There are some suspect methodology and tools around testing. The hair, saliva and sweat tests have a particularly high rate of incorrect positives and negatives. Some tests only look for the inactive metabolite that remains after someone straightens up. (THC from marijuana is converted to THC-COOH, which is fat soluble and can stay in the body for several months after use.)
It would seem best to look for the presence of THC itself which is present while a person is high and set a limit for THC based on actual impairment.
That seems pretty low. I’d heard 10ng/ml was equivalent to the legal drink-driving limit of 0.05 but that was some time ago. (Gmel G, et al. BMC Public Health 2009;9(1):40.)
I can see how these powers serve and protect Police interests. If it was about identifying driving impairment through performance they’d be testing for reaction time e.g. here or here. Or am I missing something?
