Study: THC blood levels do not characterize driving ability

According to a recent study published in Traffic Injury Prevention, the presence of THC in blood or saliva is not a reliable indicator of driving ability.

Australian researchers have evaluated the relationship between levels THC and driving performance in 14 volunteers. Participants vaporized cannabis samples of different potencies (high THC/low CBD, equal THC and CBD ratios, and placebo) and then played on a driving simulator. Blood and saliva samples were taken 30 minutes after inhalation and again 3.5 hours later.

Impossible to distinguish between drunk and sober drivers

The researchers reported that neither the presence of THC in blood or saliva is not a reliable measure of driving performance. Nearly half the participants in the study showed no impairment at the wheel 30 minutes after cannabis inhalation, despite THC levels above the commonly imposed limits. Conversely, several participants showed impairment 3.5 hours after vaporization, at a time when their THC levels were below the limits.

«Blood or saliva limits often fail to distinguish between impaired and sober drivers,» the study authors report. «Furthermore, THC concentrations in blood and saliva correlated poorly with impaired driving. ... It is almost impossible to infer how much cannabis was consumed, or when it was consumed, based solely on a given THC concentration in a biological matrix.»

L’study concludes: «Due to erratic and dependent differences in THC pharmacokinetics, as well as significant inter- and intra-individual variability, blood and oral THC concentrations, unlike blood alcohol concentrations, provide little information about the amount of cannabis consumed or the extent to which a person may be intoxicated. Collectively, these results suggest that the limits per se do not reliably represent impaired driving thresholds.»

Several studies point to shortcomings in drug testing

The results of the present study are consistent with several other studies (here or here) reporting that the presence of THC is an unreliable predictor of recent cannabis exposure or impaired performance.

A 2019 report published by Congressional Research Service, a public policy research institute of the U.S. Congress, concluded similarly: «Research studies have been unable to consistently correlate levels of cannabis consumption, or THC in a person's body, with levels of impairment. Thus, some researchers, and the National Highway Traffic Safety Administration, In their report, the authors noted that the use of THC measurement as evidence of a driver's impairment is not supported by scientific evidence to date.»