Blood and saliva are ‘inconsistent indicators’ when testing for THC impairment

3rd December 2021

The results of a ground-breaking study suggest that, while there is a strong relationship between blood alcohol concentration and driving impairment, blood and saliva samples are not a good indication of THC impairment.

Researchers at the University of Sydney’s Lambert Initiative found that blood and saliva THC concentrations are poor or inconsistent indicators of cannabis-induced impairment – thus new drug-driving laws may be necessary.

Lead author Dr Danielle McCartney, from the Lambert Initiative for Cannabinoid Therapeutics, said: “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.”

Surprisingly, for infrequent, or occasional cannabis users, some significant correlations between blood and oral fluid THC concentrations and impairment were observed.

Meanwhile, there was no significant relationship between blood THC concentration and driving performance for regular – those using the substance weekly or more often – cannabis users.

Dr McCartney added: “Of course, this does not suggest there is no relationship between THC intoxication and driving impairment.

“It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

Are the current methods to assess cannabis-related impairment valid? Do the current methods work or make the roads safer? The researchers doubt that.

Dr McCartney said: “Our results indicate that unimpaired individuals could mistakenly be identified as cannabis-intoxicated when THC limits are imposed by the law. Likewise, drivers who are impaired immediately following cannabis use may not register as such.”

Do those driving under the influence of cannabis know they are unfit to sit behind the wheel? Well, not necessarily. “Subjective intoxication” – basically how “high” the personal feels – could be misleading.

Professor Ian McGregor, the Academic Director of the Lambert Initiative, said the dangers of having ineffective measures are real.

He said: “THC concentrations in the body clearly have a very complex relationship with intoxication. The strong and direct relationship between blood-alcohol concentrations and impaired driving encourages people to think that such relationships apply to all drugs, but this is certainly not the case with cannabis.

“A cannabis-inexperienced person can ingest a large oral dose of THC and be completely unfit to drive yet register extremely low blood and oral fluid THC concentrations. On the other hand, an experienced cannabis user, might smoke a joint, show very high THC concentrations, but show little if any impairment.

“We clearly need more reliable ways of identifying cannabis impairment on the roads and the workplace. This is a particularly pressing problem for the rapidly increasing number of patients in Australia who are using legal medicinal cannabis yet are prohibited from driving.

“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.”

The issue of driving law reform is becoming more prevalent as a growing number of countries and jurisdictions continue to legalise the medicinal or recreational use of cannabis.  A growing number of studies are aiming to understand how THC impairment may affect driving ability and identify more reliable methods to identify this.

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