Effect of Blood Collection Time on Measured [Delta]^sup 9^-Tetrahydrocannabinol Concentrations: Implications for Driving Interpretation and Drug Policy

In driving-under-the-influence cases, blood typically is collected approximately 1.5- 4 h after an incident, with unknown last intake time. This complicates blood Δ^sup 9^-tetrahydrocannabinol (THC) interpretation, owing to rapidly decreasing concentrations immediately after inhalation. We evaluated how decreases in blood THC concentration before collection may affect interpretation of toxicological results. Adult cannabis smokers (≥1×/3 months, ≤3 days/week) drank placebo or low-dose alcohol (approximately 0.065% peak breath alcohol concentration) 10 min before inhaling 500 mg placebo, 2.9%, or 6.7% vaporized THC (within-individuals), then took simulated drives 0.5-1.3 h postdose. Blood THC concentrations were determined before and up to 8.3 h postdose (limit of quantification 1 µg/L). In 18 participants, observed C^sub max^ (at 0.17 h) for active (2.9 or 6.7% THC) cannabis were [median (range)] 38.2 µg/L (11.4-137) without alcohol and 47.9 µg/L (13.0 -210) with alcohol. THC C^sub max^ concentration decreased 73.5% (3.3%- 89.5%) without alcohol and 75.1% (11.5%- 85.4%) with alcohol in the first half-hour after active cannabis and 90.3% (76.1%-100%) and 91.3% (53.8%-97.0%), respectively, by 1.4 h postdose. When residual THC (from previous self-administration) was present, concentrations rapidly decreased to preinhalation baselines and fluctuated around them. During-drive THC concentrations previously associated with impairment (≥8.2 µg/L) decreased to median 2 µg/L by 4.8 h postdose; only 1 participant had THC ≥5 µg/L after 3.3 h. Forensic blood THC concentrations may be lower than common per se cutoffs despite greatly exceeding them while driving. Concentrations during driving cannot be back-extrapolated because of unknown time after intake and interindividual variability in rates of decrease.