Regarding Imuran concentration, here's the test mentioned in more detail.
Thiopurine Metabolites is a diagnostic test for immunomodulators that determines if you have the proper concentration of that medication in your system for optimum therapeutic (healing) effect, and it is helpful in establishing proper dosing for individuals. Any lab can do this test, here's an example from Labcorp:
https://www.labcorp.com/test-menu/35596/thiopurine-metabolites