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Abstract
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Are THC concentrations in blood predictive of driver impairment?
At the population level, the higher the Δ9-tetrahydrocannabinol (THC) concentrations in blood, the greater the fraction of cannabis consumers who show impairment.1 This association is clearest in occasional cannabis consumers and may differ in chronic frequent cannabis consumers who develop partial tolerance to the effects of THC. However, at the individual level, it is difficult to predict impairment in individual drivers.
At the population level, the fraction of cannabis consumers who show any degree of impairment increases with higher tetrahydrocannabinol (THC) concentrations in blood.
At the individual level, the association between THC concentration and driving performance is difficult to measure.1,2 A dissociation between blood THC concentrations and impact on psychomotor function and cognition exists for several reasons.
Are THC concentrations in oral fluid predictive of driver impairment?
Positive oral fluid test results may indicate recent cannabis use because test sensitivity is usually limited to a few hours after smoking (the time depending upon the detection threshold of the device).9 THC in oral fluid primarily represents coating of the mouth after inhalation of drug-laden smoke or vapour. It is not associated with THC concentrations in blood or driver performance.2 Two to four hours after cannabis intake, coating of the oral fluid dissipates and oral fluid THC concentrations approximately parallel blood THC concentrations, but not at the same levels.10 We cannot accurately predict blood concentrations of THC from oral fluid concentrations because of high intra-subject and inter-subject variability.
Is there a specific (per se) THC limit that allows differentiation of impaired and non- impaired drivers?
No; while impairment from THC increases at the population level, THC concentrations do not predict impairment at the individual level.
Can behavioural standardized field sobriety tests (SFSTs) alone reliably detect THC-induced driver impairment?
Current standardized field sobriety tests include horizontal gaze nystagmus (HGN), one leg stand (OLS) and walk and turn (WAT). They were developed to identify alcohol-impaired driving and do not adequately detect THC-induced driver impairment ...
Are urine drug concentrations alone appropriate to assess impairment in drivers suspected of driving under the influence of cannabis?
No, urine concentrations of cannabis metabolites simply identify past cannabis exposure and in no way identify THC impairment. Urine drug/metabolite concentrations should not be used to interpret the effect of a drug or a chemical on human behaviour.
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Keywords: cannabis impaired driving |