Blood cannabinoid molar metabolite ratios are superior to blood THC as an indicator of recent cannabis smoking.

INTRODUCTION:Cannabis use is a growing concern in transportation and workplace incidents. Because Δ9-tetrahydrocannabinol is detectable after acute psychoactive effects have resolved, it has limitations as an indicator of recent usage or potential impairment. METHODS:In an observational study of driving and psychomotor performance, we measured whole blood Δ9-tetrahydrocannabinol plus its metabolites 11-hydroxy-Δ9-tetrahydrocannabinol and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol by liquid chromatography with tandem mass spectrometry at baseline and 30 min after starting a 15-minute interval of smoking cannabis in 24 occasional and 32 daily cannabis smokers. We calculated two blood cannabinoid molar metabolite ratios: 1) [Δ9-tetrahydrocannabinol] to [11-nor-9-carboxy-Δ9-tetrahydrocannabinol] and 2) ([Δ9-tetrahydrocannabinol] + [11-hydroxy-Δ9-tetrahydrocannabinol]) to [11-nor-9-carboxy-Δ9-tetrahydrocannabinol]. We compared these to blood [Δ9-tetrahydrocannabinol] alone as indicators of recent cannabis smoking. RESULTS:Median Δ9-tetrahydrocannabinol concentrations increased from 0 ( 0.38). By comparison, a cut-point for Δ9-tetrahydrocannabinol of 5.3 µg/L yielded 88% specificity, 73% sensitivity, and 80% accuracy. CONCLUSIONS:In occasional and daily users, the blood cannabinoid molar metabolite ratios were superior to whole blood Δ9-tetrahydrocannabinol as indicators of recent cannabis smoking. We recommend measurement and reporting of Δ9-tetrahydrocannabinol, 11-hydroxy-Δ9-tetrahydrocannabinol, and 11-nor-9-carboxy-Δ9-tetrahydrocannabinol, and their molar metabolite ratios in forensic and safety investigations.