U01.21.029 Effective Renal Plasma Flow (eRPF)

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Learning Objective:
Understand and apply the formula for Effective Renal Plasma Flow (eRPF), which estimates the plasma volume flowing through the kidneys per unit time using PAH clearance.

Formulas:

eRPF = C_{PAH} = \frac{U_{PAH} \times V}{P_{PAH}}

Explanation:

  • eRPF = Effective Renal Plasma Flow
  • C_{PAH} = Clearance of para-aminohippuric acid (PAH)
  • U_{PAH} = Urine concentration of PAH
  • P_{PAH} = Plasma concentration of PAH
  • V = Urine flow rate

PAH is used because it is both filtered and secreted by renal tubules, meaning almost all PAH entering the kidney is excreted → thus clearance of PAH ≈ renal plasma flow.

Concept Summary:

  • True RPF is slightly higher than eRPF, because PAH is not 100% extracted (≈90%).
  • Correction factor: RPF = \frac{eRPF}{0.9}

Example:

Given:
U_{PAH} = 600 \text{ mg/dL}
P_{PAH} = 2 \text{ mg/dL}
V = 1 \text{ mL/min}

eRPF = \frac{600 \times 1}{2} = 300 \text{ mL/min}

✅ Answer: eRPF = 300 mL/min

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