Changes in blood cannabinoid concentrations over multiple collection times in driving under the influence of drugs casework (2025)

Δ9-Tetrahydrocannabinol (THC) is the most frequently used illicit drug in the world, yet interpretation of THC concentrations in driving under the influence of drug (DUID) cases is difficult due to possible residual THC concentrations. This study determined the concentrations of cannabinoids in blood collected across multiple time points from drivers in suspected impaired driving cases to evaluate if changes in concentrations over time can provide clarification on the time of cannabis use. This study examined cannabinoid-positive DUID cases reported from January 2019 to December 2023 to identify those that tested multiple blood draws. Thirty-five cases were identified that had multiple blood draws for a total of 81 different samples with collection times ranging from 00:32 to 12:42 hours between incident and blood draw. Cannabinoid testing was performed using a liquid chromatography-tandem mass spectrometry analysis with reporting limits of 1.0, 5.0, and 0.5 ng/mL for 11-hydroxy-THC (11-OH-THC), 11-nor-9-carboxy-THC (THC-COOH), and THC, respectively. THC concentrations (n = 81) ranged from 0.74 to 40 ng/mL. Eleven samples had an increase in THC concentration at a later collection time point. 11-OH-THC concentrations (n = 60) ranged from 1.0 to 16 ng/mL. THC-COOH concentrations (n = 81) ranged from 7.1 to 470 ng/mL. The results of this study underscore the difficulty in interpretation and drawing conclusions regarding time of cannabis use, even when multiple samples are obtained from the same subject over time from a single incident. [Author's abstract]