THC detected in blood or breath does not indicate impairment, study finds

THC detected in blood, a new study published this month provides further evidence that THC levels detected in the blood or breath of cannabis users are not a reliable indicator of ill health. The researchers also found that THC levels in blood and breath are not reliable evidence of a subject’s recent cannabis use.

In their introduction to the study, the researchers noted that “finding an objective measure of recent cannabis use that correlates with impairment has proven to be an elusive goal.” Some states have enacted laws that set legal limits themselves on the amount of THC a driver can have in their blood, similar to the nationwide limit of 0.08% blood alcohol concentration.

Critics of blood or breath THC limits argue that these limits have little effect on the level of impairment or intoxication, which can vary widely from person to person, even if the THC concentration is similar.

“These results provide further evidence that individual measurements of specific blood concentrations of delta-9-THC do not correlate with impairment and that the use of legal limits for delta-9-THC per se is currently not scientifically justified,” – wrote the authors of the study, published in the journal Scientific Reports.

To conduct the study, the researchers recruited a group of test subjects, most of whom were daily cannabis users. The researchers then determined THC levels in their blood and breath before and after inhaling marijuana.

Before inhaling marijuana, most of the subjects had residual THC levels of 5 ng/ml or higher, which exceeds the legal limit per se in several states. The authors noted that THC at such concentrations was detected despite “the absence of any impairment.” After the subjects inhaled cannabis, the researchers observed an inverse relationship between THC levels in the blood and impaired performance.

"Our results are consistent with others who have shown that delta-9-THC can be detected in the breath up to several days after recent use," - they wrote. "Given that leading breath testing technologies for recent cannabis use rely solely on the detection of delta-9-THC, this could potentially lead to false-positive test results due to the presence of delta-9-THC in breath outside the window of impairment."

New study supported by previous research

The findings are consistent with a study published late last year in the journal Neuroscience & Biobehavioral Review. In that study, researchers affiliated with the University of Sydney analysed all available research on driver performance and THC concentrations in blood and saliva.

“Higher blood THC concentrations were only weakly associated with greater driving impairment in occasional cannabis users, while no significant association was found in regular cannabis users,” – wrote lead author Dr Danielle McCartney of the Lambert Initiative for Cannabinoid Therapeutics. “This suggests that THC concentrations in blood and oral fluid are relatively poor indicators of cannabis-THC-induced impaired performance.”

In conducting the study, the researchers reviewed data from 28 publications that looked at cannabis consumption by inhalation or ingestion. They then analysed the relationship between THC concentration and driving performance, measuring driving-related skills such as reaction time and divided attention.

The researchers documented a ‘weak’ association between THC levels and impaired driving skills in people who rarely use cannabis. However, they did not observe a significant association between THC levels in blood or saliva and impairment in regular marijuana users, defined as those who use marijuana once a week or more.

“This does not mean, of course, that there is no relationship between THC intoxication and driving impairment,” – McCartney said. “It shows us that the use of THC concentrations in blood and saliva are inconsistent markers of such intoxication.”

The authors noted that the results of the study call into question the validity of widespread random mobile THC testing from saliva in Australia and the reliance on THC levels by law enforcement in the United States.

“Our results suggest that unaffected individuals may be misidentified as intoxicated by cannabis when THC limits are set by law,” – McCartney said.

“Similarly, drivers who are impaired immediately after using cannabis may not be registered as such.”

Professor Iain McGregor, academic director of the Lambert Initiative, a long-term research programme investigating the medical potential of cannabis, said that “THC concentrations in the body clearly have a very complex relationship with intoxication. The strong and direct relationship between blood alcohol concentration and impaired driving encourages people to think that such relationships apply to all drugs, but this is certainly not the case with cannabis.”

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