A meta-analysis of over 18,000 paired samples found oral fluid testing was 71% sensitive and 98% specific for detecting any blood THC, but was much less accurate at predicting whether blood THC exceeded legal per se limits.
Traffic safety policymakers; law enforcement; forensic toxicologists; drivers in jurisdictions with per se THC limits.
71% sensitive for any THC but unreliable for predicting blood levels above 5 ng/mL
What the researchers found
Oral fluid THC reliably detected presence of THC in blood (71.2% sensitivity, 97.7% specificity). However, at commonly used cutoffs, oral fluid was less sensitive and less specific for detecting blood THC above per se limits like 5 ng/mL. This would produce many false positives in random driver testing.
Why it matters
Many countries use roadside oral fluid tests to enforce cannabis-impaired driving laws. This study shows these tests are useful for identifying recent cannabis use but unreliable for determining whether drivers exceed legal blood THC limits, with major implications for enforcement fairness.
The numbers in context
Over 18,000 paired oral fluid-blood samples. Presence detection: 71.2% sensitivity, 97.7% specificity. Performance deteriorated significantly when used to predict blood levels above 5 ng/mL per se limits. High false positive rate for random driver screening.
How the study worked
Systematic review and meta-analysis of studies comparing oral fluid THC with blood THC. Individual-level data obtained from study authors and pooled for analysis. Over 18,000 paired samples analyzed. Sensitivity and specificity calculated at various oral fluid cutoff values for detecting different blood THC concentration thresholds.
What this study cannot tell us
Paired samples came from various study conditions (controlled dosing, roadside testing, clinical). The relationship between blood THC and actual driving impairment is itself imperfect. Different oral fluid collection devices may perform differently.
How to read the evidence
Strong: meta-analysis of individual-level data from over 18,000 paired samples with comprehensive statistical analysis.
When this study was published
Published in 2022.
The bigger picture
The disconnect between oral fluid THC (which reflects recent exposure) and blood THC (which correlates better with impairment) creates a fundamental problem for per se driving laws. The authors recommend against random oral fluid screening where per se blood limits exist, but support its use for investigating already-suspicious drivers.
Questions still open
- Should per se blood THC limits be replaced with impairment-based assessments? Can oral fluid testing technology be improved to better predict blood THC levels? How should cannabis driving laws balance enforcement practicality with individual rights?
Common questions
Can a roadside saliva test prove you are impaired by cannabis?
Should oral fluid testing be used for random driver screening?
Read the original research
Correlation between oral fluid and blood THC concentration: A systematic review and discussion of policy implications.
Accident; analysis and prevention, 173, 106694
Citation
Robertson, M B; Li, A; Yuan, Y; Jiang, A; Gjerde, H; Staples, J A; Brubacher, J R. (2022). Correlation between oral fluid and blood THC concentration: A systematic review and discussion of policy implications.. Accident; analysis and prevention, 173, 106694. https://doi.org/10.1016/j.aap.2022.106694
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