The fractional excretion of sodium (FENa) measures the percentage of sodium which is actually excreted through the urine over the total sodium originally filtrated by the kidney. The majority of the sodium is reabsorbed. however, if the patient is not receiving diuretics and the FENa is less than 1%, it means that the kidney is avidly trying to preserve sodium. On the contrary, if the kidney is in acute renal failure and the FENa is higher than 1% (in the absence of diuretics), this may mean that there is tubular damage.