THC measured in oral fluid was a good predictor of THC presence in blood (any amount), but sensitivity decreased at higher concentration thresholds, raising concerns for law enforcement use.
Law enforcement, traffic safety researchers, and policymakers setting cannabis driving limits.
Sensitivity decreased as THC concentration thresholds increased
What the researchers found
Using data from 7,517 drivers in the National Roadside Survey, oral fluid THC > 0 ng/mL was a good predictor of any blood THC. However, as blood and oral fluid concentrations increased above zero, sensitivity (true positive rate) decreased. Accuracy was also lower when drivers used cannabis concurrently with other drugs.
Why it matters
Law enforcement needs practical, non-invasive tools for detecting cannabis-impaired drivers. Oral fluid testing is less invasive than blood draws, but this study quantifies its limitations.
The numbers in context
7,517 drivers with both oral fluid and blood results. Oral fluid THC > 0 was a good predictor of blood THC > 0. Sensitivity decreased at higher thresholds. Multi-drug users had lower prediction accuracy.
How the study worked
Cross-sectional analysis of 7,517 drivers from the 2007 and 2013-2014 National Roadside Surveys who provided both oral fluid and blood samples. Hurdle model analysis examined predictive accuracy accounting for consumption decisions.
What this study cannot tell us
Laboratory analysis, not point-of-care devices that police would actually use. Cross-sectional design at roadside surveys, not under controlled conditions. Oral fluid THC reflects recent use timing more than impairment level.
How to read the evidence
Moderate: large roadside survey sample with paired biological specimens, but laboratory analysis rather than field conditions.
When this study was published
Published in 2019.
The bigger picture
Unlike alcohol, there is no reliable roadside impairment test for cannabis. Oral fluid testing is a step forward but has significant limitations, especially for establishing specific impairment levels rather than just recent use.
Questions still open
- Can point-of-care oral fluid devices achieve similar accuracy to laboratory analysis? Should legal THC limits be based on oral fluid or blood? Is any single biological matrix sufficient for determining cannabis impairment?
Common questions
Can police use oral fluid to test for cannabis?
Why does accuracy drop with other drug use?
Read the original research
The utility of delta 9-tetrahydrocannabinol (THC) measures obtained from oral fluid samples in traffic safety.
Traffic injury prevention, 20(7), 667-672
Citation
Romano, Eduardo; Moore, Christine; Kelley-Baker, Tara; Torres-Saavedra, Pedro A. (2019). The utility of delta 9-tetrahydrocannabinol (THC) measures obtained from oral fluid samples in traffic safety.. Traffic injury prevention, 20(7), 667-672. https://doi.org/10.1080/15389588.2019.1635690