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.