The renal clearance equation CLrenal = fu*GFR + CLsecret - CLreabs implies that increasing CLreabs would do what to CLrenal?

• A. Increase CLrenal
• B. Decrease CLrenal
• C. No effect
• D. Could increase or decrease depending on other terms

Answer: (B)

Net renal clearance is built from filtration plus secretion minus reabsorption. The formula includes a minus CLreabs term, so increasing reabsorption means more drug is returned to the blood and less is eliminated in urine. If filtration clearance and secretion stay the same, CLrenal decreases by the same amount that CLreabs increases. For instance, raising CLreabs by 3 mL/min reduces CLrenal by 3 mL/min. In extreme cases clearance can approach zero but won’t go negative.