It was a simple 5 panel 50ng test, split sample, meaning they split the sample into 2 vials. They sent the test to the lab, and I got a call stating that I came up positive for THC. I'm completely devastated. This was my dream job and to make matters, worse, the lab said my concentration was 60ng! So I literally ALMOST passed, since the cutoff is 50ng.
Since the lab gave you a quantitative value, the urine sample was sent to the GC/MS, which has a cutoff threshold of 15 ng/ml.

The simple 5 panel 50ng test is an assay used for the initial screening. Assays are a "Pass/Fail" endeavor and have no ability to yield quantitative values. Thats when the confirmation GC/MS comes into play.

As adsad pointed out, the GC/MS is highly accurate and because of its accuracy, "false-positives" are eliminated when the urine sample is run through the GC/MS. The Mass Spec (MS) would indentify the exact substance present, so any excuses about poppy seeds, OTC meds, etc; would not float.

Because of the GC/MS's accuracy and its ability to quantify, being able to mount a successful challenge to its results is virtually futile. Keep this in mind if youre planning on challenging the results of sample vial 'A' by having vial 'B' tested.