# PharmPK Discussion - Clearance

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• On 2 Jun 2004 at 13:02:24, Jean Michel linget (jmlinget.-at-.carex.fr) sent the message
`When we measure plasma clearance, we often compare to the hepatic bloodflow (for instance 21 ml/min/kg in the man). If you measure bloodclearance, due to a high blood partitioning, do you still use the samevalue of hepatic blood flow ?If we start with the value of 21 ml/min/kg as hepatic blood flow in thehuman, do we must compare the blood or the plasma clearance ?Thank you very much for your helpJean michel lingetADME/toxCareX`
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• On 7 Jun 2004 at 08:54:23, "Hans Proost" (j.h.proost.at.farm.rug.nl) sent the message
`Dear Jean Michel,You wrote:"When we measure plasma clearance, we often compare to the hepatic bloodflow (for instance 21 ml/min/kg in the man). If you measure bloodclearance, due to a high blood partitioning, do you still use the samevalue of hepatic blood flow ?"IHMO, this question depends on the rate of drug transport betweenplasma and blood cells:1) If this transport is fast, i.e. if equilibrium is maintained duringpassage of the liver, comparison of clearance to hepatic blood flowmakes sense only if the clearance is converted to blood clearance. Using thenotation proposed by Rowland, the elimination rate is proportional toboth plasma concentration and blood concentration:elimination rate = CL . C  =  CLb . Cbwhere the subscript b refers to blood and the symbols without subscriptto plasma.Assuming a fixed and known ratio of Cb and C, CLb can be calculatedfrom CL and Cb/C.2) If the transport is slow, i.e. if the concentration of drug in bloodcells is not changing during passage of the liver: in this case, hepaticblood flow should be converted to hepatic plasma flow using thehematocritvalue.I would assume that #1 is the usual case, but I am definitely not sureabout this. Any comments?Best regards,Hans ProostJohannes H. ProostDept. of Pharmacokinetics and Drug DeliveryUniversity Centre for PharmacyAntonius Deusinglaan 19713 AV Groningen, The Netherlandstel. 31-50 363 3292fax  31-50 363 3247Email: j.h.proost.-at-.farm.rug.nl`
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