Clinical and Interpretive

The fractional excretion of sodium is the percentage of the sodium filtered by the kidney which is excreted in the urine. It is measured in terms of plasma and urine sodium, rather than by interpretation of urinary sodium concentration alone, as urinary sodium concentration varies with water reabsorption.

Fractional excretion of sodium can be used to determine if hypovolemia or decreased effective circulating plasma volume is a contributor to kidney failure.