Fractional Excretion of Sodium Calculator

About this calculator

Fractional excretion of sodium (FENa) measures what percentage of the sodium filtered by the kidneys actually ends up in the urine, calculated as FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100. Creatinine appears in the formula as a reference marker because it's freely filtered and barely reabsorbed, which lets the ratio cancel out urine concentration effects and isolate how much sodium the kidneys are actively reabsorbing versus excreting.

Nephrologists and emergency physicians use FENa to distinguish prerenal azotemia (kidneys are healthy but underperfused, so they aggressively reabsorb sodium, giving FENa less than 1%) from acute tubular necrosis (the tubules themselves are damaged and can't reabsorb sodium properly, giving FENa greater than 2%) in a patient with acute kidney injury — a distinction that changes treatment, since prerenal causes respond to fluids while ATN does not.

This calculator takes urine and plasma sodium and creatinine values and returns the calculated FENa.