The concentration of para aminohippurate (PAH) is measured in one arterial blood sample (PPAH) and one urine sample(UPAH). The urine flow (V) is also measured. Renal perfusion flow is then calculated by:
The renal extraction ratio of PAH in a normal individual is approximately 0.92, and thus not exactly 1.0. Thus, this method usually underestimates RPF by approximately 10%. This margin of error is generally acceptable considering the ease with which eRPF is measured.
- Reubi FC (September 1953). "Glomerular filtration rate, renal blood flow and blood viscosity during and after diabetic coma" (PDF). Circulation Res. 1 (5): 410–3. PMID 13082682.