They do this to make sure the serum is the right level in people who take medications that can change levels based on how long you take it. You need a base number to go off of, so there’s no way to tell exactly how much someone took, but if you’ve tested every month, you’ll know how much that amount will look like in their blood over time. You can tell if someone has overdosed when there’s an amount far greater than a normal dose has entered the system, since it is not longer in the safe therapeutic range and has likely done some damage