Making a scientific case to question THC-impairment testing

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Susan Audino

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We have stronger evidence than ever that single measurements of delta-9 THC in breath or blood don’t always indicate impairment from recent cannabis use.

A new study on cannabis impairment testing, published in Scientific Reports, also explores the role of minor cannabinoids in impairment.

The research shows why it’s so crucial for regulatory and enforcement bodies to support research that will ensure public safety while protecting rights.

The study

Two California researchers, Gregory Wurz and Michael DeGregorio, recruited 74 self-reported daily cannabis users to participate in a one-year clinical study.

The average age of the participants was 25.0 years, with mean reported daily use of cannabis of 9.0 years, where most reported preferred smoking or vaping and less frequent edible delivery.

The cannabis histories of the study subjects should be especially concerning.

In one group, a majority showed baseline delta-9 THC blood concentrations that exceeded legal limits currently in place in five U.S. states.

In other words, they couldn’t legally get behind the wheel even when unimpaired.

Previous research from DeGregorio and others studied self-reports of impairment using blood and exhaled breath samples taken before smoking, and then at various times up to four hours after smoking.

Results of those studies demonstrated a positive correlation between self-assessed impairment with objective evaluations of horizontal gaze nystagmus (HGN), a common field sobriety test.

Other researchers have generally agreed that acute effects of cannabis use may produce short-term functional impairment, the extent of which depends on many factors such as age and frequency of use.

The current study builds on this data.

Specifically, delta-9 THC blood concentrations were determined before smoking a 500 milligram pre-roll. Post-smoking impairment was then evaluated and compared to baseline delta-9 THC concentrations.

Researchers also looked at the duration of impairment compared to incidence of HGN, the relationship between delta 9-THC concentration at baseline and at peak impairment in exhaled breath and blood, and identification of key cannabinoids in blood and exhaled breath.

All samples were analyzed by high-resolution mass spectrometry.

Blood analysis showed delta-9 THC concentrations exceeded 5ng/ml in 16 subjects following a 12-hour period of abstinence and with no impairment.

In both blood and exhaled breath samples, delta-9 THC concentrations were positively correlated with increasing blood levels – even in the absence of impairment.

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Researchers also considered the duration of impairment determined by self-assessments before and at various points after smoking.

Maximum impairment peaked within the first 20 minutes of smoking and rapidly dropped to 10% by three hours post-smoking.

Overall, the results suggest a significant negative correlation between chronic use and duration of impairment. In other words, as baseline concentration of delta-9 THC increased, the duration of impairment decreased.

Further, individual measurements revealed that delta-9 THC concentration in exhaled breath “cannot be reliably associated with impairment” and that “higher delta-9 THC concentrations in breath both before and after smoking … tend to be associated with higher blood concentrations.”

Do minor cannabinoids play a role?

An interesting observation from this research was the incidence of other cannabinoids in exhaled breath.

CBC, CBG, CBN and delta-9 THCV were detected in breath only during peak impairment, suggesting that delta 9-THC in isolation may not reliably indicate recent cannabis use.

Next steps

The research is becoming clear: Impairment is influenced by chronicity of cannabis use and not by a predetermined concentration of delta-9 THC either in exhaled breath or in blood.

Further, delta-9 THC may not be a good indicator nor predictor of cannabis impairment.

Minor cannabinoids such as CBC and delta 9 THCV might be more deterministic and are worthy of more aggressive research.

These studies are critically important to ensure consumers are protected from wrongful employment termination or from wrongful prosecution for driving under the influence of cannabis.

Similar to laboratory testing requirements, driving under the (presumptive) influence of cannabis likely represents yet another occasion where attempts to compare cannabis to alcohol or other intoxicants have serious limits.

Unlike alcohol, where impairment is proportionate to blood-alcohol concentration, Wurz demonstrates that cannabis impairment cannot be ascertained by a single marker or single cannabinoid concentration and is instead a function of chronicity and recency of cannabis use.

While the root cause for cannabis impairment remains elusive, regulatory and enforcement bodies should support research that will ensure safety while protecting rights.

Susan Audino, who holds a doctorate in chemistry, is a chemistry consultant and instructor for the American Association for Laboratory Accreditation. She is based in Delaware.