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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

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

Related Scores in Practice

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

Last Comprehensive Review: 2026

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