Oral fluid THC tests were highly accurate at detecting cannabis use but poor at measuring blood THC levels
An analysis of 4,596 drivers from the 2013 National Roadside Survey found that oral fluid THC testing detected blood THC positivity with 79.4% sensitivity and 98.3% specificity, but oral fluid concentration explained only 29% of the variation in blood THC concentration.
Quick Facts
What This Study Found
Researchers compared oral fluid and blood THC test results from 4,596 drivers stopped during the 2013 National Roadside Survey.
Overall, 8.9% tested positive for THC in oral fluid and 9.4% in blood.
Using blood testing as the reference standard, oral fluid testing showed 79.4% sensitivity (correctly identifying positive cases) and 98.3% specificity (correctly identifying negative cases).
However, oral fluid THC concentration was a poor predictor of actual blood THC levels. The log-transformed oral fluid concentration explained only 29% of the variation in blood THC concentration, meaning 71% of the variation was unexplained.
A 10% increase in oral fluid THC concentration was associated with only a 2.4% increase in blood THC concentration.
Key Numbers
4,596 drivers tested. 8.9% positive in oral fluid, 9.4% positive in blood. Sensitivity: 79.4% (95% CI: 75.2%-83.1%). Specificity: 98.3% (95% CI: 97.9%-98.7%). Oral fluid THC explained only 29% of blood THC variation. Each 10% oral fluid increase associated with 2.4% blood increase (95% CI: 2.1%-2.8%).
How They Did This
Cross-sectional analysis of 4,596 drivers from the 2013 National Roadside Survey who provided both oral fluid and blood samples. Sensitivity/specificity analysis for detection, plus correlation and linear regression on log-transformed concentrations for quantitative comparison.
Why This Research Matters
Roadside drug testing for cannabis relies heavily on oral fluid because blood draws are impractical. This study confirms oral fluid works well for detecting whether someone has used cannabis but cannot reliably indicate how impaired they might be, which has major implications for traffic law enforcement.
The Bigger Picture
As more jurisdictions legalize cannabis and struggle with impaired driving enforcement, the gap between detecting cannabis use and measuring impairment remains a central challenge. This study quantifies that gap: oral fluid can tell you someone used cannabis, but not how much THC is in their blood.
What This Study Doesn't Tell Us
Cross-sectional roadside data, so the timing of last cannabis use was unknown. Different oral fluid collection devices and blood sample handling could affect results. The study measured THC presence, not actual driving impairment.
Questions This Raises
- ?What oral fluid THC threshold best balances sensitivity and specificity for roadside testing?
- ?Can combining oral fluid testing with behavioral assessment improve impairment detection?
Trust & Context
- Key Stat:
- 79.4% sensitivity, 98.3% specificity, but only 29% of blood THC variation explained
- Evidence Grade:
- Moderate. Large nationally representative sample with validated testing methods, though cross-sectional design limits conclusions about individual-level impairment.
- Study Age:
- Published in 2018 using 2013 survey data. Oral fluid testing technology and roadside testing protocols have continued to evolve.
- Original Title:
- Validity of oral fluid test for Delta-9-tetrahydrocannabinol in drivers using the 2013 National Roadside Survey Data.
- Published In:
- Injury epidemiology, 5(1), 3 (2018)
- Database ID:
- RTHC-01705
Evidence Hierarchy
A snapshot of a population at one point in time.
What do these levels mean? →Frequently Asked Questions
Can a roadside oral fluid test tell if someone is impaired by cannabis?
Not reliably. This study found oral fluid tests are good at detecting whether someone has used cannabis recently, but the oral fluid THC concentration does not accurately reflect blood THC levels, which are themselves an imperfect measure of actual impairment.
What does 98.3% specificity mean?
It means that among drivers who truly had no THC in their blood, 98.3% also tested negative on the oral fluid test. In practical terms, false positives were rare, occurring in fewer than 2% of non-users.
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Cite This Study
https://rethinkthc.com/research/RTHC-01705APA
Jin, Huiyan; Williams, Sharifa Z; Chihuri, Stanford T; Li, Guohua; Chen, Qixuan. (2018). Validity of oral fluid test for Delta-9-tetrahydrocannabinol in drivers using the 2013 National Roadside Survey Data.. Injury epidemiology, 5(1), 3. https://doi.org/10.1186/s40621-018-0134-2
MLA
Jin, Huiyan, et al. "Validity of oral fluid test for Delta-9-tetrahydrocannabinol in drivers using the 2013 National Roadside Survey Data.." Injury epidemiology, 2018. https://doi.org/10.1186/s40621-018-0134-2
RethinkTHC
RethinkTHC Research Database. "Validity of oral fluid test for Delta-9-tetrahydrocannabinol..." RTHC-01705. Retrieved from https://rethinkthc.com/research/jin-2018-validity-of-oral-fluid
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.