San Diego, CA: Subjects who have not recently consumed cannabis but still have residual levels of THC in their blood perform no differently on a driving simulator than do those who are THC-negative, according to data published in the journal Clinical Chemistry.
Researchers affiliated with the University of California at San Diego assessed THC blood levels and simulated driving performance in a cohort of 190 regular cannabis consumers. Study subjects were required to have abstained from cannabis for 48 hours prior to participating in the study.
Post-abstinence, nearly half of the study participants had detectable levels of THC (above 0.5ng/ml) at baseline, with one-quarter of participants testing positive for more than 2ng/ml of THC in blood. However, those testing positive for THC showed no significant differences in their baseline driving scores as compared to those with no quantifiable THC concentrations.
“Our data argue that the concentrations we measured at baseline likely reflect steady state THC concentrations in this population, several days after last use,” the study’s authors concluded. “We also show, using quantitative data from the driving simulator, that participants who exceeded the zero-tolerance and per se cutpoints (2 and 5 ng/mL) performed in a similar manner as those below these arbitrary values. These results add to a growing body of evidence that per se THC blood statutes lack scientific credibility as prima facie evidence of impairment.”
The findings are consistent with those of other studies reporting no correlation between the detection of either THC or its metabolites in blood, urine, saliva, and breath and impaired driving performance. Nevertheless, several states have enacted laws criminalizing drivers who operate a motor vehicle with trace levels of THC or THC metabolites, regardless of whether the driver is impaired.
In an accompanying editorial, authors said that the study’s conclusions “raise serious doubts about the scientific validity of using measures of delta-9-tetrahydrocannabinol in blood to identify cannabis-impaired drivers.”
NORML has consistently argued that law enforcement should not presume that the detection of either THC or its metabolites in bodily fluids is evidence of impairment because their presence is not predictive of diminished performance. Alternatively, NORML has called for the expanded use of performance-based tests, like DRUID or Predictive Safety’s AlertMeter, which compare subjects’ cognitive skills to either their own prior performance or an aggregate baseline.
Full text of the study, “Per se driving under the influence of cannabis statutes and blood delta-9-tetrahydrocannabinol concentrations following short-term cannabis abstinence,” appears in Clinical Chemistry. Additional information on cannabis and driving is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’