DRUG CLEARANCE
Clearance is the measurement of drug elimination from the body.
1.Total Body Clearance (Clr)
Total body clearance is the rate of drug clearance per plasma drug concentration.
The concept of clearance is a portion of the drug from the total apparent volume of distribution in the body in which the drug is dissolved,is eliminated in unit time.
The following equation express this concept of total body clearance.
a) Clr = drug elimination/plasma concentration
= (dDe/dt)/Cp
Clr = VDk -----***
Clr = FD0/AUC
b)For the drugs which follows linear(First Order) kinetics,total body clearance is the sum of the clearances in the body as follows.
Clr = ClR + ClNR
where ClR is the clearance by kidneys and ClNR is the non clearance.
The relation between Clr and t1/2 is well established by substituting 0.693/t1/2 for k in equation marked *** above as follows,
Clr = VD 0.693/Clr where VD and Clr are considered as independent variables and t1/2 as a dependent variable.
As in renal desease clearance decreases t1/2 increases.Changes in VD also bring proportional changes in t1/2.
where ClR is the clearance by kidneys and ClNR is the non clearance.
The relation between Clr and t1/2 is well established by substituting 0.693/t1/2 for k in equation marked *** above as follows,
Clr = VD 0.693/Clr where VD and Clr are considered as independent variables and t1/2 as a dependent variable.
As in renal desease clearance decreases t1/2 increases.Changes in VD also bring proportional changes in t1/2.
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