What they aren't telling us is what the linearity of the instrument is. I do drug testing all the time in my lab and the line between positive and negative is clear, and it is always set much higher than the linearity of the instrument. The machine may be able to detect very small amounts, but when they get below a set point, the number cannot be guaranteed. That's why those things are tested, repeatedly.
CS
To be sure of hitting the target, shoot first and call whatever you hit the target.