Its not up to the lab to determine that. Thats a decision made by the party thats paying for the test (employer, PO, etc). The lab just provides the menu of testing options for the client to pick and choose from.

Labcorp has much more drug testing options on their menu than just the GC/MS. Whoever told you otherwise is pulling your leg.

Most employers, in an effort to preserve their bottom line, will opt for a 50 ng screen with the 15 ng GC/MS to be used only to confirm any positives showing on the screen. If the person passes the initial screen, no confirmation test would be necessary and the testing process is finished there. It's way too costly for most employers to opt for the GC/MS as the primary analysis.