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FENa Calculator

Compute FENa from urine and plasma sodium and creatinine.

How it is calculated

FENa (fractional excretion of sodium) compares how much filtered sodium ends up in the urine: FENa % = (UNa × PCr) / (PNa × UCr) × 100. U is urine and P is plasma; Na is sodium and Cr is creatinine. The two sodium values must share a unit and the two creatinine values must share a unit, though the units of sodium and creatinine can differ. The tool labels a result under 1% as pre-renal, over 2% as intrinsic and anything between as indeterminate. It is one clue in acute kidney injury, and diuretics, chronic kidney disease or other conditions can distort it. Interpretation belongs to a clinician.

Worked example

Urine sodium 20, plasma sodium 140, urine creatinine 100, plasma creatinine 2. Numerator = 20 × 2 = 40. Denominator = 140 × 100 = 14,000. 40 / 14,000 = 0.002857, times 100 gives 0.29%. This is below 1%, so the tool reports the pre-renal range, meaning the kidney is retaining sodium.

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Common mistakes

  • Swapping the plasma and urine creatinine values, which flips the result by orders of magnitude.
  • Mixing units between the two sodium measurements or between the two creatinine measurements.
  • Interpreting FENa after diuretics; they raise sodium excretion and can mimic the intrinsic range.
  • Treating the cut-offs as a diagnosis rather than one input among clinical findings.

About this calculator

This calculator computes the fractional excretion of sodium (FENa), FENa = [(urine Na × plasma creatinine) / (plasma Na × urine creatinine)] × 100, a bedside test that estimates what fraction of filtered sodium the kidneys are excreting rather than reabsorbing. A FENa below 1% typically points to a pre-renal cause of acute kidney injury (the kidneys are working normally but under-perfused, so they aggressively reabsorb sodium), while a FENa above 2% suggests intrinsic renal damage such as acute tubular necrosis, where the damaged tubules can't reabsorb sodium properly.

Nephrologists, emergency physicians, internists, and medical students use FENa at the bedside — usually from a single spot urine and blood sample — to help distinguish the two most common causes of acute kidney injury and decide on fluid resuscitation versus further renal workup. Enter urine sodium, plasma sodium, urine creatinine and plasma creatinine to calculate FENa.

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