FENa (Fractional Excretion of Sodium) Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Formula: (UNa x PCr) / (PNa x UCr) x 100. Last verified 15 June 2026.

The fractional excretion of sodium (FENa) is a urinary biomarker used to differentiate pre-renal from intrinsic causes of acute kidney injury (AKI). It expresses the fraction of filtered sodium excreted in urine as a percentage of the total filtered sodium. In pre-renal states (volume depletion, heart failure), the tubules conserve sodium avidly, resulting in FENa below 1%. In acute tubular necrosis or other intrinsic renal injuries, tubular function is impaired and FENa typically exceeds 2%. The formula requires plasma sodium, plasma creatinine, urine sodium, and urine creatinine, all from the same timed specimen. This tool is for educational reference only.

Urine Sodium (UNa, mEq/L)

Plasma Creatinine (PCr, mg/dL)

Plasma Sodium (PNa, mEq/L)

Urine Creatinine (UCr, mg/dL)

FENa (%)

0.00

Interpretation

n/a

FENa formula

FENa (%) = (UNa x PCr) / (PNa x UCr) x 100

UNa = urine sodium (mEq/L), PCr = plasma creatinine (mg/dL), PNa = plasma sodium (mEq/L), UCr = urine creatinine (mg/dL). All values should be from the same collection. The formula derives from the clearance ratio of sodium to creatinine, with creatinine serving as a surrogate for GFR.

FENa interpretation

FENa below 1%: Pre-renal AKI (volume depletion, decreased cardiac output, hepatorenal syndrome). Tubular function intact.
FENa 1-2%: Indeterminate. Consider clinical context, history, and additional tests.
FENa above 2%: Intrinsic renal injury (acute tubular necrosis, acute interstitial nephritis). Tubular sodium reabsorption impaired.
Important caveat: FENa is unreliable in patients on diuretics; use FEUrea instead.

Frequently asked questions

What is FENa and what does it measure?

The fractional excretion of sodium (FENa) measures the percentage of filtered sodium that is excreted in the urine. It reflects tubular sodium handling and is used to distinguish pre-renal acute kidney injury (where tubules avidly reabsorb sodium) from intrinsic renal disease (where tubular function is impaired and sodium is not reabsorbed efficiently).

What FENa values suggest pre-renal vs intrinsic AKI?

FENa below 1%: Suggests pre-renal AKI (avid sodium reabsorption by functioning tubules). FENa above 2%: Suggests intrinsic renal disease such as acute tubular necrosis (ATN). FENa 1-2%: Indeterminate zone. These thresholds have well-recognized exceptions.

What are the limitations of FENa?

FENa is unreliable in patients on diuretics (which cause high sodium excretion regardless of renal cause), contrast-induced nephropathy, myoglobinuria, pigment nephropathy, and early obstruction, where FENa can be below 1% despite intrinsic injury. In these cases, fractional excretion of urea (FEUrea) may be more informative.

What is FEUrea and when is it used?

FEUrea = (urine urea / plasma urea) / (urine creatinine / plasma creatinine) x 100. FEUrea below 35% suggests pre-renal AKI and is not affected by diuretics, making it preferable in patients receiving loop or thiazide diuretics.

What are the units for FENa calculation?

FENa = (UNa x PCr) / (PNa x UCr) x 100. UNa and PNa are sodium in mEq/L (or mmol/L). PCr and UCr are creatinine in mg/dL (or the same units). The units cancel in the ratio, so as long as sodium and creatinine are each in consistent units, the calculation works. Result is a percentage.

Official sources

NIDDK, NIH: Acute Kidney Injury.
NIH National Library of Medicine: Fractional Excretion of Sodium (StatPearls).

Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.