By / June 13, 2022

THC Detection in Blood or Breath Is a Bad Indicator of Impairment

A new study published last month confirms that levels of THC detected in the blood or breath of cannabis users are not a reliable indicator of impairment. The researchers also found that THC levels in blood and breath did not provide reliable evidence of a test subject’s most recent cannabis use date.

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Objectively Measuring Impairment Is Hard

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 a difficult goal to achieve.”

Some states have passed laws that set per se legal limits on 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, i.e. even the tiniest trace.

Critics of per se limits on THC concentrations in blood or breath often argue that these limits are poor indicators of the actual level of impairment or intoxication, which can vary widely from person to person to the other, despite having similar concentrations of THC.

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

Regular Cannabis Users Showed THC Levels But no Impairment

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

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

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

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

A new study supported by previous research

The results are in line with the findings of a study published late last year in the journal Neuroscience & Biobehavioral Review. In this study, researchers affiliated with the University of Sydney analyzed all 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,” said lead author Dr. Danielle McCartney. “This suggests that THC concentrations in blood and oral fluids are relatively poor indicators of cannabis-THC-induced impairment.”

To carry out this 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 skills such as reaction time and divided attention.

Only a Weak Association Between THC Levels and Impariment

The researchers found “weak” associations between THC levels and impairment in occasional cannabis users. But they found no significant association between blood or saliva THC levels and impairment in regular cannabis users, defined as those who use cannabis weekly or more often.

“Of course, that doesn’t mean there isn’t a relationship between THC intoxication and driving impairment,” McCartney said. “This shows us that the concentration of THC in blood and saliva are inconsistent markers of this intoxication.”

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

Measuring THC Levels Leads to False Positives and Negatives

“Our results indicate that non-intoxicated people could be mistakenly identified as being intoxicated by cannabis when THC limits are imposed by law,” McCartney said. “Similarly, drivers who are impaired immediately after cannabis use may not be registered as such.”

“A person inexperienced with cannabis can ingest a high oral dose of THC and be completely unfit to drive while experiencing extremely low levels of THC in blood and oral fluid,” added Professor Iain McGregor.

“On the other hand, a regular cannabis user may smoke a joint, exhibit very high THC levels, but exhibit little or no impairment. We clearly need more reliable ways to identify cannabis impairment on the roads and in the workplace.”

(Featured image by Tobi via Pexels)

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