= Drug clearance
{title2=CL}
<Drug clearance> relates the rate at which a <drug> is eliminated to a specified measured <concentration>:
$$
CL=\frac{\text{elimination rate}}{C}.
$$
If elimination is 10 mg/h at a plasma concentration of 2 mg/L, clearance is 5 L/h. This is an effective <volume> per time, rather than the physical <volume> of fluid actually removed. The concentration reference, such as plasma or whole blood, must be specified. <Population Pharmacokinetics Guidance for Industry by the FDA>[2022, model-development discussion: pharmacokinetic parameters and covariates]{ref}
For constant clearance and a one-compartment distribution <volume> $V$, the amount $A=VC$ obeys $dA/dt=-CLA/V$. Thus $k=CL/V$ and $t_{1/2}=V\ln 2/CL$. The derivation explains why a longer half-life can reflect greater distribution <volume> as well as lower clearance; half-life and clearance are not interchangeable measurements.
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