What Diege said..:thumbsup:

Only the drug panel(s) that have tested "non-negative" on the initial I/A screen undergo the confirmation GC/MS. If the sample is negative on the initial I/A, test is over as a "pass" - no confirmation GC/MS performed.

And by doing a Search in the Drug Testing forum using the terms "functionally equivalent metabolites", you'll discover that although their cutoff thresholds numerically differ, a 50 ng I/A screen is functionally equivalent to the 15 ng GC/MS.