If your sample is below 50 ng, the sample wont have to go thru the 15 ng confirmation test. It would be a huge waste of money for both the lab and the client paying for the test.

Another thing: Even though the cutoff number is lower, a 15 ng GC/MS is essentially equivalent to a 50 ng assay screen. The cutoffs are numerically different is because the assay recognizes the composition of all 31 THC metabolite concentrations, while the GC/MS looks at one 1 metabolite concentration: THC-COOH.
To pass, that one metabolite concentration, THC-COOH, must register below 15 ng.

To pass the assay screen, the whole composition of the 31 metabolite concentrations must test below 50 ng.