The science of determining levels of marijuana in the body, specifically THC or the psychoactive product of cannabis, has been improving with technology advancements throughout the past decade. By taking a vile of blood, the labs can compute the exact levels of active metabolite THC within the body at the time of the blood draw. Active metabolites in the body will affect motor skills, reaction time and brain activity.
11-hydroxy THC is the metabolite that impairs the brain. After some time, it is then broken down into 11 or 9 carboxy THC, which does not impair body function. The 2014 ruling in an Arizona Supreme Court case, State ex rel. Montgomery v. Shilgevorkyan, states that an individual cannot be prosecuted for having carboxy metabolite in their body if that person possesses a medical marijuana card.