pah clearance pah clearance para-aminohippurate (pah) clearance is a method used in renal physiology to measure renal plasma flow, which is a measure of renal function. pah clearance parameter value renal blood flow rbf = 1000 ml/min hematocrit hct = 40% glomerular filtration rate gfr = 120 ml/min renal plasma flow rpf = 600 ml/min filtration fraction ff = 20% urine flow rate v = 1 ml/min sodium inulin creatinine pah sna = 150 meq/l sin = 1 mg/ml scr = 0.01 mg/ml spah = una = 710 meq/l uin = 150 mg/ml ucr = 1.25 mg/ml upah = cna = 5 ml/min cin = 150 ml/min ccr = 125 ml/min cpah = 420 ml/min er = 90% erpf = 540 ml/min purposemeasure renal plasma flow pah is completely removed from blood that passes through the kidneys (pah undergoes both glomerular filtration and tubular secretion), and therefore the rate at which the kidneys can clear pah from the blood reflects total renal plasma flow. the concentration of 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: r p f = u p a h p p a h v v} what is calculated is the effective renal plasma flow (erpf). however, since the renal extraction ratio of pah almost equals 1, then erpf almost equals rpf. precision 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.