Dividing oral fluid THC concentration by a regression coefficient provided an acceptable estimate of blood THC levels in a population, though accuracy was poor when fewer than 15% of the population had high blood levels.
Read this if you are involved in drug-impaired driving research or policy.
Regression coefficient method best for estimating blood THC from saliva; fails below 15% prevalence
What the researchers found
Researchers compared five methods for estimating how many people in a population have high blood THC concentrations based on oral fluid (saliva) testing.
The methods ranged from simple calculations (dividing saliva concentration by an average ratio) to complex Monte Carlo simulations.
Dividing by the oral fluid/blood regression coefficient gave the best balance of accuracy and precision, making it the recommended method.
Monte Carlo simulations could give better accuracy but required good data on the distribution of oral fluid/blood ratios, which is not always available.
None of the methods worked well when fewer than 15% of the population had high blood drug concentrations.
Why it matters
Roadside saliva testing for drugged driving is more practical than blood testing. If saliva THC levels can reliably estimate blood levels, it could improve drug-impaired driving enforcement.
The numbers in context
311 cannabis users and 197 amphetamine users. Five estimation methods compared. Best method: dividing by regression coefficient. Accuracy acceptable only when 15%+ of population had high blood levels.
How the study worked
Methodological study comparing five estimation methods using data from 311 cannabis users and 197 amphetamine users from the Rosita-2 Project. Methods included simple ratio calculations and Monte Carlo simulations.
What this study cannot tell us
The relationship between oral fluid and blood THC concentrations is highly variable between individuals. No single method was highly accurate. The 15% prevalence threshold limits applicability in low-use populations.
How to read the evidence
Methodological study using existing datasets. Provides practical guidance but with significant accuracy limitations.
When this study was published
Published in 2010. Oral fluid drug testing technology and methodology have improved since then, though individual variability remains a challenge.
The bigger picture
As cannabis legalization expands, practical tools for roadside impairment assessment become increasingly important. This study contributed to the development of saliva-based drug testing protocols for traffic safety.
Questions still open
- Can newer saliva testing technologies improve accuracy? Should impairment be assessed by THC levels or by functional tests? How does the oral fluid/blood ratio vary with different cannabis consumption methods?
Common questions
Can a saliva test tell if someone is impaired from cannabis?
Why not just use blood tests?
Read the original research
Can the prevalence of high blood drug concentrations in a population be estimated by analysing oral fluid? A study of tetrahydrocannabinol and amphetamine.
Forensic science international, 195(1-3), 153-9
Citation
Gjerde, Hallvard; Verstraete, Alain. (2010). Can the prevalence of high blood drug concentrations in a population be estimated by analysing oral fluid? A study of tetrahydrocannabinol and amphetamine.. Forensic science international, 195(1-3), 153-9. https://doi.org/10.1016/j.forsciint.2009.12.011