High Traffic—The Quest for a Reliable Test of Cannabis Impairment

Abstract

In this issue of JAMA Psychiatry, Marcotte et al 1 report that field sobriety tests (FSTs) as administered by highly trained police officers are insufficient to detect cannabis-induced impairment in a double-blind, placebo-controlled, parallel randomized clinical trial involving a large sample of 184 cannabis users. Although the group receiving active doses of Δ-9-tetrahydrocannabinol (THC), the active ingredient in cannabis, performed worse on the FSTs as compared with the placebo group; about half of the participants in the placebo group were classified as impaired. These findings are in line with previous placebo-controlled studies that also reported high false-positive FST rates under placebo. 2,3 The legal implication of these findings can be major given that FSTs are currently part of the evaluation protocol in North America to detect drivers who are cannabis impaired. Yet, the lack of sensitivity of FSTs to detect THC-impaired individuals does not come as a big surprise, as FSTs have primarily been validated to detect gross alcohol impairment at high (more than 0.10%) blood alcohol concentrations. 4 To add to this problem, there is no cannabis equivalent of a breathalyzer to verify exposure induced impairment, as trace amounts of THC in biomarkers correlate poorly with cannabis-induced behavioral impairment. 5 These findings stress the need to develop a reliable roadside test that is validated to detect driver impairment that is specific to cannabis. As such, behavioral tests to detect cannabis-induced impairment do exist and have been used in placebo-controlled studies to demonstrate that acute cannabis intoxication can impair road tracking in on-road and simulator driving studies and can impair motor performance (eg, reaction time) and cognitive function (eg, attention, decisionmaking, impulse control, memory) needed for safe driving in laboratory studies. 6 Importantly, these studies measure cannabis-induced impairment as the change in performance relative to a placebo control condition. Roadside tests of cannabis impairment, such as the FSTs, however, do not have a reliable reference measure of an individual's nondrug performance and consequently aim to define an absolute impairment threshold that applies to every individual. Yet, psychomotor capacities to stand on 1 leg or to walk and turn on a white line can vary considerably between individuals, which is why many can fail an FST even when sober. Standards for cannabis impairment will therefore be hard to define for on-site behavioral tests in the absence of normative data.The lack of a reliable measure of cannabis impairment has also impacted epidemiological research designed to assess the contributory role of cannabis to crash risk in traffic. The epidemiological research so far has been underwhelming, with

Intoxication due to Δ9-tetrahydrocannabinol is characterized by disrupted prefrontal cortex activity

“…A distinction between THC exposure and THC intoxication is important. With THC intoxication, unlike with alcohol, there is a highly variable relationship between blood, breath, or saliva THC or THC metabolite concentration and THC-induced intoxication or impairment that is dependent on the individual level of tolerance to THC [ 61 , 62 ]. Therefore, it is likely that objective behavior- or brain-based metrics (e.g.…”