Thursday, June 25, 2026
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Renal Diagnostic Calculator

FENa Calculator: Fractional Excretion of Sodium

Calculate FENa from paired urine and serum sodium and creatinine values to support acute kidney injury mechanism assessment — with explicit diuretic, CKD, and intrinsic kidney disease caveats.

Quick Answer

Fractional excretion of sodium (FENa) estimates the percentage of filtered sodium excreted in urine: FENa (%) = (UNa × SCr) / (SNa × UCr) × 100. FENa below 1% often supports sodium-avid pre-renal physiology; above 2% often supports intrinsic tubular injury — but diuretics, CKD, contrast, and sepsis can invalidate interpretation. Sodium and creatinine units must match within serum and urine pairs.

Fractional Excretion of Sodium
FENa (%) = (UNa x SCr) / (SNa x UCr) x 100
UNa = urine sodium; SNa = serum sodium; SCr = serum creatinine; UCr = urine creatinine. Match units within each analyte pair.

Calculate FENa

Enter paired urine and serum sodium and creatinine values to estimate fractional excretion of sodium.

Urine values
Serum values

Serum and urine creatinine may be entered in mg/dL or micromol/L, but both creatinine values must use the same unit.

Fractional Excretion of Sodium

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Diuretics, CKD, contrast injury, sepsis, rhabdomyolysis, adrenal insufficiency, and early obstruction can limit FENa interpretation.

How to Use This FENa Calculator

1
Use urine and serum measurements collected close enough in time to represent the same physiologic state.
2
Keep sodium units matched and creatinine units matched. The formula cancels units only when paired analytes use the same units.
3
Interpret values below 1%, 1–2%, and above 2% as probability signals rather than diagnostic proof.
4
Document recent diuretic exposure because it can raise urinary sodium and weaken the pre-renal/intrinsic distinction.
Worked Example

UNa 20 mEq/L, SCr 2.0 mg/dL, SNa 140 mEq/L, UCr 100 mg/dL.

FENa = (20 × 2.0) / (140 × 100) × 100 = 0.29%, a low value that may support pre-renal physiology if the clinical context is compatible.

Interpretation Ranges

< 1%Often supports sodium-avid pre-renal physiology when the clinical setting fits — but not specific; confirm with volume assessment and urine sediment.
1–2%Indeterminate range; use urine sediment, volume assessment, medication history, FEUrea if diuretic-exposed, and time course.
> 2%Often supports intrinsic tubular injury, especially acute tubular necrosis — but not specific; confirm against diuretics, CKD, and clinical trajectory.

Diuretic and Clinical Caveats

FENa is most interpretable before diuretics and in oliguric acute kidney injury where sodium handling reflects tubular avidity. Diuretics can increase urine sodium despite low effective arterial blood volume, producing a misleadingly high FENa.

Low FENa can also occur in some intrinsic processes, including contrast nephropathy, pigment nephropathy, glomerulonephritis, and early sepsis. High FENa may occur with chronic kidney disease, bicarbonaturia, adrenal insufficiency, or recovery from acute tubular necrosis.

Pharma & clinical trial context

FENa and related fractional excretion indices appear in nephrotoxicity safety monitoring for investigational drugs, contrast-associated AKI studies, and ICU pharmacology research where tubular injury must be distinguished from hemodynamic pre-renal azotemia. Sponsors document diuretic exposure, sample timing, and FENa thresholds in renal safety monitoring plans when used as exploratory biomarkers.

Pair FENa with filtration estimates from the GFR Calculator and Creatinine Clearance Calculator, osmolality assessment via the Osmolarity Calculator, and CRRT clearance modeling with the CRRT Clearance Calculator when renal endpoints span the full nephrology calculator cluster.

KDIGO AKI staging uses serum creatinine and urine output — not FENa — as primary criteria. FENa supports mechanism assessment in case narratives and adjudication discussions but should not override protocol-defined AKI definitions or nephrology consultation requirements in clinical trials.

Evidence & sources

Frequently Asked Questions

FENa is fractional excretion of sodium. It estimates the percentage of filtered sodium excreted in urine using urine sodium, serum creatinine, serum sodium, and urine creatinine. FENa helps distinguish pre-renal azotemia from intrinsic acute kidney injury when interpreted with clinical context.
FENa is calculated as urine sodium times serum creatinine divided by serum sodium times urine creatinine, multiplied by 100. Sodium units must match each other (mEq/L or mmol/L) and creatinine units must match each other (mg/dL or µmol/L) within the serum and urine pairs.
A FENa below 1% is often associated with sodium-avid pre-renal states where the kidney retains sodium to preserve volume. However, interpretation depends on timing, diuretics, kidney disease, sepsis, contrast, rhabdomyolysis, bicarbonaturia, and other clinical factors — FENa is a probability signal, not proof.
A FENa above 2% is often associated with intrinsic tubular injury, especially acute tubular necrosis, where damaged tubules fail to reabsorb filtered sodium. Values between 1% and 2% are indeterminate and require urine sediment, volume assessment, medication history, and clinical trajectory.
Yes. Loop and thiazide diuretics increase urinary sodium excretion and can make FENa less reliable by raising urine sodium despite pre-renal physiology. Fractional excretion of urea (FEUrea) may be considered in selected diuretic-exposed patients, but it also has limitations and should not be used as a standalone diagnostic test.
FENa equals the product of urine sodium and serum creatinine, divided by the product of serum sodium and urine creatinine, times 100 to express as a percentage. The creatinine terms normalize for urine concentration and glomerular filtration, making FENa independent of urine volume.
FENa is most interpretable before diuretic administration, in oliguric acute kidney injury, and when urine and serum samples are collected close in time from a hemodynamically stable or consistently hypoperfused patient. Non-oliguric AKI and post-renal obstruction reduce reliability.
Yes. Contrast nephropathy, early sepsis-associated AKI, rhabdomyolysis, glomerulonephritis, and acute interstitial nephritis can present with FENa below 1% early in the course. Low FENa does not exclude intrinsic injury — always correlate with urine microscopy, history, and trajectory.
Yes. Chronic kidney disease, bicarbonaturia (metabolic alkalosis with bicarbonate diuresis), adrenal insufficiency, and recovery phase from acute tubular necrosis can produce FENa above 2% despite effective volume depletion. Clinical context and response to volume repletion matter more than a single FENa value.
FEUrea uses urine urea and serum urea instead of sodium in a similar formula. It may be less affected by diuretics than FENa in some settings, but it is not universally superior and has its own limitations. This calculator computes FENa only — use FEUrea when protocol or nephrology consultation specifies it.
FENa assesses tubular sodium handling relative to filtration (via creatinine proxy), not GFR directly. Serum creatinine reflects filtration function; FENa reflects whether the kidney is sodium-avid (pre-renal) or wasting sodium (intrinsic). Use the GFR Calculator and Creatinine Clearance Calculator for filtration estimates separately.
No. FENa supports AKI mechanism assessment but does not replace clinical evaluation, urine microscopy, renal ultrasound, medication review, volume assessment, nephrology consultation, or institutional AKI protocols. Never withhold treatment based on FENa alone.

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GFR GFR Calculator CrCl Creatinine Clearance CRRT CRRT Clearance Calculator Osm Osmolarity Calculator A-a A-a Gradient Calculator

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