Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis

Blood and oral fluid Δ⁹-tetrahydrocannabinol (THC) concentrations are often used to identify cannabis-impaired drivers. We used meta-analytic techniques to characterise the relationships between biomarkers of cannabis use, subjective intoxication, and impairment of driving and driving-related cognitive skills. Twenty-eight publications and 822 driving-related outcomes were reviewed. Each outcome was measured in concert with one or more biomarkers of cannabis/THC use and/or subjective intoxication. Higher blood THC and 11−OH-THC concentrations, oral fluid THC concentrations and subjective ratings of intoxication were associated with greater impairment in ‘other’ (mostly occasional) cannabis users (p’s<0.05). Blood 11−COOH-THC concentrations were associated with impairment after inhaling, but not orally ingesting, cannabis/THC. However, these ‘biomarker–performance’ relationships (R) were only very weak (blood THC_{post-ingestion}: -0.08; blood THC_{post-inhalation}: -0.10; blood 11−OH-THC_{post-ingestion}: -0.13), weak (blood 11−OH-THC_{post-inhalation}: -0.24; oral fluid THC_{post-inhalation}: -0.36; subjective intoxication: -0.29) or moderate (blood 11−COOH-THC_{post-inhalation}: -0.43) in strength. No significant biomarker-performance relationships were observed in ‘regular’ (weekly or more often) cannabis users (p’s>0.10), although the analyses were less robust. Blood and oral fluid THC concentrations are relatively poor indicators of cannabis/THC-induced impairment.