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Clearance in Pharmacology
*Clearance (from Maintenance Dose)*
Definition
Clearance is the volume of blood* cleared of drug per unit of time (ml/minute).
- It is not the amount of drug cleared in a unit of time (as it's easily confused to be).
Formula
We remind ourselves that:
Maintenance dose = (CLTC)/F x Dosing Interval
We rearrange this formula as follows:
- CL = Maintenance Dose/(Dosing Interval x C)
- Note that here we swap out the target concentration for the actual plasma concentration of the drug (we remove the bioavailability variable).
Example
Using our values from above:
- 240 mg/(12 h x 5(mg/L)) = 4 L/h
- The masses (mg) cancel and the clearance equals 4 L/h
Alternative Calculation from Elimination Constant
CL = Vdke