Blood and Saliva THC Levels Are Poor Indicators of Driving Impairment
A meta-regression of 28 studies found blood and oral fluid THC concentrations are weak to negligible predictors of driving impairment, especially in regular cannabis users.
Quick Facts
What This Study Found
Higher blood THC, 11-OH-THC, oral fluid THC, and subjective intoxication were associated with greater impairment in occasional users, but correlations were negligible to weak (r = -0.08 to -0.43). No significant biomarker-performance relationships were found in regular (weekly+) cannabis users.
Key Numbers
28 publications; 822 driving outcomes; correlations ranged from -0.08 to -0.43; no significant relationships in regular users
How They Did This
Meta-regression analysis of 28 publications and 822 driving-related outcomes examining relationships between blood/oral fluid THC concentrations, subjective intoxication, and driving performance.
Why This Research Matters
Many jurisdictions use blood or oral fluid THC levels to identify impaired drivers, similar to blood alcohol. This meta-analysis shows THC biomarkers are fundamentally different from blood alcohol as impairment indicators.
The Bigger Picture
Unlike alcohol, where blood concentration reliably predicts impairment, THC levels do not. This has profound implications for cannabis DUI laws that set per se THC limits as a proxy for impairment.
What This Study Doesn't Tell Us
Analyses in regular users were less robust due to fewer studies. Most studies used simulated rather than real-world driving.
Questions This Raises
- ?If THC levels do not predict impairment, what should replace per se THC limits in driving laws?
- ?Would performance-based impairment testing be more accurate?
Trust & Context
- Key Stat:
- No significant THC-impairment link in regular users
- Evidence Grade:
- Meta-regression of 28 publications with 822 outcomes provides strong evidence, though most underlying studies used simulated driving.
- Study Age:
- Published in 2022
- Original Title:
- Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis.
- Published In:
- Neuroscience and biobehavioral reviews, 134, 104433 (2022)
- Authors:
- McCartney, Danielle(22), Arkell, Thomas R(10), Irwin, Christopher(12), Kevin, Richard C, McGregor, Iain S
- Database ID:
- RTHC-04053
Evidence Hierarchy
Combines results from multiple studies to find an overall pattern.
What do these levels mean? →Frequently Asked Questions
Can a blood test tell if someone is too impaired to drive from cannabis?
This meta-analysis found blood and oral fluid THC levels are poor indicators of actual driving impairment. The correlations are weak at best and disappear entirely in regular cannabis users.
Why is THC different from alcohol for impairment testing?
Unlike alcohol, THC is fat-soluble and can persist in blood long after impairment resolves. THC also produces tolerance, meaning regular users can have high blood levels with minimal impairment.
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Cite This Study
https://rethinkthc.com/research/RTHC-04053APA
McCartney, Danielle; Arkell, Thomas R; Irwin, Christopher; Kevin, Richard C; McGregor, Iain S. (2022). Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis.. Neuroscience and biobehavioral reviews, 134, 104433. https://doi.org/10.1016/j.neubiorev.2021.11.004
MLA
McCartney, Danielle, et al. "Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and metabolite concentrations related to impairment? A meta-regression analysis.." Neuroscience and biobehavioral reviews, 2022. https://doi.org/10.1016/j.neubiorev.2021.11.004
RethinkTHC
RethinkTHC Research Database. "Are blood and oral fluid Δ9-tetrahydrocannabinol (THC) and m..." RTHC-04053. Retrieved from https://rethinkthc.com/research/mccartney-2022-are-blood-and-oral
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.