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Abstract
BackgroundDriving under the influence of marijuana is a serious traffic safety concern in the United States. Delta 9-tetrahydrocannabinol (THC) is the main active compound in marijuana. Although blood THC testing is a more accurate measure of THC-induced impairment, measuring THC in oral fluid is a less intrusive and less costly method of testing.MethodsWe examined whether the oral fluid THC test can be used as a valid alternative to the blood THC test using a sensitivity and specificity analysis and a logistic regression, and estimate the quantitative relationship between oral fluid THC concentration and blood THC concentration using a correlation analysis and a linear regression on the log-transformed THC concentrations. We used data from 4596 drivers who participated in the 2013 National Roadside Survey of Alcohol and Drug Use by Drivers and for whom THC testing results from both oral fluid and whole blood samples were available.ResultsOverall, 8.9% and 9.4% of the participants tested positive for THC in oral fluid and whole blood samples, respectively. Using blood test as the reference criterion, oral fluid test for THC positivity showed a sensitivity of 79.4% (95% CI: 75.2%, 83.1%) and a specificity of 98.3% (95% CI: 97.9%, 98.7%). The log-transformed oral fluid THC concentration accounted for about 29% of the variation in the log-transformed blood THC concentration. That is, there is still 71% of the variation in the log-transformed blood THC concentration unexplained by the log-transformed oral fluid THC concentration. Back-transforming to the original scale, we estimated that each 10% increase in the oral fluid THC concentration was associated with a 2.4% (95% CI: 2.1%, 2.8%) increase in the blood THC concentration.ConclusionsThe oral fluid test is a highly valid method for detecting the presence of THC in the blood but cannot be used to accurately measure the blood THC concentration.
Highlights
Driving under the influence of marijuana is a serious traffic safety concern in the United States
We examined the association between the continuous blood THC concentration and the continuous oral fluid THC concentration among drivers with positive THC in either blood or oral fluid
9.4 and 8.9% of the samples tested positive for THC in blood and oral fluid, respectively, yielding a sensitivity of 79.4% with a 95% confidence interval (CI) (75.2%, 83.1%) and a specificity of 98.3% for the oral fluid test in predicting positivity in blood THC
Summary
Driving under the influence of marijuana is a serious traffic safety concern in the United States. Because blood THC test is a more accurate measure of THC-induced impairment (Hartman et al 2016; Grotenhermen et al 2007), many US states use blood THC concentrations as the gold standard for determining marijuana-related DUI (Center for Behavioral Health Statistics and Quality 2016; Berning et al 2015). Different cut-off points of blood THC concentrations have been proposed for establishing per se laws, which make operating a motor vehicle a criminal offense for an individual if he or she has a specific amount of drug or metabolite in his or her body This threshold concentration is a legal limit, and exceeding this threshold serves as proof of legal impairment (Grotenhermen et al 2007; Wong et al 2014; National Conference of State Legislatures 2017). 20 states have established per se limit laws on marijuana-impaired driving, with four states using 5 nanograms per milliliter (ng/ml) as the threshold for THC and the others having zero tolerance (National Conference of State Legislatures 2017)
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