Oral fluid THC tests were highly accurate at detecting cannabis use but poor at measuring blood THC levels | Research

The takeaway

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.

Traffic safety researchers; law enforcement developing cannabis testing protocols; policy makers working on impaired driving legislation.

79.4% sensitivity, 98.3% specificity, but only 29% of blood THC variation explained

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot 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.

How to read the evidence

Moderate. Large nationally representative sample with validated testing methods, though cross-sectional design limits conclusions about individual-level impairment.

When this study was published

Published in 2018 using 2013 survey data. Oral fluid testing technology and roadside testing protocols have continued to evolve.

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.

Questions still open

What oral fluid THC threshold best balances sensitivity and specificity for roadside testing? Can combining oral fluid testing with behavioral assessment improve impairment detection?

Common 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.

Read the original research

Validity of oral fluid test for Delta-9-tetrahydrocannabinol in drivers using the 2013 National Roadside Survey Data.

Injury epidemiology, 5(1), 3

View on PubMed ↗Publisher’s paper ↗

Citation

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