An assay will be performed first and if the sample fails that, THEN the GC/MS will be performed on the drug(s) that failed the assay. It would be way too costly to use the GC/MS as the primary methodology.


Dont let the differing cutoff levels freak you out. Although their cutoffs numerically differ, a 50 ng assay is functionally equivalent to the 15 ng GC/MS. Their cutoffs differ because the assay recognizes all 31 metabolite concentrations combined, while the GC/MS recognizes only 1 metabolite concentration.

THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations. The most prevalent metabolite concentration is the 11-nor-delta-9-carboxy-THC (aka "THC-COOH") concentration.
In order to test negative on the assay, the combination of all 31 metabolite concentrations must register below 50 ng.
In order to test negative on the GC/MS, only that one metabolite concentration (THC-COOH) must register below 15 ng.