Para-aminohippuric acid (PAH) clearance is used clinically to estimate renal plasma flow. Why does PAH clearance slightly underestimate the true renal plasma flow?

• A PAH is partly reabsorbed in the proximal tubule
• B PAH is not completely extracted from the blood in a single pass through the kidney ✓
• C The organic anion transporters saturate at therapeutic plasma concentrations
• D A significant fraction of PAH is filtered rather than secreted

Explanation

PAH is both freely filtered and avidly secreted by the proximal tubule, so its extraction ratio is approximately 90 percent, not 100 percent. Some PAH remains in the renal venous effluent, meaning PAH clearance measures only the effective renal plasma flow, about 90 percent of true plasma flow. Reabsorption of PAH is negligible, and the transporters are far from saturation at the low diagnostic infusion concentrations used.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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