FENa Calculator (Fractional Excretion of Sodium)

Written by Great Medico

Updated September 24, 2026 •2 min read

Share Print Save

A lab ratio or imaging score can be useful only if you know what it represents. FENa Calculator (Fractional Excretion of Sodium) combines the displayed kidney/urology measurements into one focused estimate.

What this calculator does

FENa Calculator (Fractional Excretion of Sodium) uses Serum creatinine, Serum sodium, Urine creatinine, and Urine sodium to calculate fractional excretion of sodium. It is built around the exact fields displayed in this plugin rather than a generic version of the topic. Kidney calculations often depend on paired laboratory samples, stable physiology, and the population in which the equation was validated. Acute changes in kidney function or nonstandard sampling can make an apparently precise number unreliable.

How to use it

Enter or select Serum creatinine, Serum sodium, Urine creatinine, and Urine sodium. Use the units offered by the calculator and choose the option that matches the actual clinical or timing situation. Check date entries and laboratory units before relying on the output. For paired blood, urine, or dialysis measurements, use samples and treatment data from the same collection or clinical episode when the formula requires it.

How the calculation works

Fractional excretion of sodium is calculated as (urine sodium × serum creatinine) / (serum sodium × urine creatinine) × 100. The result is the percentage of filtered sodium represented by urinary excretion at the time of sampling.

Example

Using the sample values prefilled in this calculator, the primary result is 0.286 % (Fractional excretion of sodium). This worked example shows how the plugin’s own default inputs flow through the implemented method; replace them with the correct patient, laboratory, imaging, or personal values before interpreting a result.

How to interpret the result

Interpret the output as fractional excretion of sodium from the formula implemented here. Compare it with the relevant laboratory, imaging, pathology, or treatment context, and use trends or a validated clinical pathway when appropriate. One calculated value by itself cannot establish the cause of a urologic or kidney problem or predict an individual outcome.

Limitations and notes

FENa can be altered by diuretics, CKD, glomerulonephritis, pigment nephropathy, contrast injury, obstruction, and other conditions. Traditional cutoffs are context-dependent and do not reliably distinguish causes of AKI in every patient.