Study: THC detected in blood or breath is a poor indicator of impairment

A new study Published last month confirms that THC levels detected in the blood or breath of cannabis users do not constitute a reliable indicator of impairment.

Researchers also found that THC levels in blood and breath did not provide reliable proof of recent cannabis consumption by a test subject.

In their study's introduction, the researchers noted that «finding an objective measure of recent cannabis use that correlates with impairment has proven to be a difficult goal to achieve.».

Some states have enacted laws that set legal limits per se to the amount of THC a driver can have in their blood. In France, it is minimal since it is fixed at 1ng/mL of saliva, as well as any trace.

The detractors of limits per se on THC concentrations in blood or breath often argue that these limits are poor indicators of actual impairment or intoxication levels, which can vary considerably from person to person despite similar concentrations. THC.

«These results provide further evidence that single blood concentrations of delta-9-THC are not correlated with impairment, and that the use of legal limits per se for delta-9-THC is not scientifically justifiable at the present time, have written the authors of the study published in the journal Scientific Reports.

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

Before inhaling cannabis, most subjects had residual THC levels of 5 ng/ml or more, which would exceed the legal limit in France. The authors noted that such THC levels were detected despite «the absence of any impairment of faculties.» After the subjects inhaled cannabis, researchers found an inverse relationship between blood THC levels and performance impairment.

«Our results are consistent with those of other researchers who have shown that delta-9-THC can be detected in breath for up to several days after last use,» they wrote.

«Since the main breath screening technologies for recent cannabis use rely solely on detecting delta-9-THC, this could potentially lead to false positive results due to the presence of delta-9-THC in breath outside the impairment window.»

A new study supported by prior research

The results are consistent with the conclusions from a study published at the end of last year in the journal Neuroscience & Biobehavioral Reviews. In this study, researchers affiliated with the University of Sydney analyzed all available studies on driving performance and concentrations of THC in blood and saliva.

«Higher THC concentrations in the blood 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. «This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.»

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

Researchers found «weak» associations between THC levels and impairment in occasional cannabis users. However, they observed no significant associations between THC levels in blood or saliva and impairment in regular weed smokers, defined as those who use cannabis weekly or more often.

«Of course, that doesn't mean there's no link between THC intoxication and impaired driving,» McCartney said. «What this shows us is that THC concentration in blood and saliva are inconsistent markers of that impairment.»

The authors noted that the study's results call into question the validity of generalized random mobile testing for THC in saliva in Australia, and the confidence in THC levels by law enforcement in the United States.

«Our findings indicate that unimpaired individuals could be misidentified as cannabis-impaired when legal THC limits are imposed,» McCartney stated. «Similarly, drivers whose abilities are impaired immediately after cannabis use might not be registered as such.»

An inexperienced person with cannabis may ingest a strong dose THC and be completely unfit to drive, while registering extremely low THC concentrations in blood and oral fluid,» added Professor Iain McGregor.

«On the other hand, an experienced cannabis consumer may smoke a joint, present very high THC concentrations, but show little to no impairment. We clearly need more reliable ways to identify cannabis impairment on the roads and in the workplace.»