We've updated our Privacy Policy to make it clearer how we use your personal data.

We use cookies to provide you with a better experience, read our Cookie Policy

Analytical Cannabis Logo
Home > News > Science & Health > Content Piece

THC Blood Levels and Saliva Are Poor Measures of Cannabis Impairment, Review Finds

By Leo Bear-McGuinness

Published: Dec 03, 2021   
Listen with
Register for FREE to listen to this article
Thank you. Listen to this article using the player above.

The THC levels found in a person’s blood and saliva aren’t good indicators of whether that person is intoxicated or not, according to a new review of studies published in Neuroscience & Biobehavioral Reviews.

Given their findings, the researchers behind the review warn that unimpaired drivers could mistakenly be classed as dangerously high by law officers, while drivers who are dangerously impaired may pass standard cannabis intoxication tests.

Driving high

To make their findings, the researchers from the University of Sydney assessed 28 existing studies into the measurement of cannabis intoxication.

They found some significant correlations between blood and oral fluid THC concentrations and impairment. The THC metabolite 11−COOH-THC, for example, was judged to have a “moderate” correlation with impairment, despite the compound itself being non-intoxicating. But all other THC derivatives were deemed to have a “weak” relationship with impairment.

“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” Dr Danielle McCartney, lead author of the review, said in a statement.

“It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

As seven US states currently have legal THC-blood limits for driving, the researchers warn that some drivers in these areas may be unfairly penalized.

“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.”

As for the researchers’ advice for cannabis consumers thinking of getting behind a wheel, co-author Dr Thomas Arkell remarked that, “Individuals are better to wait a minimum length of time, between three and 10 hours, depending on the dose and route of administration, following cannabis use before performing safety-sensitive tasks.”

“Smartphone apps that may help people assess their impairment before driving are currently under development and may also prove useful.”

Cannabis and driving

Given the rise in cannabis legalization – and the growing concern of “high drivers” – several initiatives have popped up to develop better THC tests.

In October, researchers at Massachusetts General Hospital, Harvard Medical School, announced their EPOCH (express probe for on-site cannabis inhalation) test, a portable saliva testing kit that can reportedly detect THC in less than five minutes.

Another team from the University of Texas at Dallas developed a saliva-based THC test in 2020. Much like the EPOCH test, the method uses sensor strips coated in antibodies that bind to THC.

A breathalyzer prototype was also developed at the University of Pittsburgh in 2019. By using carbon nanotubes 100,000 times smaller than a human hair, this device could “detect THC at levels comparable to or better than mass spectrometry,” according to its developers.

However, many of these devices were designed solely to test for the presence of THC in a person’s saliva or breath. Whether those readings accurately reflect intoxication may be another matter, according to the University of Sydney researchers.


Like what you just read? You can find similar content on the topic tags shown below.

Policy Science & Health Testing

Stay connected with the latest news in cannabis extraction, science and testing

Get the latest news with the FREE weekly Analytical Cannabis newsletter