Which components contribute to renal clearance in the equation CLrenal = fu*GFR + CLsecret - CLreabs?

• A. Filtration via fu*GFR only
• B. Filtration via fu*GFR plus active secretion (CLsecret) minus reabsorption (CLreabs)
• C. Secretion only
• D. Reabsorption only

Answer: (B)

Renal clearance is the balance of what the kidney removes from the blood and what it returns to it. The filtration term uses the unbound fraction of the drug times the glomerular filtration rate, because only unbound drug is filtered into the tubules. So filtration contributes fuGFR to clearance. In addition, the tubules can actively secrete drugs into the urine, adding CLsecret to the clearance. At the same time, some drugs are reabsorbed from the tubular fluid back into the blood, which reduces the amount excreted and thus subtracts CLreabs from total clearance. Put together, the net renal clearance is fuGFR plus CLsecret minus CLreabs. If a drug has no secretion or reabsorption, the equation reduces to filtration alone; if there are secretion or reabsorption processes, they adjust the total accordingly.