How is renal clearance approximated for drugs that are filtered but not secreted or reabsorbed?

• A. ClR ≈ fu × CLint
• B. ClR ≈ GFR / fu
• C. ClR ≈ Q × fu
• D. ClR ≈ fu × GFR

Answer: (D)

The key idea is that the renal clearance of a drug that is only filtered (no secretion or reabsorption) is determined by how much unbound drug can be filtered at the glomerulus. Only the unbound fraction (fu) of the drug passes through the filtration barrier, so the filtration rate is fu × GFR. Since renal clearance is the rate excreted divided by plasma concentration, this filtration rate equals the clearance: ClR ≈ fu × GFR. If the drug were fully unbound (fu ≈ 1), this would approach GFR; if it’s bound (fu < 1), it’s proportionally lower. Secretion would increase ClR beyond fu × GFR, while reabsorption would decrease it.