Cannabis, crashes and blood: challenges for observational research
Notice bibliographique
Résumé
The cannabis crash responsibility study by Brubacher et al. 1 is timely, considering that cannabis for recreational use became lawful in Canada on 17 October 2018 2, raising expectations of increased adult use 3. Some governmental agencies prohibit cannabis use for 24 hours, 28 days or ever before safety-sensitive work 4-7. The occupational medicine recommendation is 24 hours (or longer if impaired) 8. Canadian impaired driving law uses a two-tier system of concentration per-se limits for delta-9-tetrahydrocannabinol (THC) in blood: greater offence at 5 ng/ml THC or 2.5 ng/ml in combination with 0.05 g% ethanol; lesser offence at 2 ng/ml THC 9. International discussion has not produced scientific consensus on what threshold THC concentration should be enforced, varying from 0.2 ng/ml (Sweden) to 5.0 ng/ml (Colorado) 10. For alcohol, per-se limits work because of distribution throughout the total body water compartment, with good correlations between blood and brain concentrations, degree of impairment and crash risk 10. Lipophilic THC does not have well-established correlations: after smoking, maximum blood THC concentration is reached by approximately 10 minutes, then drops precipitously by 90% in 1–2 hours, with low concentrations measurable for up to 12 hours or longer 11. THC clears the bloodstream but accumulates in lipids and brain tissue 12, causing dose-dependent subjective effects for up to 8 hours 13, 14 (10 hours with oral administration 14, 15) and cognitive effects up to 24 hours 15. Because of rapidly declining THC concentration, a driver might have been substantially above a per-se limit at the time of driving or crash but fall below the limit by the time of blood draw 16. Postmortem blood THC concentrations more accurately reflect concentration at the time of crash, because circulation and metabolism ceases with death. In a study of traffic fatalities, blood THC concentrations of at least 5 ng/ml in killed drivers yielded an odds ratio (OR) of 6.6 for crash responsibility 17. Meta-analysis of cannabis crash studies calculated a non-statistically significant OR in non-fatal crashes but a statistically significant OR of 2.1 in fatal crashes, including both killed and surviving drivers 18. The underpowered study by Brubacher et al. does not demonstrate a statistically significant increase in OR, as only 20 of 1825 analyzed subjects had 5 ng/ml or more THC in blood, probably reflecting delayed sample collection. Nevertheless, the authors claim that their findings ‘suggest that the impact of cannabis on road safety is relatively small at the present time’ (p. 1621), despite $1.09 billion in costs, 75 collision deaths and 4407 injuries attributed to Canadian cannabis-related collisions in 2012 19, and the fact that data collection ended almost 2 years before recreational cannabis became lawful. Like many observational studies, Brubacher et al. rely on blood THC concentrations in living drivers without driver-specific data determinative of impairment and crash risk, such as THC dose, route of administration, time of last use and cannabis use history. True crash risk associated with cannabis use cannot be understood based solely upon low THC concentrations in the blood of living drivers.
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Prédiction distillée sur la base complète
Imitation des enseignantsNi prévalence calibrée, ni vérité terrain. Validation humaine à venir. Apprise à partir de 10 348 étiquettes directes de Codex et de 10 348 étiquettes directes de Gemma. Le mode candidate est l'union des têtes enseignantes seuillées; le consensus est leur intersection. Ces sorties portent le statut machine_predicted_unvalidated et ne sont ni des étiquettes humaines ni des étiquettes directes de modèles de pointe.
Scores Codex et Gemma par catégorie
| Catégorie | Codex | Gemma |
|---|---|---|
| Métarecherche | 0,001 | 0,000 |
| Méta-épidémiologie (sens strict) | 0,000 | 0,000 |
| Méta-épidémiologie (sens large) | 0,000 | 0,000 |
| Bibliométrie | 0,000 | 0,000 |
| Études des sciences et des technologies | 0,000 | 0,000 |
| Communication savante | 0,000 | 0,000 |
| Science ouverte | 0,000 | 0,000 |
| Intégrité de la recherche | 0,001 | 0,002 |
| Charge utile insuffisante (le modèle a refusé de juger) | 0,000 | 0,000 |
Scores machine (provisoires)
Les deux têtes enseignantes du modèle étudiant, lues sur ce travail. Un score ordonne la base pour la relecture; il n'affirme jamais une catégorie, et le statut de validation accompagne chaque rangée tel quel.
Scores de référence d'un modèle non mature (critères de maturité non atteints, 7 itérations). Un score ordonne; il n'affirme jamais une catégorie.
score_only:v0-immature-baseline · tel quel depuis la passe de notation : score_only signifie que le nombre peut ordonner les travaux, et qu'aucune étiquette de catégorie n'en découleClassification
machine, non validéePrédiction automatique; un appel candidat d’une seule tête enseignante, pas un consensus.
Le détail, modèle par modèle et score par score, se trouve en fin de page sous « Comment cette classification a été obtenue ».