Field sobriety tests alone classified 49% of placebo recipients as impaired, but combining them with a 2 ng/mL oral fluid THC cutoff reduced false positives to zero.
Read this if you drive after using cannabis or are interested in how cannabis impairment should be detected by law enforcement.
49% false positiveProportion of placebo recipients incorrectly classified as impaired by field sobriety tests alone
What the researchers found
In 191 participants who smoked placebo or cannabis (5.9% or 13.4% THC), there was no relationship between THC concentrations in blood, oral fluid, or breath and driving simulator performance at any timepoint. Field sobriety tests classified 81% of THC recipients as impaired 71 minutes after smoking, but also classified 49% of placebo recipients as impaired. Combining a 2 ng/mL THC oral fluid cutoff with positive FST findings eliminated false positives among controls 86 minutes after smoking.
Why it matters
This is one of the most rigorous studies addressing the fundamental challenge of cannabis DUI enforcement. The finding that nearly half of sober people fail field sobriety tests underscores why toxicology confirmation is essential.
The numbers in context
- 191 participants randomized
- THC doses: placebo, 5.9%, 13.4%
- 5-hour observation period
- No correlation between THC levels and driving performance (P>0.05)
- FST: 81% of THC group deemed impaired at 71 min
- FST: 49% of placebo group deemed impaired at 71 min
- Combined FST + 2 ng/mL oral fluid THC: 0% false positives at 86 min
How the study worked
Placebo-controlled, randomized, double-blind study. Participants smoked cannabis or placebo ad libitum. Serial blood, oral fluid, and breath samples collected over 5 hours. Driving simulator testing (SDLP, coherence). Law enforcement officers conducted field sobriety tests.
Who was studied
191 participants randomized to placebo or cannabis (5.9% or 13.4% THC)
What this study cannot tell us
Driving simulator performance may not reflect real-world driving. Ad libitum smoking introduces dose variability. Single cannabis session; chronic users may differ. Field sobriety tests administered by a limited number of officers.
How to read the evidence
Randomized, double-blind, placebo-controlled experimental study with objective and subjective measures. High-quality evidence.
When this study was published
Published in 2023. Among the most rigorous studies on cannabis DUI detection methodology.
The bigger picture
The 49% false positive rate for field sobriety tests alone is striking and has serious implications for justice. Combining subjective assessments with objective toxicology dramatically improves accuracy and should inform DUI enforcement policy.
Questions still open
- Should jurisdictions require toxicology confirmation before cannabis DUI charges?
- How should the oral fluid THC cutoff be set for different populations (chronic vs occasional users)?
- Could breath-based THC testing eventually replace oral fluid for roadside detection?
Read the original research
Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests.
Clinical chemistry, 69(7), 724-733
Citation
Fitzgerald, Robert L; Umlauf, Anya; Hubbard, Jacqueline A; Hoffman, Melissa A; Sobolesky, Philip M; Ellis, Shannon E; Grelotti, David J; Suhandynata, Raymond T; Huestis, Marilyn A; Grant, Igor; Marcotte, Thomas D. (2023). Driving Under the Influence of Cannabis: Impact of Combining Toxicology Testing with Field Sobriety Tests.. Clinical chemistry, 69(7), 724-733. https://doi.org/10.1093/clinchem/hvad054