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FENa Calculator

The fractional excretion of sodium (FENa) compares how much of the filtered sodium the kidneys are excreting. It is traditionally used when assessing acute kidney injury, to help distinguish a prerenal pattern (the kidneys holding on to sodium because perfusion is low) from a tubular injury pattern.

Formula: FENa = (urine Na × plasma creatinine) ÷ (plasma Na × urine creatinine) × 100  — sodium in mEq/L, creatinine in mg/dL (any consistent creatinine unit on both sides).

Enter the values or tick the options above — the result updates automatically.

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How to interpret the result

FENaTraditional interpretation in acute kidney injury
Below 1%Prerenal pattern — sodium conservation by intact tubules responding to low perfusion.
1–2%Indeterminate — can be seen in either pattern or during transition.
Above 2%Tubular injury pattern (for example acute tubular necrosis).

Important limits: FENa is unreliable after diuretics (the FEUrea is often used instead), in chronic kidney disease, in glomerulonephritis, and in several other settings. It is one clue among many — urine microscopy, the clinical story, and the response to treatment all matter.

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Frequently asked questions

What FENa suggests a prerenal cause?

A FENa below 1% is the classic prerenal pattern, meaning the tubules are intact and conserving sodium. Above 2% suggests tubular injury; between 1% and 2% is indeterminate.

Why is FENa unreliable after diuretics?

Diuretics force the kidney to excrete sodium, so the FENa rises even when the underlying problem is prerenal. In that situation the fractional excretion of urea (FEUrea) is traditionally used instead, because urea handling is less affected by common diuretics.

Do the urine and blood samples need to be taken together?

Ideally yes — the urine and blood sodium and creatinine should come from about the same time, before large fluid or diuretic interventions change the picture.

Can FENa be used in chronic kidney disease?

With caution at best. In CKD the remaining nephrons normally excrete a larger fraction of filtered sodium, so the classic cut-offs do not apply cleanly.

Educational use only. This calculator is an aid to learning and does not replace clinical judgment, local reference ranges, or specialist advice. Always interpret a result alongside the patient, the full laboratory picture, and your laboratory's own reference ranges.