Para-aminohippuric acid (PAH) is infused into a healthy volunteer until its plasma concentration reaches 1 mg/dL. At steady state, urine PAH concentration is 65 mg/dL and urine flow rate is 2 mL/min. What does this clearance value represent?

• A Effective renal plasma flow ✓
• B Glomerular filtration rate
• C Total renal blood flow
• D Maximal tubular secretory capacity for PAH

Explanation

At low plasma concentrations, PAH is both freely filtered and avidly secreted by the proximal tubule via organic anion transporters, so roughly 90 percent of the PAH arriving in renal plasma is excreted in one pass. Its clearance therefore approximates effective renal plasma flow, about 625 mL/min. It underestimates true RPF because about 10 percent of renal blood flow perfuses non-excretory tissue such as fat and the capsule. Dividing by the hematocrit, not multiplying, converts it toward total renal blood flow.

Reference: Ganong's Review of Medical Physiology, 27th ed.

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