What Is a FENa Calculator?
A FENa calculator computes the fractional excretion of sodium — the percentage of sodium filtered by the kidneys that is actually excreted in the urine. It is a widely used tool in the workup of acute kidney injury (AKI) to help distinguish a prerenal cause (such as dehydration or low blood flow) from intrinsic kidney damage such as acute tubular necrosis (ATN). Enter the serum and urine sodium and creatinine values, and the calculator returns the FENa percentage and its typical interpretation.
How to Use the FENa Calculator
- Urine sodium (UNa) — in mEq/L.
- Serum sodium (PNa) — in mEq/L.
- Urine creatinine (UCr) — in mg/dL.
- Serum creatinine (PCr) — in mg/dL.
- Calculate — see the FENa percentage and interpretation.
The FENa Formula
FENa (%) = [(UNa × PCr) ÷ (PNa × UCr)] × 100
The formula compares how much sodium is cleared relative to how much creatinine is cleared, which corrects for changes in urine concentration.
Interpreting FENa
| FENa | Typical Interpretation |
|---|---|
| Less than 1% | Prerenal AKI (kidneys avidly retaining sodium) |
| 1%–2% | Indeterminate / overlap |
| Greater than 2% | Intrinsic AKI such as ATN |
Important Caveats
- FENa is most reliable in oliguric AKI and is less useful in chronic kidney disease.
- Diuretics can falsely elevate FENa; in that situation the fractional excretion of urea (FEUrea) may be more reliable.
- Some conditions (contrast nephropathy, early obstruction, glomerulonephritis) can show a low FENa despite intrinsic injury.
- FENa is one data point and must be interpreted alongside the full clinical picture.
Note: This calculator is an educational tool for clinicians and students and does not replace clinical judgment or institutional protocols.
Frequently Asked Questions
What is FENa used for?
FENa helps differentiate prerenal acute kidney injury from intrinsic causes like acute tubular necrosis by measuring how much filtered sodium the kidneys excrete.
What is the FENa formula?
FENa (%) = [(urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine)] × 100.
What FENa indicates prerenal AKI?
A FENa below 1% suggests a prerenal cause, where the kidneys are conserving sodium in response to reduced perfusion. A value above 2% suggests intrinsic injury such as ATN.
When is FENa unreliable?
FENa can be misleading in patients on diuretics, with chronic kidney disease, or with conditions like contrast nephropathy. The fractional excretion of urea may be used instead when diuretics are involved.
Is this FENa calculator free?
Yes — it is completely free, requires no signup, and returns the percentage with interpretation.