Youre worrying yourself over nothing.

If youre passing your home tests with the first void of the day, you'll pass the actual U/A. They wouldnt run the GC/MS on the sample once it passes the initial screen anyway. Its a waste of the labs time and the clients money.

But even if for shits & giggles they do run the confirmation GC/MS, the sample would still test negative. Thats because a 15ng GC/MS is functionally equivalent to the 50ng Immunoassay screen. The cutoff levels are numerically different because the Immunoassay recognizes the composite of all 31 THC metabolite concentrations, while the GC/MS only recognizes one metabolite concentration: THC-COOH.
To pass the Immunoassay, the whole composite must test below 50ng.
To pass the GC/MS, only that one metabolite concentration (THC-COOH) must register below 15ng. The cutoff has to be set at a lower number to make the two types of tests functionally equivalent.