Testing for THC Impairment

Why Setting a Legal "High" Driving Limit Makes No Sense

A simple “They looked stoned” isn’t enough to convict someone of impaired driving under the influence of cannabis. Some states have set a threshold level for THC, the high-inducing chemical in cannabis, in the bloodstream. As it turns out, however, this threshold is arbitrary. It’s impossible to know how much THC is affecting the brain without fancy brain-imaging techniques, and assuming impairment correlates with THC levels is wrong. Because of the fat-soluble nature of THC and other cannabinoids, as opposed to the aerosolized nature of alcohol, there are numerous challenges for THC detection in the body and the interpretation of how it relates to impairment.

Unlike alcohol, where levels in the blood reflect effects on the brain, you can have a high blood THC level with very little reaction in the brain; it largely depends on the time blood is drawn in relation to when you last consumed cannabis. Most importantly, you can’t predict impairment by blood THC concentration alone. Genetics (and even your last meal!) can influence the proportion of THC that reaches the brain, as well as the speed, strength and duration of its effect. Use history also affects how stoned you become. Long-term toking reduces the number of brain receptors for THC to act upon, so equivalent THC levels in the blood of a novice may lead to substantially greater impairment than that of a seasoned weed veteran. Fewer targets, weaker action. And let’s not forget that predicting cannabis intoxication becomes even more challenging when additional cannabinoids are thrown into the mix, like cannabidiol (CBD), which can counteract some of THC’s effects.

Most legislators, law enforcement and scientists agree that trying to infer impairment from a blood THC level is pointless. But when Colorado legalized recreational cannabis, they set an arbitrary limit of five nanograms THC per milliliter of blood as a compromise b

etween those pushing for zero tolerance and the cannabis lobby, who feared that THC released from the body’s fat cells into the blood long after consumption could trigger a positive test result. In states like Washington and Montana, THC levels above this threshold result in an automatic DUI, whereas Colorado uses this number to simply infer impairment.

Law enforcement uses blood to establish THC levels because limitations in other detection methods have traditionally lacked in sensitivity and reliability. But getting blood samples is invasive and requires a warrant, which delays the time to sample collection. New detection strategies had to be devised. Urine was proposed, but detection is fraught with handling challenges; cannabinoids can absorb into the sample container, break down in light and decompose if too warm. At lower temperatures, on the other hand, they become unevenly distributed in the sample, leading to falsely high results. Cannabinoids can also precipitate out of solution entirely. Spit can be used to test THC levels, but remains subject to many of the same challenges as urine.

Breath tests are emerging as the most promising means of assessing THC levels. But because THC doesn’t easily vaporize, like alcohol—unless heated, of course—the amount of THC that can be detected through the breath has been traditionally too low, and the window for detection too short. Not surprisingly, then, a bulk of research funding in states with legal cannabis is devoted to improving breathalyzer technology.

Proponents of legal weed shouldn’t look upon these efforts with scorn. These detection devices will hopefully lead to safer roads, improved transparency between law enforcement and the community and help establish a clearer understanding between THC and impairment. The real frustration should be over the five nanograms THC per milliliter of blood threshold and its automatic DUI charge in some states. That just makes no sense.

Josh Kaplan

Josh Kaplan earned his Ph.D. in behavioral neuroscience from Oregon Health & Science University in 2015. He is currently a freelance writer and a senior fellow at the University of Washington conducting medicinal cannabis research. Links to his work can be found at neurokaplan.com.

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