several reasons. Blood is not too far removed from the event that we're trying to measure. It's also related in almost all cases to the therapeutic effects that are eventually achieved by the drug since the blood is thought to be an equilibrium or related to the drug appearance at the site of activity. So in all respects, the blood answers most people's questions very adequately and very efficiently.
It also happens that blood levels for regular drugs, not endogenous substances, have some very nice properties. I mean, either it's a straight line relationship between what you're trying to measure and the dose or at worst, it's a nonlinear function where, on this particular graph, a nonlinear elimination would make the curve go upwards which actually increases the sensitivity of the test. And by sensitivity in this respect, I'm saying that a test done in a nonlinear range is much more likely to fail the product. So it becomes extremely sensitive to small differences. So in effect, even a nonlinear drug tends to make products fail rather than passing products that are quite different.
The therapeutic or pharmacodynamic effects