL. FENa = (65 × 2.5)/(135 × 40) × 100 = (162.5)/(5400) × 100 = 3.0% (suggests ATN, but patient on furosemide). FEUrea = (500 × 2.5)/(55 × 40) × 100 = (1250)/(2200) × 100 = 56.8% (borderline, actually >50%, suggests ATN). Wait recalc: 1250/2200 = 0.568 × 100 = 56.8% (>50%, ATN pattern). That suggests ATN, not prerenal. But clinical picture suggests prerenal (worsening heart failure, JVP elevated, oliguria). FEUrea 56% may be due to furosemide (some effect on urea reabsorption). This is a gray zone. Plan: Optimize heart failure (nitroglycerin drip to reduce preload, dobutamine for inotropy). Reduce furosemide to 40 mg BID (to minimize further renal injury). Monitor Cr, urine output. If Cr continues to rise, consider ATN (possible from prolonged hypoperfusion)." Example 2: Cirrhosis, HRS (low FEUrea, supports prerenal): "Patient is a 58-year-old with alcoholic cirrhosis, ascites, presenting with AKI (Cr 2.8, baseline 1.0). BUN 18 (low due to liver disease), Na 128. Urine: UNa 8 mEq/L, UCr 60 mg/dL, urine urea 300 mg/dL. FENa = (8 × 2.8)/(128 × 60) × 100 = (22.4)/(7680) × 100 = 0.29% (<1%, prerenal pattern). FEUrea = (300 × 2.8)/(18 × 60) × 100 = (840)/(1080) × 100 = 77.8% (>50%, ATN pattern). Discordant: FENa suggests prerenal (HRS), FEUrea suggests ATN. Which to trust? In cirrhosis, FEUrea may be falsely elevated due to low BUN (hepatic synthetic dysfunction). The low FENa (0.29%) is more reliable in this case (no diuretics). Diagnosis: Hepatorenal syndrome (prerenal). Plan: IV albumin 1 g/kg/day x2 days, midodrine 5-10 mg TID, octreotide 100 mcg SC TID (or terlipressin if available). Avoid large-volume paracentesis without albumin. Nephrology consult." Example 3: Post-cardiac surgery ATN (FEUrea >50%, confirms ATN): "Patient is a 68-year-old male post-CABG, AKI (Cr 3.0, baseline 1.2), oliguric (200 mL/8h). No diuretics. BUN 65. Urine: UNa 70 mEq/L, UCr 45 mg/dL, urine urea 600 mg/dL. FENa = (70 × 3.0)/(140 × 45) × 100 = (210)/(6300) × 100 = 3.33% (>2%, ATN). FEUrea = (600 × 3.0)/(65 × 45) × 100 = (1800)/(2925) × 100 = 61.5% (>50%, ATN). Concordant: ATN. Plan: Supportive care. Avoid fluids (CVP 12, pulmonary edema). Remove nephrotoxins. Monitor K⁺, HCO₃, BUN. Nephrology consult for possible RRT if hyperkalemia >6.5, acidosis pH <7.2, uremia BUN >100, or fluid overload refractory to diuretics.

Origins & History

Carvounis and the FEUrea Solution

Dr. Christos P. Carvounis (1946-2013) was a nephrologist at SUNY Stony Brook (State University of New York at Stony Brook). In 2002, he recognized a persistent clinical problem: many patients with AKI were on diuretics (especially furosemide), rendering FENa unreliable. He hypothesized that urea reabsorption, which occurs mainly in the proximal tubule and collecting duct, would be less affected by loop diuretics (which act on the thick ascending limb). He designed a prospective study of 110 patients with AKI who had received diuretics, comparing FENa and FEUrea. The results confirmed that FEUrea maintained diagnostic accuracy while FENa did not. His 2002 Kidney International paper has been cited over 500 times and remains the primary reference for FEUrea. FEUrea is now widely used in clinical practice, especially in cardiorenal syndrome, hepatorenal syndrome, and post-operative AKI, where diuretics are common.

Key Contributors and Timeline

Year	Contributor(s)	Institution	Contribution
2002	Carvounis CP, Nisar S, Guro-Razuman S	State University of New York (SUNY) at Stony Brook	Landmark study validating FEUrea for AKI in diuretic-treated patients. Established thresholds (<35% prerenal, >50% ATN).
2012	Pépin MN, Bouchard J, Legault L, Ethier J	Université de Montréal, Canada	Validation in cirrhotic patients. Highlighted limitations in low BUN states.
2012	KDIGO AKI Guideline Work Group	International (Kidney Disease: Improving Global Outcomes)	Formal guideline mention of FEUrea as alternative to FENa (Grade 2B).
2013	Musso CG, Jauregui JR, Macías Núñez JF	Hospital Italiano de Buenos Aires, Argentina	Validation in elderly patients (n=62). Confirmed utility but suggested lower thresholds (<30% prerenal).
2020-2024	Ongoing research	Multiple	Comparative studies of FEUrea vs new biomarkers (NGAL, KIM-1, TIMP-2 × IGFBP7). FEUrea remains a low-cost alternative.

Future Directions

FEUrea remains a useful, low-cost test in the era of expensive biomarkers. However, its limitations (dependence on protein intake, liver function, ADH) mean it will likely complement, not replace, clinical judgment. Newer AKI biomarkers (NGAL, KIM-1, TIMP-2 × IGFBP7) offer earlier detection and better specificity for ATN, but they are not universally available and are more expensive. FEUrea, being calculated from routine labs, will continue to be used in resource-limited settings and as an adjunct to clinical assessment. Future research may refine FEUrea thresholds for specific populations (cirrhosis, CKD, elderly) and establish its role in predicting renal recovery.

Last Comprehensive Review: 2026-07-17

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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)

FEUrea (%)

Interpretation

Likely Diagnosis

Physiologic Basis

Action

<35%

Prerenal azotemia (avid urea reabsorption)

Volume depletion, decreased cardiac output, hepatorenal syndrome

In prerenal states, reduced renal blood flow and increased ADH enhance urea reabsorption in proximal tubule and collecting duct. FEUrea <35% indicates appropriate conservation.

Volume resuscitation (unless contraindicated). Treat underlying cause. Monitor response (urine output, Cr, FEUrea should rise toward normal).

35-50%

Indeterminate (gray zone)

Early ATN, CKD with prerenal, recovery phase, diuretic effect (mild), mixed disorders

Partial impairment of urea reabsorption but not complete loss. May represent transitional state between prerenal and ATN.

Repeat FEUrea in 4-6 hours. Assess urine sediment (granular casts suggest ATN). Consider FENa if no diuretics. Clinical correlation essential.

>50%

Intrinsic renal (ATN) – or established tubular injury

Acute tubular necrosis (ischemic, toxic), sepsis-associated AKI, post-obstructive AKI (recovery phase), severe tubulointerstitial nephritis

Impaired urea reabsorption due to tubular damage. Loss of proximal tubule urea transporters (UT-A1, UT-A3) and collecting duct responsiveness to ADH.

Avoid volume overload. Remove nephrotoxins. Treat underlying cause. Monitor for complications (hyperkalemia, acidosis, uremia). Nephrology consult if severe or not improving.

Population

Prerenal Threshold

ATN Threshold

Reason for Modification

Alternative

Cirrhosis

<25-30% (lower due to reduced urea synthesis)

>40-45%

Liver disease reduces BUN production, lowering FEUrea baseline. Thresholds may need adjustment; limited data.

FENa (if no diuretics) or clinical assessment. Hepatorenal syndrome (HRS) typically has FEUrea <35% (prerenal).

High protein intake / TPN

<35% (unchanged)

>50% (unchanged, but may be higher due to increased urea production)

High protein increases urea production, potentially raising FEUrea even in prerenal states? Actually, both urine urea and BUN rise proportionally, so FEUrea may be unchanged. But if tubular function impaired, FEUrea may rise more.

Clinical assessment; repeat FEUrea after 4-6 hours if indeterminate.

CKD (eGFR <30)

Limited data; prerenal FEUrea may be <35%

ATN FEUrea may be >50% but not well studied

CKD affects urea handling; thresholds not well validated. Baseline FEUrea may be elevated due to reduced functional nephrons.

FENa also unreliable; use clinical judgment, urine sediment, and trend in Cr.

SIADH

May be falsely low (<35%) even without prerenal state

Not applicable

ADH increases urea reabsorption via UT-A1/UT-A3, lowering FEUrea. A low FEUrea in SIADH does NOT indicate prerenal AKI.

FENa is more reliable in SIADH (typically >1% due to mild natriuresis). Use FENa instead of FEUrea.

Post-obstructive AKI

Variable (early obstruction may have low FEUrea; post-obstruction diuresis has high FEUrea)

>50% common in post-obstructive diuresis

After relief of obstruction, there is a urea and sodium diuresis (impaired reabsorption). High FEUrea does not necessarily indicate ATN.

Clinical context (relief of obstruction, diuresis). Manage fluids carefully (replace urine output).

Neonates/infants

Limited data; <30% may be prerenal

>40% may indicate ATN

Immature tubular function; urea handling differs from adults.

Use clinical assessment; consult pediatric nephrology.

Scenario

FEUrea Result

Likely Diagnosis

Management

Heart failure on furosemide, AKI (Cr rise 1.0→2.0), oliguric, JVP elevated, SBP 110

25% (<35%, prerenal pattern)

Prerenal AKI due to worsening heart failure (decreased cardiac output). FENa would be >2% (diuretic effect, misleading).

Optimize heart failure therapy (vasodilators, inotropes). Reduce or hold furosemide if not essential. Do NOT give fluids (volume overload). Consider ultrafiltration if refractory.

Sepsis, AKI (Cr rise 0.8→2.5), oliguric, MAP 65 on norepinephrine, received furosemide 40 mg 12 hours ago

75% (>50%, ATN pattern)

Sepsis-associated AKI (ATN). FENa would be unreliable due to furosemide, but FEUrea >50% confirms ATN.

Treat sepsis (antibiotics, source control). Avoid nephrotoxins. Maintain MAP >65 (vasopressors). Monitor for need for renal replacement therapy (RRT). Nephrology consult.

Cirrhosis, ascites, AKI (Cr 1.2→2.5), on spironolactone (not loop diuretic), SBP 90, JVP flat

FEUrea 22% (<35%)

Hepatorenal syndrome (HRS) – prerenal pattern. FENa also low (<1%), but FEUrea confirms prerenal.

IV albumin 1 g/kg x2 days, vasoconstrictors (midodrine + octreotide, or terlipressin if available). Avoid large-volume paracentesis without albumin. Nephrology consult.

Post-cardiac surgery, AKI (Cr 1.0→2.2), oliguric, no diuretics

FEUrea 65% (>50%)

Ischemic ATN (post-cardiopulmonary bypass). FENa would also be >2%.

Supportive care (avoid overload, remove nephrotoxins). Monitor for hyperkalemia. Renal replacement therapy if severe.

Contrast-induced AKI (post-cath), Cr 1.0→2.5, non-oliguric, no diuretics

FEUrea 32% (<35%)

Contrast-induced nephropathy often presents with low FEUrea (non-oliguric ATN variant).

Do NOT give fluids (already given peri-procedure). Avoid further nephrotoxins. Most cases resolve spontaneously in 5-7 days. FEUrea may be misleading; rely on clinical context.

Aminoglycoside toxicity (gentamicin), Cr 1.0→2.2, non-oliguric, no diuretics

FEUrea 28% (<35%)

Non-oliguric ATN (aminoglycosides). FEUrea often low despite tubular injury.

Discontinue gentamicin. Avoid other nephrotoxins. Supportive care. FEUrea may normalize as injury resolves? Actually, may stay low. Use urine sediment (granular casts) to confirm ATN.

Year

Contributor(s)

Institution

Contribution

2002

Carvounis CP, Nisar S, Guro-Razuman S

State University of New York (SUNY) at Stony Brook

Landmark study validating FEUrea for AKI in diuretic-treated patients. Established thresholds (<35% prerenal, >50% ATN).

2012

Pépin MN, Bouchard J, Legault L, Ethier J

Université de Montréal, Canada

Validation in cirrhotic patients. Highlighted limitations in low BUN states.

2012

KDIGO AKI Guideline Work Group

International (Kidney Disease: Improving Global Outcomes)

Formal guideline mention of FEUrea as alternative to FENa (Grade 2B).

2013

Musso CG, Jauregui JR, Macías Núñez JF

Hospital Italiano de Buenos Aires, Argentina

Validation in elderly patients (n=62). Confirmed utility but suggested lower thresholds (<30% prerenal).

2020-2024

Ongoing research

Multiple

Comparative studies of FEUrea vs new biomarkers (NGAL, KIM-1, TIMP-2 × IGFBP7). FEUrea remains a low-cost alternative.

Fractional Excretion of Urea (FEUrea)

When to Use

What is FEUrea?

Primary Clinical Indications

Contraindications / Limitations

FENa vs FEUrea: Comparison

How it Works

FEUrea Formula (Carvounis 2002)

Interpretive Thresholds (Carvounis 2002, KDIGO)

Modified Thresholds for Special Populations

Clinical Pearls

Critical Pearl #1: FEUrea is Most Useful When FENa is Unreliable (Diuretics)

Critical Pearl #2: Low FEUrea in Cirrhosis May Be Falsely Low (Hepatorenal Syndrome vs ATN)

Critical Pearl #3: High FEUrea in SIADH Does NOT Indicate ATN

Critical Pearl #4: FEUrea May Be Normal in Early ATN (Like FENa)

Common Pitfalls in FEUrea Interpretation

Calculating FEUrea Step-by-Step (Practical Guide)

Next Steps

Step-by-Step Clinical Action Based on FEUrea

Clinical Scenarios and FEUrea Interpretation

Sample Clinical Documentation for FEUrea Cases

The Evidence

Primary Source: Carvounis (2002)

Fractional excretion of urea in the differential diagnosis of acute renal failure

Validation Studies

Diagnostic performance of fractional excretion of urea and sodium in the evaluation of acute kidney injury in patients with cirrhosis

Fractional excretion of urea (FEUrea) as a marker of prerenal acute kidney injury in the elderly

Guideline Recommendations

Clinical Practice Guideline for Acute Kidney Injury (AKI)

Origins & History

Carvounis and the FEUrea Solution

Key Contributors and Timeline

Future Directions

Related Tools

In Recent Clinical News

Fractional Excretion of Urea (FEUrea)

When to Use

What is FEUrea?

Primary Clinical Indications

Contraindications / Limitations

FENa vs FEUrea: Comparison

How it Works

FEUrea Formula (Carvounis 2002)

Interpretive Thresholds (Carvounis 2002, KDIGO)

Modified Thresholds for Special Populations

Clinical Pearls

Critical Pearl #1: FEUrea is Most Useful When FENa is Unreliable (Diuretics)

Critical Pearl #2: Low FEUrea in Cirrhosis May Be Falsely Low (Hepatorenal Syndrome vs ATN)

Critical Pearl #3: High FEUrea in SIADH Does NOT Indicate ATN

Critical Pearl #4: FEUrea May Be Normal in Early ATN (Like FENa)

Common Pitfalls in FEUrea Interpretation

Calculating FEUrea Step-by-Step (Practical Guide)

Next Steps

Step-by-Step Clinical Action Based on FEUrea

Clinical Scenarios and FEUrea Interpretation

Sample Clinical Documentation for FEUrea Cases

The Evidence

Primary Source: Carvounis (2002)

Fractional excretion of urea in the differential diagnosis of acute renal failure

Validation Studies

Diagnostic performance of fractional excretion of urea and sodium in the evaluation of acute kidney injury in patients with cirrhosis

Fractional excretion of urea (FEUrea) as a marker of prerenal acute kidney injury in the elderly

Guideline Recommendations

Clinical Practice Guideline for Acute Kidney Injury (AKI)

Origins & History

Carvounis and the FEUrea Solution

Key Contributors and Timeline

Future Directions

Related Tools

In Recent Clinical News