Fitness to drive and cannabis

Forensic Sci Int. 2014 Jun 4;242C:1-8. doi: 10.1016/j.forsciint.2014.05.014. [Epub ahead of print]

Fitness to drive and cannabis: Validation of two blood THCCOOH thresholds to distinguish occasional users from heavy smokers.

Fabritius M1, Augsburger M1, Chtioui H2, Favrat B3, Giroud C4.

Author information

• 1Forensic Toxicology and Chemistry Unit (UTCF), University Center of Legal Medicine (CURML), Rue du Bugnon 21, 1011 Lausanne, Switzerland.
• 2Division of Clinical Pharmacology, Centre Hospitalier Universitaire Vaudois, rue du Bugnon 17, Lausanne, Switzerland.
• 3Unit of Psychology and Traffic Medicine, University Center of Legal Medicine, Rue Saint-Martin 26, 1005 Lausanne, Switzerland; Unit of Psychology and Traffic Medicine, University Center of Legal Medicine, CMU, Rue Michel-Servet 1, 1211 Geneva 4, Switzerland.
• 4Forensic Toxicology and Chemistry Unit (UTCF), University Center of Legal Medicine (CURML), Rue du Bugnon 21, 1011 Lausanne, Switzerland. Electronic address: Christian.giroud@chuv.ch.

Abstract

Many studies based on either an experimental or an epidemiological approach, have shown that the ability to drive is impaired when the driver is under the influence of cannabis. Baseline performances of heavy users remain impaired even after several weeks of abstinence. Symptoms of cannabis abuse and dependence are generally considered incompatible with safe driving. Recently, it has been shown that traffic safety can be increased by reporting the long-term unfit drivers to the driver licensing authorities and referring the cases for further medical assessment. Evaluation of the frequency of cannabis use is a prerequisite for a reliable medical assessment of the fitness to drive. In a previous paper we advocated the use of two thresholds based on 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THCCOOH) concentration in whole blood to help to distinguish occasional cannabis users (≤3μg/L) from heavy regular smokers (≥40μg/L). These criteria were established on the basis of results obtained in a controlled cannabis smoking study with placebo, carried out with two groups of young male volunteers; the first group was characterized by a heavy use (≥10 joints/month) while the second group was made up of occasional users smoking at most 1 joint/week. However, to date, these cutoffs have not been adequately assessed under real conditions. Their validity can now be evaluated and confirmed with 146 traffic offenders' real cases in which the whole blood cannabinoid concentrations and the frequency of cannabis use are known. The two thresholds were not challenged by the presence of ethanol (40% of cases) and of other therapeutic and illegal drugs (24%). Thus, we propose the following procedure that can be very useful in the Swiss context but also in other countries with similar traffic policies: if the whole blood THCCOOH concentration is higher than 40μg/L, traffic offenders must be directed first and foremost toward medical assessment of their fitness to drive. This evaluation is not recommended if the THCCOOH concentration is lower than 3μg/L and if the self-rated frequency of cannabis use is less than 1 time/week. A THCCOOH level between these two thresholds cannot be reliably interpreted. In such a case, further medical assessment and follow-up of the fitness to drive are also suggested, but with lower priority.