THC-Positive Trauma Patients: Why Injury Severity Is Not a Crash-Risk Estimate
A study of injured people cannot, by itself, tell us how often cannabis users crash compared with drivers who do not use cannabis. The distinction between injury severity after a crash and the risk of having a crash is essential when interpreting trauma-registry research.
The population the researchers selected
Borst and colleagues examined five San Diego County adult trauma-center registries, identifying 11,491 motor-vehicle or motorcycle crash patients with completed toxicology screening over the study period. They compared toxicology categories and reviewed separate medical-examiner data for scene deaths.
This was observational human research among selected injured patients. It did not sample the general driving population or provide a denominator of miles driven by comparable exposed and unexposed drivers.
What was associated with THC positivity?
The study reported increasing THC positivity over time and associations with patient characteristics and injury severity. THC-positive patients differed in age and sex and were less likely to use seatbelts. There was no difference in in-hospital mortality between the compared groups.
Both the significant and nonsignificant results matter. A difference in an injury-severity score is not the same outcome as death, and neither is a direct measurement of whether cannabis caused the crash.
Testing and selection affect the inference
The included patients had completed toxicology screening. The exposure category does not independently establish the degree of functional impairment at the time of driving. Nor should the separate medical-examiner cohort be merged with hospital survivors as though they were one uniformly sampled dataset.
Co-exposures also matter. THC alone, alcohol above the study’s threshold, combined THC and alcohol, and stimulant-related categories answer different questions. A finding from one category should not be relabeled as the effect of THC alone.
How to use the study in a collision review
Ask whether the opinion concerns trends in positive tests, severity among injured patients, population crash risk, or responsibility for a particular collision. Identify the actual specimen, test, and exposure definition in the records, along with protective-device use and other relevant circumstances.
The registry study can inform injury-prevention questions. It cannot supply an individualized impairment determination or a population crash-risk multiplier merely by comparing people who have already been injured.
Related evidence
See our related analysis of toxicology evidence and its limits.
Primary research
Driving under the influence: a multi-center evaluation of vehicular crashes in the era of cannabis legalization. Trauma Surgery & Acute Care Open, 2021. DOI: 10.1136/tsaco-2021-000736.
