Which statement about PAH clearance and effective renal plasma flow is true?

• A. PAH clearance equals true renal blood flow (RBF)
• B. PAH clearance yields GFR
• C. PAH clearance approximates ERPF because PAH is freely secreted and not significantly reabsorbed
• D. PAH clearance is used to measure renal blood flow directly

Answer: (C)

PAH clearance serves as an estimate of renal plasma flow because PAH is filtered and actively secreted in the proximal tubule and is not significantly reabsorbed. This means nearly all PAH entering the kidney is removed from the plasma and excreted in urine, so the rate at which PAH is cleared from plasma (the clearance) closely matches the effective renal plasma flow (ERPF). It does not measure total renal blood flow directly, since that would require accounting for the plasma as a portion of blood and is larger than ERPF by the factor 1/(1 minus hematocrit). Not used for GFR, which is measured by inulin or creatinine clearance.