Have we come across a presenter who commented that the ‘clearance rate of a drug is high or low’?
A question arises: Is clearance an elimination rate of a given drug?
Some of us remain unclear regarding the concepts of elimination rate and clearance of drugs.
Elimination rate is the amount of drug (e.g. mg) that is eliminated in the plasma per unit time (e.g. min). Hence the unit of drug elimination rate is mg/min. In linear pharmacokinetics, the elimination rate is not a constant but changes with the drug ‘burden’ in our body.
Therefore, the drug elimination rate is high when more drug is present in our body while the rate is low when there is less drug in our body!
In other words, drug elimination rate is proportional to the plasma concentration of drug, isn’t it?
Drug elimination rate ∝ plasma concentration (C)
Another question arises: what is the proportionality constant?
The proportionality constant is in fact the plasma clearance (CL) of the drug!
Drug elimination rate (mg/min) = CL (mL/min) × C (mg/mL)
In other words, plasma clearance is a constant in linear pharmacokinetics, associating the relationship between drug elimination rate and plasma concentration.
We can also describe the plasma clearance as the volume of plasma that is cleared of the drug per unit time! All the units make sense too!
In summary, next time when we present or write our report, we should describe that ‘the clearance of a drug is high and low’ and not the ‘clearance rate’.
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Resource Person: Eric Chan, PhD