Even though the cutoff numbers are different, a 50 ng assay screen is essentially equivalent to the 15 ng GC/MS. The difference is the assay screen recoginzes the composite of the 31 metabolite concentrations. THC enters the body in its ingested form and exits the body as 31 metabolite concentrations, the most prevelant form being the THC-COOH metabolite concentration.
In order to test negative, the whole composite must test below 50 ng.

The GC/MS has a lower cutoff at 15 ng because the machine only recognizes 1 metabolite concentration (THC-COOH). In order to test negative, that one metabolite concentration must test below 15 ng.