As I understand your complaint, your initial assay was positive for amphetamine (Amp) by screen (IA). Confirmatory testing by GC/MS particularized to methamphetamine (MAmp). You are prescribed Didrex.

Didrex (Benzphetamine) metabolizes into d-amphetamine, same as methamphetamine,* and could (in fact should) return positive on both initial screen and confirmatory test (i.e. false positive for meth).

Since you do not use meth and have previously recorded use of Didrex I would suggest you direct your contest to the documented pharmacolgic facts noted foregoing as opposed to procedural errors noted.

If we can be of further assistance please advise.

* See Didrex as example.