Person smokes but at the moment has a theoretical cutoff of THC at 20 ng/mL. Person passes for substances except amphetamine. Sample is sent to the lab for confirmation (which we will assume can detect to 15 ng/mL THC).

Does person now fail for THC? I have run GC/MS myself and cant imagine why they would not
Heres why it would not:
A 50 ng Immunoassay is equivalent to the 15 ng GC/MS. The cutoff numbers are different because the Immunoassay recognizes all 31 metabolite concentrations.
As you may know, THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations, with the most prevalent form being the THC-COOH metabolite concentration. In order to test negative on the I/A, the whole composite of the 31 metab concentrations must register below 50 ng.

The GC/MS cutoff is 15 ng because the machine recognizes only one metabolite concentration: the THC-COOH concentration. In order to test negative, that one metabolite concentration must register below 15 ng.

Therefore if a given sample tests negative on the I/A, that same sample will also test negative on the GC/MS.

But once the sample passes the I/A, it will never see the GC/MS. It would be a huge waste of money for both the client an the lab; and the GC/MS is far from being cheap. Therefore, the GC/MS is only used to confirm positives for only the drugs that tested positive on the I/A.