Quote Originally Posted by yedmundi


Many labs will use a cheap kit, and if there is a positive, they will do a more expensive test using more sensitive equipment.


The good news is that sometimes the cheap test is more sensitive that the law requires. So it will generate a positive result, even if the amount is below the legal detection threshold. That is why they then to a quantitative test next. So you can test positive, but be officially negative if the amount is low enough. That is the only saving grace. But it depends on what the standard you are being measured against.
Its a common misconception that the confirmation GC/MS test is more sensitive. That misconception stems from the fact that the GC/MS has a numerically lower cutoff threshold than your typical Immunoassay screen.
The GC/MS is not more sensitive - its just more accurate than the initial I/A screen. And even though their cutoffs numerically differ, A 50 ng/ml I/A screen is functionally equivalent to the 15 ng GC/MS. Their cutoffs numerically differ because the I/A screen recognizes all 31 metabolite concentrations combined, while the GC/MS recognizes only one metabolite concentration.

THC enters the body in its ingested form and exits the body as 31 different metabolite concentrations. The most prevalent form of these metabolite concentrations is THC-COOH.

In order to test negative on the I/A screen, the whole combination of the 31 metabolite concentrations must test below 50 ng.
In order to test negative on the GC/MS, only one metabolite concentration (the THC-COOH concentration) must register below 15 ng. The cutoff has to be set numerically lower to make the two types of tests functionally equivalent.