Another study, published in this month, has added evidence to show that THC levels found in blood or the breaths of marijuana users are not an indicator of impairment. The researchers also discovered that the levels of THC detected in breath and blood did not indicate how recent a subject 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 seThe legal limit on how much THC a driver can have in their blood is similar to the national 0.08% blood alcohol content limit.

Critics Per seLimits on THC levels in breath or blood have been argued to have no bearing on impairment or intoxication. However, THC concentrations can differ widely between people despite having similar levels.

“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.

The researchers used a small group of daily cannabis smokers to conduct their study. After inhaling marijuana, the scientists measured their THC levels.

Prior to inhaling cannabis, the majority of subjects tested positive for residual THC levels greater than 5ng/ml. Per seThere are several legal limits in many 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.”

A New Study Supported by Past Research

This is consistent with the results of the study that was published in late 2013 in the journal. Neuroscience & Biobehavioral Review.The University of Sydney researchers analysed the data from all studies that had been done on driving and the THC concentrations found 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.”

The researchers examined data from 28 studies on the inhaled and ingested effects of cannabis. They used measures such as divided attention to analyze the correlation between THC content and driving performance.

The researchers documented “weak” associations between THC levels and impairment among infrequent cannabis users. However, they found no evidence of impairment in regular cannabis users (defined as someone who uses marijuana weekly or more frequently).

“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.”

According to the authors, the results of the study raise serious questions about the reliability and validity of random mobile THC testing in Australia for saliva.

“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.”