I know that immunoassay and GC/MS are functionally the same but isn't there a chance that the level of the particular molecule they are looking for with GC/MS (THC-COOH or something, I believe?) will be higher than 15 ng since it is the most prevalent metabolite of THC that remains in the body? Isn't that why they use that molecule for GC/MS? I understand how both tests work and honestly, if a false positive is caused by something I may have taken just randomly, I definitely want them to do GC/MS to confirm that I'm not positive. Just a little confused why they picked that particular one to use. Also, if I'm on a prescription medication and get a positive for another type of drug, do they use GC/MS across the board for all drugs to confirm the presence of that one? The way I understood it, GC/MS is very expensive and they only use it to confirm a presumptive positive for one certain drug molecule and not all of them.