3. Toxicology: detection of THC impairment
The aim is to address main toxicological issues that have arisen in the context of DUIC.
1) Are THC concentrations in drivers’ blood predictive of the magnitude of driver impairment? At the population level, there is evidence from experimental studies that the fraction of cannabis users that shows any degree of impairment increases with higher THC concentrations in blood. The correlation between THC concentration in blood and magnitude of impairment in individual cannabis users however is low.
2) Are THC concentrations in oral fluid predictive of driver impairment? THC in oral fluid primarily represents coating of the mouth following inhalation of drug-laden smoke or vapour and is not associated with THC concentrations in blood or driver performance. Two to four hours after cannabis intake, coating of the oral fluid dissipates and then oral fluid THC concentrations parallel but are not the same concentration as blood THC concentrations.
3) Is there a specific THC limit that allows differentiation of impaired and non-impaired drivers? No, as explained under 1, there are several reasons why THC concentrations in blood do not predict impairment at the individual level.
4) Can behavioural field sobriety tests alone reliably detect THC induced driver impairment? Standardized field sobriety tests do not adequately detect THC induced driver impairment. Other behavioural tests to detect THC induced impairment are under active development throughout the world.
At present, no biological or behavioural test exists to reliably differentiate THC-impairment from an individual’s non-drug driving performance