How do I arrive at the formula for total clearance? (pharmacokinetics)

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In Rang & Dale (10 ed) on page 152, the formula for total clearance is given by:

$$Cl_{tot}=\frac{Q}{AUC_{0-\infty}}$$

$CL_{tot}=$total clearance

Q=initial dose given

$AUC_{0-\infty}$=Area under the concentration/time curve from time 0 to infinity.

No explanation for this formula is given, at least not as far as I can see, it's just stated as some kind of obvious fact. How do I arrive at this formula (i.e, where is the derivation?), is it given by some kind of elementary rule?

The concentration/time curve should be an example of exponential decay, so in principle, you shouldn't have to be well versed in pharamacology or parmacokinetics to arrive at this formula I think.

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