Justice Department Researcher Says ‘We May Need Better Tests’ For Marijuana Impairment, Questioning ‘Per Se’ THC Limits For Driving

A Justice Department research says states may need to “get away from that idea” that marijuana impairment can be tested based on the concentration of THC in a person’s system.

Frances Scott, a physical scientist at the National Institute of Justice (NIJ) Office of Investigative and Forensic Sciences under DOJ, discussed the challenges of cannabis impairment testing in an episode of the Justice Today podcast that was posted late last month.

Scott questioned the efficacy of setting “per se” THC limits for driving that some states have enacted, making it so a person can be charged with driving while impaired based on the concentration of cannabis components in their system. Ultimately, there may not be a way to assess impairment from THC like we do for alcohol, she said.

One complication, Scott said, is that “if you have chronic users versus infrequent users, they have very different concentrations correlated to different effects.”

“So the same effect level, if you will, will be correlated with a very different concentration of THC in the blood of a chronic user versus an infrequent user,” she said.

That issue was also examined in a recent federally funded study that identified two different methods of more accurately testing for recent THC use that accounts for the fact that metabolites of the cannabinoid can stay present in a person’s system for weeks or months after consumption.

“The problem is we’ve funded this research that pretty conclusively shows that the THC concentration in the blood is not particularly well-correlated with impairment for driving,” Scott said. “One of the outstanding questions is trying to figure out, is there a good proxy, a good metric, that we can use?”

“If it’s not delta-9 THC, is there another one of those cannabinoids that is a good metric, or do we need to kind of move away from that? With alcohol, we per se laws—that’s the 0.08 [blood alcohol content], right?” she said. “If we demonstrate that your blood alcohol content is over 0.08, that’s all I have to do to prove impairment, right? That’s also well-understood.”

“Maybe what we need to do is kind of get away from that idea that we can sort of have a number when it comes to marijuana and have that mean that you’re impaired,” the researcher said. “And it may get into some different types of measures than we’re used to doing. So maybe it’s not a blood measure or a breath measure.”

While the Justice Department is continuing to support research into the development of a possible breathalyzer test for cannabis, there’s also grant funding going to alternative screening options—including saliva tests and peripheral vision assessments to determine if certain eye functioning could be associated with impairment from THC.

“We may need better tests,” Scott—who has also recently discussed the seemingly arbitrary way that Congress arrived as a 0.3 percent THC limit to define federally legal hemp—said.

The THC impairment question has been a major focus for lawmakers and the research community, particularly as it concerns driving laws.

Last summer, a congressional report for a Transportation, Housing and Urban Development, and Related Agencies (THUD) bill said that the House Appropriations Committee “continues to support the development of an objective standard to measure marijuana impairment and a related field sobriety test to ensure highway safety.”

Sen. John Hickenlooper (D-CO) sent a letter to the Department of Transportation (DOT) in 2022 seeking an update on that status of a federal report into research barriers that are inhibiting the development of a standardized test for marijuana impairment on the roads. The department was required to complete the report by November under a large-scale infrastructure bill that President Joe Biden signed, but it missed that deadline and it’s unclear how much longer it will take.

A study published in 2019 concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were not statistically more likely to be involved in an accident compared to people who haven’t used marijuana.

Separately, the Congressional Research Service in 2019 determined that while “marijuana consumption can affect a person’s response times and motor performance … studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”

Another study from 2022 found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.

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Photo courtesy of Martin Alonso.