FeNa Calculator (Fractional Excretion of Sodium)
Work out the fractional excretion of sodium (FeNa) to help tell apart the causes of acute kidney injury — chiefly a prerenal cause such as dehydration from an intrinsic one such as acute tubular necrosis. FeNa is the percentage of the sodium filtered by the kidney that ends up in the urine. Enter the paired serum and urine sodium and creatinine (spot samples, conventional US units).
About FeNa
The fractional excretion of sodium is the proportion of filtered sodium excreted in the urine, used to help distinguish prerenal from intrinsic acute kidney injury. FeNa = (UNa × PCr) ÷ (PNa × UCr) × 100. Interpret alongside the clinical picture and urinalysis.
Frequently asked questions
What is the FeNa formula?
FeNa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100. The sodium values are in mmol/L (the same as mEq/L) and the creatinine values in mg/dL; because each pair is a ratio, the units cancel as long as they are consistent. It uses a single paired blood and spot-urine sample.
What do the FeNa values mean?
A FeNa under 1% suggests a prerenal cause — the kidneys are avidly holding on to sodium, as in dehydration, heart failure or other low-perfusion states. A FeNa over 2% suggests an intrinsic renal cause, classically acute tubular necrosis, where the tubules can no longer reabsorb sodium. Values between 1% and 2% are indeterminate and interpreted with the wider picture.
When is FeNa unreliable?
FeNa is most useful in oliguric acute kidney injury. Diuretics increase sodium excretion and can push a prerenal FeNa above 1%, giving a falsely “intrinsic” result — in that setting the fractional excretion of urea (FeUrea) is preferred. A low FeNa can also occur in some intrinsic causes such as contrast nephropathy, acute glomerulonephritis and early sepsis, so it is one clue, not a diagnosis.
Do the units matter?
Use consistent units within each pair: sodium in mmol/L for both serum and urine, and creatinine in mg/dL for both. If your urine creatinine is reported in mmol/L, convert it to mg/dL (multiply by 11.3) or make sure both creatinines share the same unit — the ratio is what counts. Because FeNa is a ratio of ratios, a consistent choice cancels out and the percentage is the same in either system. The four fields on this page are labelled for the conventional combination: sodium in mmol/L and creatinine in mg/dL.
What is FeNa used for?
FeNa helps classify acute kidney injury at the bedside and guides the next step — for example, a trial of fluids for a prerenal picture versus looking for and treating tubular injury. It is used alongside the history, examination, urinalysis and other tests such as the eGFR. This calculator is an educational reference, not medical advice.
This is a reference tool, not a diagnosis or medical advice. Results depend on the laboratory, assay and clinical context, and reference intervals vary between labs and by age and sex — always read your result against the range printed on your own report and discuss it with a qualified healthcare professional.
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