Study: Blood Cannabis Levels May Not Accurately Reflect Driver Impairment

*WASHINGTON* – A new study demonstrates that many regular marijuana users
exceed legal blood cannabis limits for U.S. drivers despite showing no
evidence of impairment. The findings, published today in the Association
for Diagnostics & Laboratory Medicine’s *Clinical Chemistry* journal,
contribute to evidence that cannabis blood limit laws lack scientific
credibility and are not an accurate determinant of when drivers should face
criminal charges or not.

Previous research that evaluated the effect of delta-9-tetrahydrocannabinol
(THC, the main psychoactive compound in cannabis) on motor vehicle crashes
concluded that there is no increase of crash risk because of detectable
THC. Despite evidence showing no correlation between the detection of THC
in the blood and driving impairment, 6 states in the United States have per
se laws using 2 or 5 ng/mL of THC as the cut-off point for driving under
the influence of cannabis, while 12 have a zero-tolerance law. These
cut-off points are considered face value evidence of driving impairment,
which means that even if it has been several days since an individual’s
last use of the drug and they show no behavioral impairment, they may still
face legal risks, up to and including felony charges.

To address this issue, a team of researchers led by Dr. Thomas D. Marcotte,
professor of psychiatry at the University of California, San Diego, and
co-director of the Center for Medicinal Cannabis Research, set out to
investigate the blood concentrations of THC in regular cannabis users, as
well as the simulated driving performance for participants who exceeded per
se cut-off points compared with those who were below these values. The
researchers measured baseline concentrations of THC in 190 regular cannabis
users after instructing participants to abstain from cannabis for at least
48 hours. Following abstention, the researchers also evaluated driving
performance in this group using a driving simulator.

From this, the team found that many regular users of cannabis exceed zero
tolerance and per se THC cut-off point concentrations days after their last
use. Specifically, 43% of participants exceeded zero-tolerance statutes at
baseline, while 24% had baseline blood THC concentrations that were greater
than or equal to the per se cut-off of 2 ng/mL, and 5.3% had blood
concentrations greater than or equal to 5 ng/mL. Based on the results from
the driving simulation, participants with elevated baseline concentrations
of THC did no worse on a driving simulator compared with participants who
were below per se cut-off points. Altogether, the results add to a growing
body of evidence showing that current per se THC blood limit laws lack
scientific credibility as face-value evidence of impairment.

“More work needs to be done to address how to best identify drivers who are
under the influence of cannabis and are unsafe to drive,” the study authors
wrote. “At present, the best protocol is a combination of observations in
the field and toxicology testing.” They added that “an essential component
of improving highway safety is collaborations between law enforcement and
the scientific community to develop standards that are unbiased and
potentially lifesaving.”
About the Association for Diagnostics & Laboratory Medicine (ADLM)

Dedicated to achieving better health for all through laboratory medicine,
ADLM (formerly AACC) unites more than 70,000 clinical laboratory
professionals, physicians, research scientists, and business leaders from
110 countries around the world. Since 1948, ADLM has championed the
advancement of laboratory medicine by fostering scientific collaboration,
knowledge sharing, and the development of innovative solutions that enhance
health outcomes.
About *Clinical Chemistry*

*Clinical Chemistry* is a leading international journal of laboratory
medicine, featuring nearly 400 peer-reviewed studies every year. The
research is advancing areas of healthcare ranging from genetic testing and
drug monitoring to pediatrics and appropriate test utilization.