Yes a screen and test are not the same, a screen or EMIT is a immunoassay or dip test like a home test. EMIT recognizes a composite of all 31 metabolite concentrations. The psychoactive ingredient in marijuana is 11-nor-D-9-tetrahyrocanibinol (THC). This is oxidized by the body and comes out as 31 different metabolites. The most prevalent form is 11-nor-D-9-tetrahyrocanibinolic acid (THCA). Thats why it's at a higher cut-off level 50 ng/mL.
The GC/MS cutoff number is lower at 15 ng, because the machine only identifies one of the 31 metabolites, which is the 11-nor-D-9-tetrahyrocanibinolic acid, mentioned previously. To pass, that one metabolite must be below the 15 ng cutoff level.
If your positive on a screen it will be sent for a GC/MS for a conformation