Fractional Excretion of Sodium (FENa)

FENa Calculator

Excretion Fraction Analyzer

Serum Parameters

Serum Sodium (SNa)

Serum Creatinine (SCr)

Urine Parameters

Urine Sodium (UNa)

Urine Creatinine (UCr)

Renal Shunt Analysis

Enter paired serum and urine electrolytes to differentiate AKI causes.

Guidelines & Evidence

Verified

Last Review: 2026-07-17

When to Use

What is FENa?

The fractional excretion of sodium (FENa) is the percentage of filtered sodium that is excreted in the urine. It reflects the kidney's ability to reabsorb sodium, which is typically increased in prerenal states (the kidney tries to conserve sodium) and decreased in intrinsic acute tubular necrosis (ATN) where tubular reabsorptive capacity is impaired. FENa is calculated using simultaneous serum and urine samples and is expressed as a percentage. It is most useful in oliguric patients (urine output <400 mL/day) with acute kidney injury (AKI) to differentiate prerenal azotemia (FENa <1%) from intrinsic ATN (FENa >2%).

Primary Clinical Indications

Differentiating prerenal AKI from intrinsic ATN – Core indication; helps avoid unnecessary volume resuscitation in ATN (which may cause volume overload) or premature discontinuation of fluids in prerenal states

Oliguric AKI evaluation – Most useful when urine output is <400 mL/day (or <0.5 mL/kg/hour for >6 hours). Less reliable in non-oliguric AKI

Acute kidney injury in hospitalized patients – Common scenario: post-operative AKI, sepsis-associated AKI, contrast-induced nephropathy, drug-induced AKI (aminoglycosides, amphotericin B, vancomycin)

Preoperative risk stratification – Patients with chronic kidney disease (CKD) may have FENa >1% at baseline; interpreting FENa requires knowledge of baseline renal function

Guidance for fluid management – Low FENa (<1%) suggests volume-responsive AKI; high FENa (>2%) suggests fluid-unresponsive ATN (avoid over-resuscitation)

Monitoring response to therapy – As prerenal AKI resolves, FENa should increase toward normal; persistent low FENa suggests ongoing hypoperfusion despite apparent hemodynamic stability

Contraindications / Limitations

Diuretic use (most common limitation) – Loop diuretics (furosemide, bumetanide, torsemide) and thiazides increase urinary sodium excretion, falsely elevating FENa. A patient with prerenal AKI on furosemide may have FENa >2%, mimicking ATN. Use fractional excretion of urea (FEUrea) instead (less affected by diuretics).

Chronic kidney disease (CKD) – Patients with baseline CKD have impaired sodium reabsorption, so FENa may be >1% even in prerenal states. In CKD, FEUrea may be more accurate, but clinical correlation is essential.

Non-oliguric AKI – In non-oliguric AKI (e.g., aminoglycoside toxicity, early ATN), FENa may be <1% despite intrinsic injury. FENa is less reliable in non-oliguric patients.

Contrast-induced nephropathy (CIN) – CIN often presents with FENa <1% (prerenal pattern) despite intrinsic tubular injury. Do NOT interpret low FENa as prerenal if CIN is suspected.

Glomerulonephritis – FENa may be <1% in glomerulonephritis (due to reduced GFR, intact tubular function). FENa does not differentiate between glomerular and tubular diseases; use urine sediment (RBC casts) instead.

Sepsis-associated AKI – Can present with either low or high FENa depending on phase (early sepsis may have low FENa due to hypoperfusion; late sepsis with ATN may have high FENa). Single FENa may be misleading; serial measurements and clinical correlation are needed.

Obstructive uropathy – FENa may be >2% (post-obstructive diuresis) or <1% (early obstruction with intact tubular function). Not diagnostic; use renal ultrasound.

Loop diuretic use within past 24-48 hours – Even a single dose of furosemide can elevate FENa for 24-48 hours. If diuretics were given, FEUrea is preferred.

Spot urine vs timed collection – FENa using spot urine is valid (creatinine cancels out), but requires simultaneous blood collection. Using a spot urine without concurrent serum creatinine invalidates the calculation.

FENa vs FEUrea vs Renal Failure Index vs Urine Sodium

Test	Formula	Normal/Interpretation	Advantages	Limitations
FENa	(UNa × SCr) / (SNa × UCr) × 100	<1% prerenal, >2% ATN, 1-2% indeterminate	Widely studied, standard of care	Invalid with diuretics, CKD, non-oliguric AKI
FEUrea	(UUrea × SCr) / (SUrea × UCr) × 100	<35% prerenal, >50% ATN (some variation)	Less affected by diuretics (urea reabsorption is passive, not inhibited by loop diuretics)	Less studied, requires BUN (not always available on chemistry panel), affected by protein intake, liver disease, steroids
Renal Failure Index (RFI)	(UNa × SCr) / UCr	<1 prerenal, >2 ATN (similar to FENa but not indexed to SNa)	Simpler (no need for SNa)	Less accurate than FENa (SNa variation affects interpretation), same diuretic/CKD limitations
Urine Sodium (UNa)	None (direct measurement)	<20 mEq/L prerenal, >40 mEq/L ATN	Simple, no calculation	Overlap significant (20-40 mEq/L gray zone). Affected by diuretics, sodium intake, volume status. Less accurate than FENa.
Urine Osmolality	None (direct measurement)	>500 mOsm/kg prerenal, <350 mOsm/kg ATN	Simple, no calculation	Overlap, affected by many factors (protein intake, ADH, diuretics, CKD)
Urine Sediment	Microscopy	Hyaline casts (prerenal), granular casts/muddy brown casts (ATN)	Direct visualization of tubular injury	Requires experienced technician, may be absent in early ATN

How it Works

FENa Formula (Espinel 1976)

FENa (%) = (Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine) × 100 Where: • Urine Sodium (UNa) = spot urine sodium concentration (mEq/L) • Serum Creatinine (SCr) = serum creatinine (mg/dL or μmol/L) • Serum Sodium (SNa) = serum sodium (mEq/L) • Urine Creatinine (UCr) = spot urine creatinine (mg/dL or μmol/L) Example (Prerenal AKI): UNa = 10 mEq/L, SCr = 2.0 mg/dL, SNa = 140 mEq/L, UCr = 100 mg/dL FENa = (10 × 2.0) / (140 × 100) × 100 = (20) / (14,000) × 100 = 0.00143 × 100 = 0.14% Example (ATN): UNa = 60 mEq/L, SCr = 3.0 mg/dL, SNa = 135 mEq/L, UCr = 50 mg/dL FENa = (60 × 3.0) / (135 × 50) × 100 = (180) / (6,750) × 100 = 0.0267 × 100 = 2.67%

Interpretive Thresholds (Espinel 1976, KDIGO)

FENa (%)	Interpretation	Clinical Context	Mechanism	Action
<1.0%	Prerenal azotemia (or early intrinsic AKI)	Volume depletion (diarrhea, vomiting, bleeding, burns, diuretics prior to AKI), decreased cardiac output (heart failure, shock), hepatorenal syndrome	Intact tubular function; kidney reabsorbs sodium maximally (avid sodium conservation) due to low effective arterial volume	Volume resuscitation (IV fluids, blood transfusion, vasopressors). Avoid nephrotoxins. Monitor response (FENa should rise as perfusion improves)
1.0-2.0%	Indeterminate (gray zone)	Early ATN, non-oliguric ATN, CKD with superimposed prerenal AKI, or recovery phase of ATN	Mixed picture: some tubular injury but persistent sodium conservation	Repeat FENa in 4-6 hours. Assess clinical status (volume, blood pressure, urine output). Consider FEUrea (may clarify). If oliguric, treat as prerenal initially (fluid challenge) while monitoring for overload.
>2.0%	Intrinsic renal (ATN) – or post-renal with established injury	Established ATN (ischemic, toxic), sepsis-associated AKI, post-obstructive AKI, glomerulonephritis (rarely), interstitial nephritis	Impaired tubular sodium reabsorption due to tubular necrosis, loss of brush border, or tubular damage	Avoid volume overload (do not give fluids unless hypovolemic). Remove nephrotoxins (aminoglycosides, NSAIDs, contrast). Monitor for hyperkalemia, acidosis. Renal consult if severe or not improving.

Modified Thresholds for Special Populations

Population	Prerenal Threshold	ATN Threshold	Reason for Modification	Alternative Test
Patients on diuretics	<0.5% (if loop diuretics, FENa usually >2% even with prerenal)	Not reliable	Diuretics block sodium reabsorption, increasing urinary sodium excretion. FENa falsely elevated.	FEUrea (<35% prerenal, >50% ATN) or clinical assessment
Chronic Kidney Disease (CKD)	<0.5% (baseline FENa may be 1-3% due to impaired reabsorption)	>3-4% (higher threshold)	CKD impairs sodium reabsorption even at baseline. A prerenal insult in CKD may have FENa 1-2% (normal range).	FEUrea (<35% prerenal) or use serial FENa trend (rising from baseline suggests ATN)
Neonates/infants	<2.5% (some use <3%)	>4%	Immature tubular function; neonates normally have higher FENa (2-3% at baseline).	Clinical assessment, FEUrea (limited pediatric data)
Contrast-induced nephropathy	Not applicable (CIN often presents with FENa <1% despite tubular injury)	Not reliable	Contrast causes vasoconstriction and direct tubular toxicity; early phase may mimic prerenal pattern.	Clinical context (contrast exposure within 24-72 hours, oliguria, rise in Cr). No good alternative test.
Sepsis-associated AKI	Variable (early: <1%; late: >2%)	Variable	Sepsis causes initial hypoperfusion (prerenal) then ATN.	Serial FENa (trend), urine sediment (muddy brown casts appear late), clinical status
Hepatorenal syndrome (HRS)	<1% (HRS is a prerenal state, despite normal urine output)	Not applicable	HRS is functional prerenal AKI due to splanchnic vasodilation; FENa low.	Diagnosis of exclusion; FENa helps differentiate from ATN (which would be >2%)
Post-obstructive AKI	Variable (early obstruction: low FENa; post-obstruction diuresis: high FENa)	Variable	After relief of obstruction, there is a natriuresis (salt wasting) that elevates FENa.	Renal ultrasound; if post-obstructive diuresis, manage fluids carefully (replace urine output with 0.45% NaCl).

Clinical Pearls

Critical Pearl #1: FENa is UNRELIABLE in Patients on Diuretics – Use FEUrea Instead

The single most important limitation of FENa is the effect of diuretics. Loop diuretics (furosemide, bumetanide, torsemide) and thiazides directly inhibit sodium reabsorption, increasing urine sodium concentration. A patient with prerenal AKI (volume depleted) who received furosemide 40 mg IV 6 hours ago may have FENa >2-5%, falsely suggesting ATN. This leads to inappropriate withholding of fluids and possible delay in treatment. Solution: Use fractional excretion of urea (FEUrea) which is less affected by diuretics (urea reabsorption is passive and not directly inhibited by loop diuretics). FEUrea formula: (UUrea × SCr) / (SUrea × UCr) × 100. Interpretation: <35% suggests prerenal, >50% suggests ATN. If FEUrea not available: Interpret FENa with caution. A FENa >2% in a patient who received diuretics within 24-48 hours does NOT reliably indicate ATN. Consider the clinical context: (1) Did the patient have a clear prerenal insult (vomiting, diarrhea, bleeding) before diuretics? (2) Is there another cause of ATN (nephrotoxin, contrast, sepsis)? (3) What is the urine output? Oliguria (<0.5 mL/kg/hour) suggests prerenal if other signs are present. If still uncertain, a fluid challenge (500-1000 mL crystalloid) may be diagnostic (if urine output increases and FENa decreases, prerenal likely). But be cautious in patients at risk of volume overload (heart failure, CKD).

Critical Pearl #2: FENa in CKD: Thresholds Are Higher

Patients with chronic kidney disease (CKD) have impaired sodium reabsorption even at baseline due to nephron loss and adaptive changes in remaining nephrons. A patient with baseline CKD stage 3 (eGFR 30-59) may have a normal FENa of 2-3%. If they develop a prerenal insult (e.g., gastroenteritis), their FENa may rise only to 3-4% (not the classic <1%). Using standard thresholds would misclassify prerenal AKI as ATN. Recommendation: - Obtain a baseline FENa if possible (prior to acute illness). A rising FENa from 2% to 5% suggests ATN, while a rise to 3% (unchanged) may still be prerenal. - Use FEUrea (less affected by CKD? Actually, CKD affects urea handling too, but FEUrea <35% still suggests prerenal, though data limited). - Use clinical judgment: presence of granular casts (ATN) vs hyaline casts (prerenal) on urine microscopy. - If FENa is 1-2% in a patient with CKD and no other cause of ATN (nephrotoxins, sepsis), treat as prerenal initially (fluid challenge) while monitoring for volume overload.

Critical Pearl #3: Oliguria is Required for Optimal FENa Interpretation

FENa is most accurate in oliguric AKI (urine output <400 mL/day or <0.5 mL/kg/hour for >6 hours). In non-oliguric AKI (e.g., aminoglycoside-induced ATN, early cisplatin toxicity, some cases of contrast nephropathy), FENa may be <1% despite intrinsic tubular injury. The kidney maintains sodium reabsorption even when urine output is preserved. Clinical examples: - Aminoglycoside toxicity (gentamicin, tobramycin): Patients often have non-oliguric AKI with FENa <1% (misleading). Do NOT attribute low FENa to prerenal; the clinical context (recent aminoglycoside use, rising Cr, urinary casts) indicates ATN. - Contrast-induced nephropathy: Often non-oliguric with FENa <1%. - Sepsis-associated AKI: Early sepsis may have normal urine output with low FENa. Action: In non-oliguric AKI, do NOT rely on FENa alone. Use urine microscopy (muddy brown granular casts, renal tubular epithelial cells), clinical context (nephrotoxin exposure, sepsis, contrast), and serial creatinine trends. FEUrea may also be helpful (though less studied).

Critical Pearl #4: Urine Sediment is Complementary to FENa

FENa is a functional test; urine microscopy is a structural test. Combining them improves diagnostic accuracy. Prerenal AKI: FENa <1% + urine sediment: hyaline casts only (no granular casts). Response to fluids. ATN: FENa >2% + urine sediment: muddy brown granular casts, renal tubular epithelial cells (RTEs). Does NOT respond to fluids (or may worsen volume overload). Mixed picture: FENa indeterminate (1-2%) + few granular casts: possible early ATN or recovery phase. If urine microscopy shows many granular casts (ATN pattern) but FENa is <1%: Possible (1) early ATN (before tubular sodium handling is impaired), (2) non-oliguric ATN (aminoglycosides, contrast), (3) CKD with superimposed ATN. Do NOT dismiss the granular casts; they indicate tubular injury regardless of FENa. If FENa >2% but urine sediment shows only hyaline casts: Possible (1) diuretic use (furosemide, thiazides), (2) CKD (baseline FENa elevated), (3) post-obstructive AKI, (4) early ATN without casts yet. Use clinical correlation.

Common Pitfalls in FENa Interpretation

Using FENa in patients on diuretics – Most common error. Always ask: "Has the patient received furosemide, bumetanide, torsemide, hydrochlorothiazide, or chlorthalidone in the past 48 hours?" If yes, use FEUrea or interpret FENa with extreme caution.

Using FENa in non-oliguric AKI – Non-oliguric ATN (aminoglycosides, contrast) can have FENa <1%. FENa has low sensitivity in non-oliguric patients.

Using FENa in CKD – Standard thresholds do not apply. Baseline FENa may be >1% (even 2-5%) in advanced CKD. Use FEUrea or clinical judgment.

Using FENa without simultaneous serum creatinine – The formula requires concurrent serum and urine samples. Using a serum creatinine from 12 hours prior invalidates the result (creatinine changes rapidly in AKI).

Using FENa in patients with acute glomerulonephritis – GN often presents with FENa <1% (due to reduced GFR, intact tubules). A low FENa does NOT rule out GN; look for RBC casts, proteinuria, hypertension, edema.

Using FENa in patients with hepatorenal syndrome (HRS) – HRS is a prerenal state (low FENa). But HRS is not volume-responsive (giving fluids does NOT improve renal function). Do NOT misinterpret low FENa as "needs more fluids" in HRS; the treatment is vasoconstrictors (midodrine/octreotide, terlipressin) and albumin.

Using FENa in post-obstructive AKI – After relief of obstruction, there is a natriuresis (salt wasting) that elevates FENa >2% even if no ATN. Do NOT misinterpret as ATN; manage post-obstructive diuresis carefully (replace urine output with 0.45% NaCl).

Using FENa in sepsis-associated AKI – FENa can be low (early sepsis, hypoperfusion) or high (late sepsis, ATN). A single FENa is not diagnostic; serial measurements and clinical context are needed.

Failing to correct for urine creatinine units – If serum creatinine is in mg/dL and urine creatinine is in mg/dL, the units cancel correctly. If one is in μmol/L and the other in mg/dL, conversion is needed (1 mg/dL = 88.4 μmol/L). Always check units.

Assuming FENa always distinguishes prerenal from ATN – FENa accuracy is 80-90% in ideal circumstances (oliguric, no diuretics, no CKD). There is a 10-20% false-positive/false-negative rate. Never rely on FENa alone; integrate with history, physical exam, urine microscopy, and clinical context.

FEUrea (Fractional Excretion of Urea) – Alternative When FENa Fails

FEUrea (%) = (Urea Urine × SCr) / (Urea Serum × UCr) × 100 Interpretation: • <35%: Prerenal AKI (avid urea reabsorption) • 35-50%: Indeterminate • >50%: Intrinsic ATN (impaired urea reabsorption) Advantages over FENa: • Less affected by loop diuretics (urea reabsorption is passive, not inhibited by furosemide) • May be more accurate in CKD (less dependent on sodium handling) • Can be calculated if BUN and urine urea are available (common in routine labs) Disadvantages: • Requires BUN (not always on basic metabolic panel) • Affected by protein intake, liver disease (urea production varies), steroids, and other factors that alter urea handling • Less validated than FENa; fewer studies establishing thresholds

Next Steps

Step-by-Step Clinical Action Based on FENa

Management Algorithm by FENa Result

FENa Result	Likely Diagnosis	Initial Management	Fluid Management	Nephrology Consult?	Special Considerations
<1% (Prerenal)	Volume depletion, decreased cardiac output, hepatorenal syndrome	Identify cause (vomiting, diarrhea, bleeding, heart failure, sepsis). Fluid challenge (500-1000 mL) if no contraindications.	Isotonic fluids (0.9% NaCl, balanced crystalloids). For heart failure: vasopressors (if cardiogenic shock) or diuretics (paradoxical? No, treat underlying heart failure).	If no response to fluids and no clear cause, or if hepatorenal syndrome suspected.	If no response to fluids, consider early ATN (prerenal → intrinsic conversion). Check urine sediment for granular casts.
1-2% (Indeterminate)	Early ATN, CKD with prerenal, recovery phase, non-oliguric ATN	Repeat FENa in 4-6 hours. Assess urine sediment. Consider FEUrea.	Cautious fluid challenge (250-500 mL) if hypovolemia suspected, but monitor for overload (especially in CKD or heart failure).	If FENa remains indeterminate after 24 hours, or if Cr continues to rise, or if urine sediment shows granular casts.	Most challenging group. Serial FENa (trending up suggests ATN, trending down suggests prerenal).
>2% (ATN)	Established ATN (ischemic, toxic), sepsis-associated AKI, diuretic effect, CKD, post-obstructive	Exclude diuretics and CKD. Remove nephrotoxins (NSAIDs, aminoglycosides, vancomycin, contrast, amphotericin B). Treat underlying cause (sepsis, hypotension).	Avoid volume overload. Do NOT give fluids unless clearly hypovolemic (unlikely with ATN).	Consult nephrology if: (1) Cr continues to rise, (2) Need for renal replacement therapy (uremia, refractory hyperkalemia, severe acidosis), (3) Uncertain diagnosis.	Monitor for complications: hyperkalemia (treat with calcium, insulin/glucose, kayexalate, dialysis), metabolic acidosis (bicarbonate if severe, pH <7.2), uremia (dialysis if BUN >100, pericarditis, encephalopathy).

Sample Clinical Documentation for FENa Cases

Example 1: Prerenal AKI (oliguric, no diuretics): "Patient is a 72-year-old male with diarrhea x 3 days, oliguria (urine output 200 mL/12h), hypotension (SBP 85), tachycardia (HR 110). Labs: Cr 2.5 (baseline 1.0), BUN 55, Na 140. Urine: UNa 12 mEq/L, UCr 120 mg/dL. FENa = (12 × 2.5)/(140 × 120) × 100 = (30)/(16,800) × 100 = 0.18%. Urine sediment: hyaline casts only. Interpretation: Prerenal AKI due to volume depletion (diarrhea). Plan: 1. IV fluids (0.9% NaCl) 1 L bolus, then 150 mL/hour. Target MAP >65, urine output >0.5 mL/kg/hour. 2. Recheck Cr, FENa in 6 hours. Expect Cr to decrease, FENa to rise (toward 1-2%) with volume repletion. 3. Treat diarrhea (loperamide if infectious cause excluded). 4. No nephrotoxins (hold NSAIDs, ACE inhibitors)." Example 2: ATN (post-cardiac surgery, oliguric): "Patient is a 65-year-old male with post-operative AKI after CABG. Oliguric (urine output 150 mL/8h). Labs: Cr 3.0 (baseline 1.2), BUN 45, Na 138. No diuretics for 48 hours. Urine: UNa 55 mEq/L, UCr 45 mg/dL. FENa = (55 × 3.0)/(138 × 45) × 100 = (165)/(6,210) × 100 = 2.66%. Urine sediment: muddy brown granular casts (2+). Interpretation: Acute tubular necrosis (ischemic, post-cardiac bypass). Plan: 1. Avoid volume overload (patient euvolemic, CVP 10). Do NOT give fluids. 2. Remove nephrotoxins: hold NSAIDs, ACE inhibitors, metformin. 3. Monitor for hyperkalemia (K⁺ 5.1, recheck q6h). 4. Consult nephrology (Cr rising, oliguria). 5. Prepare for possible renal replacement therapy if uremia, hyperkalemia >6.5, severe acidosis (pH <7.2), or fluid overload develops." Example 3: Diuretic effect (misleading FENa): "Patient is a 68-year-old female with heart failure, on furosemide 40 mg daily. Presents with AKI (Cr 2.0, baseline 1.3), oliguria? Actually, urine output 500 mL/24h (non-oliguric). Urine: UNa 65 mEq/L, UCr 30 mg/dL. FENa = (65 × 2.0)/(140 × 30) × 100 = (130)/(4,200) × 100 = 3.10%. Urine sediment: hyaline casts only. Interpretation: FENa >2% suggests ATN, but patient is on furosemide (loop diuretic), which elevates UNa. True diagnosis is prerenal (decreased effective arterial volume due to heart failure) with diuretic-induced natriuresis. Action: Use FEUrea instead. Calculate: BUN 30, Urine urea 400 mg/dL, SCr 2.0, UCr 30 mg/dL. FEUrea = (400 × 2.0)/(30 × 30) × 100 = (800)/(900) × 100 = 88% (suggests ATN). But FEUrea may also be affected? In heart failure, urea reabsorption is increased (prerenal), but loop diuretics have less effect on urea than sodium. Low FEUrea (<35%) would have supported prerenal. High FEUrea here may still be due to diuretic? This is complex. Better approach: Clinical assessment. The patient has heart failure, not volume depleted. Treatment: optimize heart failure therapy (diuretics, vasodilators) rather than volume expansion. FENa is not helpful here; rely on clinical judgment and urine output.

Origins & History

Carlos H. Espinel and the FENa Revolution

Dr. Carlos H. Espinel, a nephrologist at Georgetown University Medical Center (Washington, D.C.), first described the FENa test in 1976. At the time, clinicians relied on urine sodium concentration alone, which had significant overlap between prerenal and ATN (prerenal UNa <20 mEq/L; ATN UNa >40 mEq/L; gray zone 20-40 mEq/L). Espinel recognized that indexing urine sodium to creatinine clearance (fractional excretion) would account for variations in urine concentration and provide a more accurate assessment of tubular function. He derived the formula using simultaneous serum and urine samples and validated it in 35 patients. The test was rapidly adopted and remains a core component of AKI evaluation. Dr. Espinel passed away in 2015, but his legacy continues in every FENa calculation performed worldwide.

Key Contributors and Timeline

Year	Contributor(s)	Institution	Contribution
1976	Espinel CH	Georgetown University Medical Center, Washington, D.C.	Original description of FENa test in JAMA. Established thresholds <1% prerenal, >2% ATN.
1978	Miller TR, Anderson RJ, Linas SL, et al.	University of Colorado, Denver	Large prospective validation (n=62). Confirmed Espinel's thresholds. Identified limitations in non-oliguric AKI and GN.
1980	Espinel CH, Gregory AW	Georgetown University	Extended validation to 103 patients; proposed FEUrea as alternative when FENa unreliable.
1987	Carvounis CP, Nisar S, Guro-Razuman S	State University of New York (SUNY), Stony Brook	Studied effect of diuretics on FENa; showed that loop diuretics elevate FENa, reducing diagnostic accuracy. Recommended FEUrea in diuretic-treated patients.
1990s-2000s	Multiple investigators	Various	Validation of FENa in CKD, contrast nephropathy, sepsis, and other specific AKI etiologies. Meta-analyses confirmed overall accuracy but noted limitations.
2012	KDIGO AKI Guideline Work Group	International (Kidney Disease: Improving Global Outcomes)	Formal guideline recommendation for FENa (Grade 2B). Highlighted caveats (diuretics, CKD, non-oliguria).
2016-2024	Ongoing research	Multiple	Development of new biomarkers (NGAL, KIM-1, TIMP-2 × IGFBP7) that may complement or replace FENa in the future.

Future Directions: Biomarkers vs FENa

While FENa remains a cornerstone of AKI evaluation, newer biomarkers (neutrophil gelatinase-associated lipocalin [NGAL], kidney injury molecule-1 [KIM-1], tissue inhibitor of metalloproteinase-2 × insulin-like growth factor-binding protein 7 [TIMP-2 × IGFBP7]) may provide earlier detection of ATN. FENa typically becomes abnormal 24-48 hours after insult, whereas biomarkers can detect tubular injury within 4-12 hours. However, biomarkers are more expensive and not universally available. FENa, being inexpensive and derived from routine labs, will likely remain in use for the foreseeable future, especially in resource-limited settings.

Last Comprehensive Review: 2026-07-17

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Test

Formula

Normal/Interpretation

Advantages

Limitations

FENa

(UNa × SCr) / (SNa × UCr) × 100

<1% prerenal, >2% ATN, 1-2% indeterminate

Widely studied, standard of care

Invalid with diuretics, CKD, non-oliguric AKI

FEUrea

(UUrea × SCr) / (SUrea × UCr) × 100

<35% prerenal, >50% ATN (some variation)

Less affected by diuretics (urea reabsorption is passive, not inhibited by loop diuretics)

Less studied, requires BUN (not always available on chemistry panel), affected by protein intake, liver disease, steroids

Renal Failure Index (RFI)

(UNa × SCr) / UCr

<1 prerenal, >2 ATN (similar to FENa but not indexed to SNa)

Simpler (no need for SNa)

Less accurate than FENa (SNa variation affects interpretation), same diuretic/CKD limitations

Urine Sodium (UNa)

None (direct measurement)

<20 mEq/L prerenal, >40 mEq/L ATN

Simple, no calculation

Overlap significant (20-40 mEq/L gray zone). Affected by diuretics, sodium intake, volume status. Less accurate than FENa.

Urine Osmolality

None (direct measurement)

>500 mOsm/kg prerenal, <350 mOsm/kg ATN

Simple, no calculation

Overlap, affected by many factors (protein intake, ADH, diuretics, CKD)

Urine Sediment

Microscopy

Hyaline casts (prerenal), granular casts/muddy brown casts (ATN)

Direct visualization of tubular injury

Requires experienced technician, may be absent in early ATN

FENa (%)

Interpretation

Clinical Context

Mechanism

Action

<1.0%

Prerenal azotemia (or early intrinsic AKI)

Volume depletion (diarrhea, vomiting, bleeding, burns, diuretics prior to AKI), decreased cardiac output (heart failure, shock), hepatorenal syndrome

Intact tubular function; kidney reabsorbs sodium maximally (avid sodium conservation) due to low effective arterial volume

Volume resuscitation (IV fluids, blood transfusion, vasopressors). Avoid nephrotoxins. Monitor response (FENa should rise as perfusion improves)

1.0-2.0%

Indeterminate (gray zone)

Early ATN, non-oliguric ATN, CKD with superimposed prerenal AKI, or recovery phase of ATN

Mixed picture: some tubular injury but persistent sodium conservation

Repeat FENa in 4-6 hours. Assess clinical status (volume, blood pressure, urine output). Consider FEUrea (may clarify). If oliguric, treat as prerenal initially (fluid challenge) while monitoring for overload.

>2.0%

Intrinsic renal (ATN) – or post-renal with established injury

Established ATN (ischemic, toxic), sepsis-associated AKI, post-obstructive AKI, glomerulonephritis (rarely), interstitial nephritis

Impaired tubular sodium reabsorption due to tubular necrosis, loss of brush border, or tubular damage

Avoid volume overload (do not give fluids unless hypovolemic). Remove nephrotoxins (aminoglycosides, NSAIDs, contrast). Monitor for hyperkalemia, acidosis. Renal consult if severe or not improving.

Population

Prerenal Threshold

ATN Threshold

Reason for Modification

Alternative Test

Patients on diuretics

<0.5% (if loop diuretics, FENa usually >2% even with prerenal)

Not reliable

Diuretics block sodium reabsorption, increasing urinary sodium excretion. FENa falsely elevated.

FEUrea (<35% prerenal, >50% ATN) or clinical assessment

Chronic Kidney Disease (CKD)

<0.5% (baseline FENa may be 1-3% due to impaired reabsorption)

>3-4% (higher threshold)

CKD impairs sodium reabsorption even at baseline. A prerenal insult in CKD may have FENa 1-2% (normal range).

FEUrea (<35% prerenal) or use serial FENa trend (rising from baseline suggests ATN)

Neonates/infants

<2.5% (some use <3%)

>4%

Immature tubular function; neonates normally have higher FENa (2-3% at baseline).

Clinical assessment, FEUrea (limited pediatric data)

Contrast-induced nephropathy

Not applicable (CIN often presents with FENa <1% despite tubular injury)

Not reliable

Contrast causes vasoconstriction and direct tubular toxicity; early phase may mimic prerenal pattern.

Clinical context (contrast exposure within 24-72 hours, oliguria, rise in Cr). No good alternative test.

Sepsis-associated AKI

Variable (early: <1%; late: >2%)

Variable

Sepsis causes initial hypoperfusion (prerenal) then ATN.

Serial FENa (trend), urine sediment (muddy brown casts appear late), clinical status

Hepatorenal syndrome (HRS)

<1% (HRS is a prerenal state, despite normal urine output)

Not applicable

HRS is functional prerenal AKI due to splanchnic vasodilation; FENa low.

Diagnosis of exclusion; FENa helps differentiate from ATN (which would be >2%)

Post-obstructive AKI

Variable (early obstruction: low FENa; post-obstruction diuresis: high FENa)

Variable

After relief of obstruction, there is a natriuresis (salt wasting) that elevates FENa.

Renal ultrasound; if post-obstructive diuresis, manage fluids carefully (replace urine output with 0.45% NaCl).

FENa Result

Likely Diagnosis

Initial Management

Fluid Management

Nephrology Consult?

Special Considerations

<1% (Prerenal)

Volume depletion, decreased cardiac output, hepatorenal syndrome

Identify cause (vomiting, diarrhea, bleeding, heart failure, sepsis). Fluid challenge (500-1000 mL) if no contraindications.

Isotonic fluids (0.9% NaCl, balanced crystalloids). For heart failure: vasopressors (if cardiogenic shock) or diuretics (paradoxical? No, treat underlying heart failure).

If no response to fluids and no clear cause, or if hepatorenal syndrome suspected.

If no response to fluids, consider early ATN (prerenal → intrinsic conversion). Check urine sediment for granular casts.

1-2% (Indeterminate)

Early ATN, CKD with prerenal, recovery phase, non-oliguric ATN

Repeat FENa in 4-6 hours. Assess urine sediment. Consider FEUrea.

Cautious fluid challenge (250-500 mL) if hypovolemia suspected, but monitor for overload (especially in CKD or heart failure).

If FENa remains indeterminate after 24 hours, or if Cr continues to rise, or if urine sediment shows granular casts.

Most challenging group. Serial FENa (trending up suggests ATN, trending down suggests prerenal).

>2% (ATN)

Established ATN (ischemic, toxic), sepsis-associated AKI, diuretic effect, CKD, post-obstructive

Exclude diuretics and CKD. Remove nephrotoxins (NSAIDs, aminoglycosides, vancomycin, contrast, amphotericin B). Treat underlying cause (sepsis, hypotension).

Avoid volume overload. Do NOT give fluids unless clearly hypovolemic (unlikely with ATN).

Consult nephrology if: (1) Cr continues to rise, (2) Need for renal replacement therapy (uremia, refractory hyperkalemia, severe acidosis), (3) Uncertain diagnosis.

Monitor for complications: hyperkalemia (treat with calcium, insulin/glucose, kayexalate, dialysis), metabolic acidosis (bicarbonate if severe, pH <7.2), uremia (dialysis if BUN >100, pericarditis, encephalopathy).

Year

Contributor(s)

Institution

Contribution

1976

Espinel CH

Georgetown University Medical Center, Washington, D.C.

Original description of FENa test in JAMA. Established thresholds <1% prerenal, >2% ATN.

1978

Miller TR, Anderson RJ, Linas SL, et al.

University of Colorado, Denver

Large prospective validation (n=62). Confirmed Espinel's thresholds. Identified limitations in non-oliguric AKI and GN.

1980

Espinel CH, Gregory AW

Georgetown University

Extended validation to 103 patients; proposed FEUrea as alternative when FENa unreliable.

1987

Carvounis CP, Nisar S, Guro-Razuman S

State University of New York (SUNY), Stony Brook

Studied effect of diuretics on FENa; showed that loop diuretics elevate FENa, reducing diagnostic accuracy. Recommended FEUrea in diuretic-treated patients.

1990s-2000s

Multiple investigators

Various

Validation of FENa in CKD, contrast nephropathy, sepsis, and other specific AKI etiologies. Meta-analyses confirmed overall accuracy but noted limitations.

2012

KDIGO AKI Guideline Work Group

International (Kidney Disease: Improving Global Outcomes)

Formal guideline recommendation for FENa (Grade 2B). Highlighted caveats (diuretics, CKD, non-oliguria).

2016-2024

Ongoing research

Multiple

Development of new biomarkers (NGAL, KIM-1, TIMP-2 × IGFBP7) that may complement or replace FENa in the future.

Fractional Excretion of Sodium (FENa)

When to Use

What is FENa?

Primary Clinical Indications

Contraindications / Limitations

FENa vs FEUrea vs Renal Failure Index vs Urine Sodium

How it Works

FENa Formula (Espinel 1976)

Interpretive Thresholds (Espinel 1976, KDIGO)

Modified Thresholds for Special Populations

Clinical Pearls

Critical Pearl #1: FENa is UNRELIABLE in Patients on Diuretics – Use FEUrea Instead

Critical Pearl #2: FENa in CKD: Thresholds Are Higher

Critical Pearl #3: Oliguria is Required for Optimal FENa Interpretation

Critical Pearl #4: Urine Sediment is Complementary to FENa

Common Pitfalls in FENa Interpretation

FEUrea (Fractional Excretion of Urea) – Alternative When FENa Fails

Next Steps

Step-by-Step Clinical Action Based on FENa

Management Algorithm by FENa Result

Sample Clinical Documentation for FENa Cases

The Evidence

Original Description: Espinel (1976)

The FENa test. Use in the differential diagnosis of acute renal failure

Validation in Larger Cohorts

Urinary diagnostic indices in acute renal failure: a prospective study

Differential diagnosis of acute renal failure

Guideline Recommendations

Clinical Practice Guideline for Acute Kidney Injury (AKI)

Origins & History

Carlos H. Espinel and the FENa Revolution

Key Contributors and Timeline

Future Directions: Biomarkers vs FENa

Related Tools

In Recent Clinical News

Fractional Excretion of Sodium (FENa)

When to Use

What is FENa?

Primary Clinical Indications

Contraindications / Limitations

FENa vs FEUrea vs Renal Failure Index vs Urine Sodium

How it Works

FENa Formula (Espinel 1976)

Interpretive Thresholds (Espinel 1976, KDIGO)

Modified Thresholds for Special Populations

Clinical Pearls

Critical Pearl #1: FENa is UNRELIABLE in Patients on Diuretics – Use FEUrea Instead

Critical Pearl #2: FENa in CKD: Thresholds Are Higher

Critical Pearl #3: Oliguria is Required for Optimal FENa Interpretation

Critical Pearl #4: Urine Sediment is Complementary to FENa

Common Pitfalls in FENa Interpretation

FEUrea (Fractional Excretion of Urea) – Alternative When FENa Fails

Next Steps

Step-by-Step Clinical Action Based on FENa

Management Algorithm by FENa Result

Sample Clinical Documentation for FENa Cases

The Evidence

Original Description: Espinel (1976)

The FENa test. Use in the differential diagnosis of acute renal failure

Validation in Larger Cohorts

Urinary diagnostic indices in acute renal failure: a prospective study

Differential diagnosis of acute renal failure

Guideline Recommendations

Clinical Practice Guideline for Acute Kidney Injury (AKI)

Origins & History

Carlos H. Espinel and the FENa Revolution

Key Contributors and Timeline

Future Directions: Biomarkers vs FENa

Related Tools

In Recent Clinical News