A new study published this month adds further evidence that levels of THC detected in the blood or breath of cannabis users is not a reliable indicator of impairment. Researchers also found that levels of THC in blood and breath did not provide reliable evidence of how recently a test subject had used cannabis.

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 per se legal limits on the amount of THC a driver may have in their blood, similar to the 0.08% blood alcohol concentration limit in effect nationwide.

Critics of per se limits on THC concentrations in blood or breath have argued that such limits have little bearing on the level of impairment or intoxication, which can vary widely from person to person despite similar levels of THC concentration.

“These findings provide further evidence that single measurements of specific delta-9-THC blood concentrations do not correlate with impairment, and that the use of per se legal limits for delta-9-THC is not scientifically justifiable at the present time,” wrote the authors of the study published by the journal Scientific Reports.

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

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

“Our findings are consistent with others who have shown that delta-9-THC can be detected in breath up to several days since last use,” they wrote. “Because the leading technologies for breath-based testing for recent cannabis use rely solely on the detection of delta-9-THC, this could potentially result in false positive test outcomes due to the presence of delta-9-THC in breath outside of the impairment window.”

New Study Backed by Previous Research

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

“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,” wrote lead author Dr. Danielle McCartney of the Lambert Initiative for Cannabinoid Therapeutics. “This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.”

To conduct the study, the researchers reviewed data from 28 publications that studied the consumption of inhaled or ingested cannabis. They then analyzed the association between THC concentration and driving performance, using measures of driving-related skills such as reaction time and divided attention.

The researchers documented “weak” associations between THC levels and impairment among infrequent cannabis users. But they observed no significant association between blood or saliva THC levels and impairment among regular pot users, defined as those who used cannabis weekly or more often.

“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” McCartney said. “It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

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

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

Professor Iain McGregor, the academic director of the Lambert Initiative, a long-term research program studying 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 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,” McGregor added. “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.”