In the renal dose adjustment equation, Fe represents what?

• A. Percent of drug cleared by kidneys unchanged
• B. Fraction excreted by liver
• C. Protein binding percentage
• D. Volume of distribution

Answer: (A)

Fe reflects the fraction of the drug that is eliminated by the kidneys as unchanged drug. In renal dose adjustment, this tells you how much renal clearance contributes to total elimination. If a large portion of the drug is cleared renally (high Fe), renal impairment will markedly reduce clearance and increase exposure, so dosing or dosing intervals must be adjusted. If renal excretion is minimal (low Fe), renal impairment has less effect on overall clearance. The other options don’t describe this renal-specific excretion fraction: the fraction excreted by the liver relates to hepatic clearance, protein binding percentage affects distribution and clearance but is not Fe, and volume of distribution is a separate PK parameter that does not denote renal excretion.