I know some about drug testing. First of all the sample is screened for most of the drugs everyone is aware of. Heroin, meth, MJ, cocaine, and numerous controlled prescription drugs. It will then test positive for say amphetamine, it is a screen, not a positive test.
Once the screen tests positive then the sample is retested only for the positive hits, so if it tested positive for amphetamines which would be the screen then it is retested for all of the amphetamines until a positive match is found.
You can have a positive screen and a positive test and still test negative. There are standards that are set by NIDA. Look it up. There are levels set for confirmation on every drug. Marijuana for example is 50 nanograms, meaning if you test positive on the screen for MJ, positive on the test in your sample and if it comes out 49 nanograms per ML of blood that is a negative. The NIDA standards are set so that you won't get a false positive and are medically accepted as "you did it" for a lack of better terms.
You can't test positive unless you used the drug. The intent of the NIDA standards are to make sure that doesn't happen
I have arrested people who had meth in their posession, admitted to smoking it and using on a regular basis and still test negative because of the required levels to confirm a positive test.
I enjoy watching the conspiracy stories out there. It is like the guy you arrest that has meth in his pocket and says, dude these are not my pants.
Rich