Scientists say they’ve identified an alternative way to test for recent marijuana use that’s significantly more accurate than standard THC blood tests that sometimes misrepresent a person’s potential impairment depending on frequently they use cannabis. And they’re actively working to build on that research with an expanded study.
The initial study, which was funded in part by the National Institute on Drug Abuse (NIDA) and published in the journal Clinical Toxicology, could have key criminal justice implications, as police currently rely on basic THC blood tests for evidence of possible intoxication in criminal investigations, such as after car accidents.
But as the researchers from the University of Colorado (UC) point out, that testing standard can be unreliable, especially for frequent cannabis users.
“Since THC accumulates and lingers in fat tissue, daily cannabis users may maintain constant elevations of THC in the blood even long after the psychoactive effects abate,” Michael Kosnett, an associate adjunct professor and cannabis researcher at UC’s Colorado School of Public Health, said in a press release.
A more accurate method of testing for recent marijuana use, the researchers found, is by analyzing the molar metabolite ratio of THC to THC-COOH in the blood. If a person’s ratio of the active and inactive metabolites meets or exceeds a 0.18 cut-off point, “you can feel pretty confident (with 98 percent specificity) that the person just used within the past 30 minutes.”
They tested this approach by having 24 occasional and 32 daily marijuana consumers participate in a driving simulator exercise. Participants had their blood analyzed at a baseline and then 30 minutes after a 15-minute smoking interval.
The study revealed that the molar metabolite ratio of THC to THC-COOH, at a 0.18 cut-off, yielded results with 98 percent specificity (meaning there is a two percent false positive rate), 93 percent sensitivity (meaning the test only fails to detect recent use seven percent of the time) and 96 percent accuracy (a combination of those two rates).
By comparison, testing for THC alone “yielded 88 percent specificity, 73 percent sensitivity and 80 percent accuracy.”
“The principle behind the study is basically, you’re looking at active drug versus inactive drug—because THC and hydroxy-THC are active and carboxy-THC is inactive. So it makes sense that soon after you smoke cannabis, you’re going to have relatively more of the active forms present,” Kosnett told Marijuana Moment. “As the ratio of active forms to inactive forms decreases, the more likely that the active form present represents the tail-end or residual of smoking that occurred many hours ago.”
In a prior study, the researchers also assessed driving ability during a simulation, and notably they found that daily cannabis consumers had an average five times the THC concentration in their blood after the 30-minute mark compared to occasional users—yet the latter group “showed evidence of decrement in their driving skills, whereas that wasn’t statistically significant in the daily users.”
One limitation of the more recent study was that the blood was tested 30 minutes after consuming, but in real life scenarios such as after a car accident, it could take longer before a person is tested. The researchers say they are working to analyze the molar metabolite ratio of more participants at different time intervals.
Another limitation, Kosnett said, is that you “can’t conclude that just because the ratio was elevated in and of itself with this test that the person was impaired.”
“We’re working on that too. But I think, even right now, this work is helpful,” he said. “When you are using a test that has implications for people’s jobs or for convictions, you generally want to have something that’s very specific. With this test’s specificity of 98 percent, we can have high confidence that this isn’t a false positive.”
The researcher said they are currently recruiting a larger pool of participants for a follow-up study that will assess the molar metabolite ratio at different time intervals, rather than just after the 30-minute mark.
“We’re going to be checking again with a larger data set, which is always better,” he said.
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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