Legit Prescriptions and GC/MS Confirmation

I was wondering if you show up positive for a prescription, if it would cause them to confirm (re-test at 15 ng/ml on gc/ms) ALL the drugs that tested negative the the original assay? I've been passing my home tests, but am worried sick that the confirmation test will pick up the thc that I (hopefully) tested negative for. - Also is it true that 50/ng/ml on the home test is the equivalent to 15ng/ml ongc/ms due to the metabolites they look for? Any help would mean the world !