How is the fraction excreted renally (fe) defined in pharmacokinetics?

• A. fe = Dose excreted in urine / Dose administered
• B. fe = Clr / Cl
• C. fe = 1 - (Clr/Cl)
• D. fe = Renal clearance of the drug only.

Answer: (B)

The key idea is how much of a drug’s total elimination comes from the kidneys. Total clearance (Cl) is the sum of all routes that remove the drug from the body, and renal clearance (Clr) is the portion cleared by the kidneys. The fraction excreted renally is defined as fe = Clr / Cl. This dimensionless ratio tells you what portion of overall elimination is due to renal processes. For example, if Cl is 15 L/hr and Clr is 9 L/hr, fe = 9/15 = 0.6, meaning 60% of elimination is renal.

The other ways to express related ideas aren’t the standard definition of fe. Dose excreted in urine divided by dose administered gives how much of the dose ends up in urine, which depends on absorption and distribution and isn’t the same as the clearance-based fraction. The expression 1 − (Clr/Cl) would be the non-renal fraction of clearance, not fe itself. And stating renal clearance of the drug only is not a fraction at all.