Study Shows Roadside THC Tests Do Not Indicate Impairment

As the laws surrounding cannabis relax around the world, drug impairment and driving is a growing concern. Each country has its own method of dealing with this issue. For many governments, the best option has been to use roadside THC saliva tests. However, when it comes to determining cannabis impairment, saliva samples are not accurate. According to a study released from the Lambert Initiative, the amount of THC present in your system does not always indicate intoxication. With new evidence available, the conversation on cannabis and driving is changing; now it’s time for the government to catch up. 

Current test

It’s far easier to tell if someone is driving drunk, versus driving stoned. However, we approach cannabis impairment and driving in the same way we do alcohol. Despite the differences between these two substances, the testing methods are the same. Roadside testing can include a standard field sobriety test or if the officer deems it necessary, a saliva sample. If the saliva sample tests positive for cannabis, it is followed by a blood test at the police station. In Canada, two tests have been approved for cannabis drug screening:

1. Dräger DrugTest® 5000 when used with a Dräger DrugTest® 5000 STK-CA
2. SoToxa™, an Abbot SoToxa™ Test Cartridge used with an Abbot SoToxa™ Oral Fluid Collection Device.

Despite being aware of the limitations and gaps associated with these devices, they were approved in 2018. 

Limitations and Gaps

According to Canada’s Oral Fluid Drug Screening Policy, “Roadside oral fluid drug screening is another tool for law enforcement to help reduce the problem of drug-impaired driving. It is not the answer to the problem but when employed in conjunction with the Standardized Field Sobriety Test and the Drug Evaluation and Classification program, it can help to reduce the number of drug-impaired drivers.” 

There has always been an awareness of the fact that this type of testing is not accurate for every situation; false negatives and positives are to be expected. These devices don’t test for impairment, they test for concentration. This is clearly stated in the policy; “Oral fluid drug screening does not provide the concentration of the drug, but only detects the presence of particular substances in concentrations that exceed an established threshold.” 

The Study

The amount of THC present in your saliva and blood does not indicate intoxication; thanks to the Researchers at the Univesity of Sydney Lambert Initiative, we now have the evidence to back this up. 

• Researchers began by collecting data surrounding the effects of cannabis and driving. Using meta-analytic techniques, they conducted an in-depth and comprehensive review of 28 publications. 
• With this information, they were able to “characterize the relationships between biomarkers of cannabis use, subjective intoxication, and impairment of driving and driving-related cognitive skills.” This included nine driving-related cognitive skills such as dividing attention, information processing, and reaction time.

The Results

According to the Lead Author of the study, Dr. Danielle McCartney, “Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users while no significant relationship was detected in regular cannabis users. This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.”

Lead author Dr. Danielle McCartney from the Lambert Initiative. Photo Courtesy of the University of Sydney

Cannabis impairment cannot be measured through THC levels in bodily fluid. It simply doesn’t work that way. A new cannabis user can ingest a low dose of THC, yet become extremely intoxicated; someone with more experience can ingest a significantly higher amount, yet feel less impaired from the effect. 

What now?

While it might be applicable in a few situations, THC fluid testing is not a reliable tool. Thus, the subject of drug-impaired driving needs more research. Professor Iain McGregor, Academic Director of the Lambert Initiative had this to say on the matter; “We clearly need more reliable ways of identifying cannabis impairment on the roads and the workplace. The increase in legal recreational use of cannabis across multiple jurisdictions worldwide is also making the need for reform of cannabis-driving laws more urgent.”