Why Low Oral-Fluid THC Does Not Rule Out Other Drug Exposure
A low oral-fluid THC concentration does not establish that cannabis was the only substance present. Spanish roadside data found other drugs frequently detected alongside THC, including among samples with relatively low THC values.
The denominator changes the meaning
Herrera-Gómez and colleagues analyzed administrative roadside testing data from 2011–2016. Among 65,244 confirmed drug-positive tests, 51,869 were THC-positive. These were selected testing records, not a random sample of all drivers with complete behavioral assessments.
Just over half of THC-positive tests also contained another tested substance. Cocaine and amphetamine-class drugs were prominent co-detections. The overall co-detection percentage should not be interpreted to mean that both drug classes were present together in every such sample.
A concentration is not a complete exposure history
Other substances were common among the lower oral-fluid THC concentrations. These findings caution against interpreting a relatively small THC number as a complete description of the toxicological situation.
The authors found age-related differences in positivity patterns. Some adjusted gender comparisons were nonsignificant even though men greatly outnumbered women in the testing records. Raw sample composition and an adjusted association answer different questions.
What the records cannot establish
The database lacked detailed information on negative drivers and alcohol breath results. Testing circumstances included targeted enforcement as well as other reasons for testing. Selection and missing information limit population prevalence estimates and causal interpretation.
No comparison in this study establishes how much a specific combination impaired an individual or caused a collision. Oral fluid is also not interchangeable with blood. In a case review, identify all tested and untested substances, the collection circumstances, and documented performance before offering an opinion about the role of THC.
Related reading: why urine and blood results are not interchangeable.
Primary research
Francisco Herrera-Gómez; Mercedes García-Mingo; Mónica Colás; Juan Carlos González-Luque; F Javier Alvarez. Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016. BMJ Open (2019). DOI: 10.1136/bmjopen-2018-026648.
