The GC/MS wont test for THC if youve tested negative for it on the initial assay screen, and on the off-chance that they did, you will still test negative. Thats because the 50 ng assay is functionally equivalent to the 15 ng GC/MS. The cutoff levels are numerically different because the assay detects all 31 different metabolite concentrations combined, while the GC/MS only detects one metabolite concentration (THC-COOH).

THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations, the most prevalent concentration being THC-COOH.

In order to test negative on the assay, all 31 concentrations combined must test below 50 ng.

In order to test negative on the GC/MS, only one metabolite concentration (THC-COOH) must test below 15 ng/ml.