Even Multiple Blood Draws Cannot Reliably Determine When Someone Last Used Cannabis

In 35 DUID cases with multiple blood draws (81 total samples), THC concentrations ranged from 0.74 to 40 ng/mL. Notably, 11 samples showed an increase in THC concentration at a later collection time point, which undermines the assumption that THC levels consistently decline after use. Collection times ranged from 32 minutes to over 12 hours after the incident.

Retrospective analysis of cannabinoid-positive driving under the influence cases from 2019-2023 that had multiple blood draws. Samples were analyzed using LC-MS/MS with reporting limits of 0.5 ng/mL for THC, 1.0 ng/mL for 11-OH-THC, and 5.0 ng/mL for THC-COOH.

Many jurisdictions set per se THC limits for driving, similar to blood alcohol limits. This study demonstrates a fundamental problem with that approach: unlike alcohol, THC blood levels do not reliably decline in a predictable pattern, making it impossible to back-calculate the level at the time of driving.

As more jurisdictions implement cannabis-impaired driving laws, the scientific limitations of blood THC testing become increasingly important. This study provides real-world DUID casework evidence that even the gold-standard approach of multiple blood draws cannot reliably establish the time of cannabis use, challenging the validity of per se THC limits.

Relatively small sample (35 cases). Cannot control for subjects' cannabis use between blood draws in all cases. No information on the specific cannabis products used, route of administration, or chronic use history. Forensic casework data with inherent variability in collection timing and conditions.