A federal agency this spring will convene government officials, forensics experts, academics, industry representatives, law enforcement and standards organizations for what it describes as “an open and candid discussion” about “the path forward to realize meaningful cannabis breathalyzer technology and implementation.”The two-day event, hosted by the National Institute of Standards and Technology (NIST), which is part of the U.S. Department of Commerce, is scheduled to be held in Boulder, Colorado, on April 16 and 17.[.]Unlike with alcohol, there’s currently no widely accepted field test to determine whether someone is under the influence of marijuana.In 2023, a federally funded report by researchers at NIST and the University of Colorado Boulder concluded that evidence does “not support the idea that detecting THC in breath as a single measurement could reliably indicate recent cannabis use.”“A lot more research is needed to show that a cannabis breathalyzer can produce useful results,” Kavita Jeerage, a NIST materials research engineer and co-author of the report, said at the time. “A breathalyzer test can have a huge impact on a person’s life, so people should have confidence that the results are accurate.”More recently, a U.S. Department of Justice researcher cast doubt on whether a person’s THC levels are even a reliable indicator of impairment.States may need to “get away from that idea,” Frances Scott, a physical scientist at the National Institute of Justice (NIJ) Office of Investigative and Forensic Sciences under DOJ, said on a podcast early last year.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 is that “if you have chronic users versus infrequent users, they have very different concentrations correlated to different effects,” Scott said. “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.”That issue was also examined in a federally funded study last year 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 consensus is that there is no linear relationship of blood THC to driving,” [a study preprint posted on The Lancet by an eight-author team representing Canada’s Centre for Addiction and Mental Health, Health Canada and Thomas Jefferson University in Philadelphia identified and assessed a dozen peer-reviewed studies concluded]. “This is surprising given that blood THC is used to detect cannabis-impaired driving.”
Most states where cannabis is legal measure THC intoxication by whether or not someone’s blood THC levels are below a certain cutoff. The study’s findings suggest that relying on blood levels alone may not accurately reflect whether someone’s driving is impaired.
“Of the 12 papers included in the present review,” authors wrote, “ten found no correlation between blood THC and any measure of driving, including [standard deviation of lateral position (SDLP)], speed, car following, reaction time, or overall driving performance.
[.]
Evan [sic] as far back as 2015, a U.S. National Highway Traffic Safety Administration (NHTSA) concluded that it’s “difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects,” adding that “it is inadvisable to try and predict effects based on blood THC concentrations alone.”
In a separate report last year, NHTSA said there’s “relatively little research” backing the idea that THC concentration in the blood can be used to determine impairment, again calling into question laws in several states that set “per se” limits for cannabinoid metabolites.
