= Drug elimination
<Drug elimination> removes a parent <drug> from the body's measured parent-drug pool through <drug metabolism> or excretion. Redistribution from <blood> to a <tissue> changes plasma concentration but does not by itself eliminate the drug from the body. A mass balance separates input, output and transfers between compartments. <Population Pharmacokinetics Guidance for Industry by the FDA>[2022, pharmacokinetic model development and concentration-time analysis]{ref}
For a one-compartment model with no input and first-order elimination, $dA/dt=-kA$ gives $A(t)=A(0)e^{-kt}$. Equal fractions disappear in equal time intervals. By contrast, a saturated elimination route can approach removal of equal amounts per interval. The latter produces a different concentration-time curve and cannot be assumed to have a concentration-independent half-life.
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