Fractional Excretion of Electrolytes

The kidneys normally reabsorb most of the sodium that enters the glomerular filtrate. However, sodium reabsorption and potassium excretion are frequently impaired in renal failure, and phosphorus excretion may be increased in the polyuric phase of renal disease. Bicarbonate excretion may also be excessive, contributing to the acidosis that commonly accompanies renal disease. Calculations can be done to determine the fractional excretion of electrolytes, defined as the amount of an electrolyte excreted by the kidneys relative to the amount that is in the glomerular filtrate (representative of the serum level). Calculation of fractional excretions may assist in differentiating the two main types of renal tubular acidosis, which are due to either bicarbonate loss or hydrogen ion retention. However, usefulness of calculating fractional excretions is often limited because of nonrenal factors such as variability in diets and excretion by routes other than the kidney. See fractional excretion section in the Protocol Manual for further information.