Roadside cannabis impairment tests unreliable according to study

cannabis impairment tests might be unreliable

There is new evidence that the current cannabis impairment tests in Canada are unreliable. This is according to an article by the Department of Justice’s National Institute of Justice (NIJ) in the United States.

Police in this country have two ways to test for cannabis-impaired drivers: testing bodily fluids for THC and standardised field sobriety tests (SFST). The NIJ’s study determined that both SFSTs and THC levels in biofluids are unreliable indicia of impairment.

The study, carried out by researchers from RTI International, raises serious questions about the efficacy of the prevailing means to test for marijuana impairment at the roadside. It remains to be seen whether the study’s findings will have any influence on driving legislation in the US or Canada.

Cannabis impairment tests under the microscope

Personally, the outcome of the NIJ’s research comes as no surprise. Before recreational cannabis was legalised in Canada in 2018, I argued that THC levels in blood and urine do not tell you if someone is too impaired to drive. Police forces now use drivers’ saliva at the roadside to test for THC, although questions remain about how effective this is. I also said that SFSTs had some serious flaws when it came to predicting marijuana impairment.

This study adds to a growing list of data that calls into question the efficacy of Criminal Code provisions related to cannabis-impaired driving. Currently, it is illegal to drive with 2.5 ng or more of THC per 1 ml of blood. Police also use field sobriety tests, such as the walk and turn, the horizontal gaze nystagmus, and the one-leg stand, to detect marijuana-impaired drivers at the roadside.

What did the study consist of?

According to the NIJ’s study, “there is little evidence correlating a specific THC level with impaired driving, making marijuana per se laws controversial and difficult to prosecute”. It also states that SFSTs are not effective in detecting marijuana impairment.

The experiment consisted of six double-blind clinical dosing sessions. Researchers administered known doses of THC to 20 participants before testing their blood, urine, and oral fluid samples. The doses administered were cannabis brownies containing 0, 10 mg, and 25 mg of THC and vapor containing 0, 5 mg, and 20 mg of THC. The doses were spaced at least one week apart.

Researchers then assessed the participants’ cognitive and psychomotor performances using a common impairment tests. The battery of tests included: paced serial addition, digit symbol substitution, a divided attention test, smartphone app tasks, and standardized field sobriety tests.

The participants’ performance in the cannabis impairment tests were negatively impacted after all oral and vaped doses of cannabis except for the 5 mg of vaped THC. The peak of the effects was observed between zero and two hours for vaped THC and three to five hours for edibles.

The results

Intriguingly, the researchers found that standardised field sobriety tests – the one-leg stand, walk and turn, and modified Romberg balance tests – were “not sensitive to cannabis intoxication for any of the study participants”. This suggests that the use of SFSTs in cannabis impairment tests, or at some of them, are ineffective indicia of impairment.

As for the biofluid results, samples of the participants’ blood, urine and saliva were collected every hour for eight hours after consuming cannabis. The samples were tested for THC and non-psychoactive components, cannabidiol and cannabinol. According to the study, “the levels of all three targeted cannabis components…did not correlate with cognitive or psychomotor impairment measures for oral or vaporized cannabis administration.” In other words, how the participants performed on the tests bore no relation to the amount of cannabis components in their systems.

This runs completely contrary to the basis for our cannabis-impaired driving laws, that the amount of THC in your body directly determines your culpability. What effect this study will have in Canada remains to be seen but it’s hard to ignore evidence backed by a US government agency.

Owing to its illicit nature for many years, there is still so much we do not know about cannabis’s effects on cognitive and motor functions. It is vital that our driving laws are backed by science in the interests of justice. Policies must be based on cold, hard facts rather than long-held preconceptions. This study adds to the growing list evidence that our current cannabis impairment tests are deeply flawed.

Download the study here:

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