A motorcyclist was fatally injured when he collided with a city bus in a business district. The weather was clear and fair, the posted speed limit was 30 mph and traffic was reported to be light at the time of the collision. The bus (which was empty at the time and traveling at approximately 25 mph) was making a left turn into a bus garage terminal when the collision occurred. The driver died on impact.

Witnesses stated that the motorcyclist attempted to decelerate as the bus turned, but the back tire of the motorcycle rose into the air, causing the bike to flip over and collide with the bus. A multi-camera DOT video recording from the bus revealed no apparent events or unusual distractions. Witnesses reported that the motorcycle was traveling at a high rate of speed.

  

Based on these findings, Dr. Sawyer elected to conduct a pharmacokinetic assessment to determine when the marijuana was smoked and if the reported cannabinoid blood levels were consistent with impairment sufficient to degrade the motorcyclist's reaction time and judgment at the time of the accident. In Toxicology, the metabolite ratios of THC (Tetrahydrocannabinol, the psychoactive ingredient in marijuana) can be measured for this purpose. Dr. Sawyer relied on controlled studies which documented the ratios of active THC to the carboxy-THC metabolite to determine the time elapsed since dosing. This, in turn, would establish when it was administered and whether the blood level was sufficient to contribute to the accident.

Pharmacokinetic Methodology

The two primary metabolites at issue are the 11-hydroxy (active) and delta-9-carboxy (inactive) THC metabolites. Initially after smoking, active THC blood levels rapidly rise with peak levels occurring at approximately five minutes and then decreasing. Simultaneously, the delta-9-carboxy THC metabolite slowly increases (accumulates) as THC is transformed into the inactive metabolite. After 10 to 15 minutes, levels of these two metabolites are approximately equal.

The total and relative amounts of these components were significant in this case. Since the motorcyclist's postmortem blood analyses revealed slightly more active THC vs. carboxy-THC levels (2.8 and 2.7 ng/ml, respectively), Dr. Sawyer was able to offer a supported opinion that the motorcyclist smoked marijuana approximately 10 to 15 minutes before his death.

Additionally, Dr. Sawyer cited a recent peer-reviewed study by Ramaekers, et. al.^1,2 which demonstrated that the blood levels of 11-hydroxy THC and delta-9-carboxy-THC (two marijuana metabolites) are generally equal at approximately 10 to 15 minutes after smoking. He further cited studies which documented adverse motor control and cognitive effects using up to 20 subjects in a controlled setting. Primary findings included:

• Peak impairment occurred within the first hour after smoking.
• Impairment was significant at THC levels between 2 and 5 ng/ml. For reference, this level is similar to a blood alcohol concentration (BAC) of 0.08%.
• Cognitive function deficits (based on the "Tower of London" test) and impaired reaction time.
• Impact at low levels was relevant to the safe operation of a motor vehicle.

Dr. Sawyer assembled a detailed report and presented his testimony to the court. His findings offered objective reasoning (supported by peer-reviewed studies and substantial toxicological literature) that the motorcyclist displayed impaired judgment with respect to operating his motorcycle at high speed down a city street. The reaction of downshifting rather than braking just prior to the crash strongly suggested an impaired response. Dr. Sawyer further noted that event reconstruction experts estimated a speed in excess of 100 mph before impact. All of these factors were consistent with THC impairment based on the blood analyses. As a consequence, the motorcyclist presented an unreasonable scenario that the bus driver could not have avoided. 

Plaintiffs' counsel vehemently denied Dr. Sawyer's findings and filed strongly worded pre-trial motions to exclude his testimony, accusing Dr. Sawyer of presenting testimony which "...lacks foundation, is speculative, is based on hearsay and invades the province of the jury." 

In his rebuttal, Dr. Sawyer addressed these attacks point for point. In particular, he noted that plaintiffs' assertion that the level of THC was insufficient to induce impairment was incorrect, citing numerous sources from NTSB (National Transportation Safety Board)^3,4 and others demonstrating that the THC dose was more than sufficient to impair judgment, reaction time and corrective actions. 

Plaintiffs also asserted that a third THC metabolite (11-hydroxy delta-9 THC) should have been present at one-fifth the level of delta-9 carboxy THC, but was not detected in the blood analyses. Dr. Sawyer pointed out that the amount in the motorcyclist's blood was below the laboratory level of detection (LOD). Thus, it did not mean that the chemical was absent, but that it was below the minimum level that the laboratory instrument could detect. Dr. Sawyer further noted that if the motorcyclist had smoked the cited quantity of marijuana 10 to 15 minutes before the accident as outlined, the THC blood levels would have been precisely as measured in the autopsy report.