All urine testing methodologies have some sort of cutoff threshold, so there really is no "zero-tolerance" test.
The test youve took is a 10 panel screen with a confirmation GC/MS. The assay screen is always performed first. It would be too cost-prohibitive on the part of both the lab and client to run the GC/MS as the primary analysis.
Therefore, the GC/MS is only used on any of the panels that have tested "non-negative" on the initial screen. It would run into hundreds, if not thousands of dollars to run the GC/MS on all of the drugs on a given panel (particularly in this case, the 10 panel).

Drug tests are just not conducted in that manner.