USMLE RPF calc using PAH

[kimjongillest]

Hi everyone,

I am having trouble understanding what should be a basic concept. When measuring renal plasma flow, Urine PAH x urine output is divided by serum PAH. Why do we divide by serum PAH? I can't seem to understand how that would give you information about secretion (vs Urine PAH/Urine+Serum PAH)

Thank you in advance for any help!

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[KVBane]

Usolute/Ssolute x Volume is the formula for Clearance
Since PAH is completely removed from the plasma by the kidney (20% filtration, 80% secretion), it has almost complete clearance (approx. 92%)

Since we know the clearance of a substance is almost complete - we can use that to figure out how much plasma is flowing to the kidney - since clearance depends on RPF.

PAH calculates eRPF - which is about a 10% underestimation of real RPF.