FENa Calculator (Fractional Excretion of Sodium)
The fractional excretion of sodium (FENa) is the percentage of filtered sodium that is excreted in the urine, calculated from paired urine and plasma sodium and creatinine. In oliguric acute kidney injury, a FENa under 1% conventionally suggests a pre-renal cause and over 2% suggests intrinsic renal disease such as acute tubular necrosis.
Quick answer
FENa is the percentage of filtered sodium excreted in the urine: FENa = (urine sodium Γ plasma creatinine) Γ· (plasma sodium Γ urine creatinine) Γ 100. In oliguric acute kidney injury, a FENa under 1% conventionally points to a pre-renal cause and over 2% to intrinsic renal disease such as acute tubular necrosis, though diuretics and other factors can confound it.
FENa inputs
Use urine and plasma samples taken at the same time. FENa is most useful in oliguric acute kidney injury.
How to use the FENa calculator
- 1Enter the paired lab values. Enter urine sodium and plasma sodium (both mmol/L), and urine creatinine and plasma creatinine (both mg/dL), from samples taken at the same time.
- 2Read the FENa percentage. The tool applies the formula and shows the FENa as a percentage.
- 3Interpret in context. Under 1% conventionally suggests pre-renal, over 2% intrinsic renal, and 1β2% is indeterminate β interpreted with the clinical picture by a clinician.
Medical disclaimer: This is general health information, not medical advice. It does not diagnose or treat any condition, and the results are estimates based on public reference formulas. Always consult a qualified doctor about your individual health. If you think you may have a medical emergency, contact your local emergency services immediately.
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Book a consultationFrequently asked questions
What does a low FENa mean?β
In oliguric acute kidney injury, a FENa under 1% conventionally suggests a pre-renal cause β the kidney is avidly conserving sodium because of reduced perfusion. It is a guide, not a diagnosis, and must be read with the clinical picture.
Why can diuretics make FENa misleading?β
Diuretics increase sodium excretion, which can raise FENa above 1% even in a pre-renal state. When a patient is on diuretics, the fractional excretion of urea (FEUrea) is sometimes used instead, because urea handling is less affected.
Does FENa always separate pre-renal from intrinsic causes?β
No. A FENa under 1% can also occur in some intrinsic causes (such as contrast nephropathy, pigment nephropathy, or early or partial obstruction), so the result is interpreted alongside the history, examination and other tests.
Is my data stored?β
No. The calculator runs entirely in your browser; nothing you enter is sent to a server.
Sources & validation
This calculator reproduces the published FENa score, validated for differentiating pre-renal from intrinsic causes of acute kidney injury.
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