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Clinical — Renal
FeNa Calculator
Enter urine and plasma sodium and creatinine to get the fractional excretion of sodium (FeNa) for assessing acute kidney injury.
Fractional excretion of sodium
—
FeNa, as a percentage.
| Interpretation | — |
|---|---|
| Urine / plasma sodium | — |
| Urine / plasma creatinine | — |
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Enter your figures above to see the step-by-step working.
For clinical reference — diuretics and other factors distort FeNaEverything computed client-side — nothing leaves this page
How the FeNa Calculator Works
The fractional excretion of sodium (FeNa) is the percentage of the sodium filtered by the kidneys that ends up in the urine. It helps distinguish the cause of acute kidney injury. The calculator uses paired urine and plasma sodium and creatinine measurements.
A low FeNa (under 1%) suggests the kidneys are avidly holding on to sodium, as in prerenal states from low blood flow. A high FeNa (over 2%) points to intrinsic kidney damage such as acute tubular necrosis. Diuretics and other factors can distort the result, so interpret in context.
FeNa = (urine Na × plasma Cr) ÷ (plasma Na × urine Cr) × 100
Frequently Asked Questions
What does the FeNa tell me?
It helps sort out why kidney function has dropped. Under 1% suggests a prerenal cause — the kidney is conserving sodium because of reduced blood flow. Over 2% suggests intrinsic damage such as acute tubular necrosis.
When is FeNa unreliable?
Diuretics increase sodium excretion and can raise FeNa despite a prerenal state; chronic kidney disease and other conditions also distort it. In diuretic use, the fractional excretion of urea (FeUrea) is sometimes used instead.
What samples do I need?
A spot urine and a blood sample taken around the same time, each measured for sodium and creatinine. The calculator combines the four values into the fractional excretion.
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