The only true synthetics that are common for us with chronic pain are the fentanyl
, Sufentanil, Alfentanil, etc.) and methadone, which are come from very different chemical processes. The chemistry is not too complicated, if you have any background in it.
As for a difference is testing for them, that's largely not true. Most or all the semisynthetics are broken down into common metabolites (e.g.