Re: “Trends in Alcohol and Other Drugs Detected in Fatally Injured Drivers in the United States, 1999–2010”

Driving under the influence of drugs is a growing public safety concern, even as rates of alcohol-impaired driving have declined significantly in recent decades (1). The Office of National Drug Control Policy prioritized drugged driving among its top research initiatives (2). A recent article by Brady and Li (3) described alcohol and drug trends in 1999–2010 in fatally injured drivers who died within an hour of a crash on the basis of Fatality Analysis Reporting System (FARS) data. Much of the article focused on increased detection of cannabis or marijuana markers in drivers' blood; unfortunately, inaccurate terminology led to incorrect reporting. Cannabis, cannabinol, and cannabinoid are not interchangeable terms.

The authors cite the FARS coding manual in describing drug test results entered into the FARS database. The FARS Analytical User's Manual (4, p.256) describes attribute codes entered in the database. In the 1991–1992 manual, code “06” was imprecisely labeled “cannabinol”; but versions from 1993 and later corrected the label to “cannabinoid,” also changing the codes to 600–695. “Cannabinoids” refers to a class of compounds that act on cannabinoid receptors. More than 100 cannabinoids are present in cannabis (5); the primary psychoactive cannabinoid is Δ⁹-tetrahydrocannabinol (THC). Cannabinol is a specific, minor phytocannabinoid with much lower psychoactivity than THC, present in low concentrations in smoke and detected for up to 2.1 hours in the blood of frequent smokers (6). Detection of cannabinol in blood coincides with detection of THC, although THC is detectable for much longer periods at higher concentrations. Thus, cannabinol is often associated with recent intake, but not direct psychoactive impairment. Throughout the article (3), the authors describe drug test results for cannabinol rather than reporting on cannabinoids. Cannabinol is not among the specific coded cannabinoid subtypes listed within the FARS coding system; if detected, it might be categorized under code 695 (“cannabinoid, type unknown”). However, most laboratories do not yet test for cannabinol, rendering it virtually unrepresented within the FARS—particularly given its short detection window.

Although the terminology interchange is apparent throughout the article, table, and figures, an instance of particular note is where Skopp and Pötsch (7) are cited for the assertion that “[i]t is possible for a driver to test positive for cannabinol in the blood up to 1 week after use” (3, p. 697). Again, cannabinol is detected in blood for only brief periods postdose. Furthermore, that study examined serum, not blood; and cannabinol was not among the analytes tested (7). The apparent intended data refer to a “heavy, regular” (>1 joint/day) smoker with serum THC quantifiable 120 hours (5 days) after last smoking (7, p. 161).

We wish to emphasize that the FARS coding system does not differentiate between active and inactive cannabinoid metabolites. Inactive phase I and phase II metabolites 11-nor-9-carboxy-THC and 11-nor-9-carboxy-THC-glucuronide fall under the same cannabinoid classification as active THC, although they may be present in blood for 30 hours or more (6), beyond the acute intoxication period (8). As briefly discussed in the article, this necessitates cautious interpretation. Correlation does not imply causality, particularly with extended cannabinoid detection windows. This represents one of the most challenging aspects of cannabis-driving research. Even the primary impairing psychoactive compound, THC, could be detected for a month (at low concentrations) in some chronic, frequent smokers' blood (9). This results from THC's high lipophilicity and large THC body burdens in frequent smokers following daily or near-daily cannabis smoking. THC is stored in fatty tissues and gradually released back into blood for excretion over a long time frame.

The authors are correct that the drugged driving issue is critical. A great deal of misinformation is currently circulating in the media, particularly related to cannabis-impaired driving. Scientific studies are often misrepresented (albeit typically unintentionally), leading to incorrect assertions. It is crucial for scientists to report data accurately to help prevent disseminating misinformation.