Testing Challenges: No BAC for THC

Utilizing urine to detect THC presents unique challenges – the collection cannot be accomplished roadside, must be monitored to preserve integrity, but cannot be forced, and implies some indignity for both the observer and the subject.  THC in urine, though, likely represents a metabolite of THC that fails to demonstrate a level of impairment at the time of vehicle operation.

The least invasive collection technique of bodily substances, breath testing provides the shortest detection window for THC, and if a breath quantification for THC impairment existed, as it does for alcohol, detecting THC impairment could be relatively simple.  To date, though, no such measure exists.  Similar to a breath test, oral fluid testing also presents a short detection window, but provides a relatively quick process, can be completed roadside, and is not invasive. Standardized field sobriety tests (SFSTs) and drug recognition expert (DRE) evaluations may provide clues to THC impairment, but fail to provide conclusive proof of THC involvement.

While criminal justice practitioners possess broad experience with the standards for determining alcohol impairment, THC impairment represents an emerging challenge, particularly as more states legalize medical and/or recreational marijuana.  Blood alcohol content (BAC) serves as an accurate measurement of alcohol impairment because the substance is a relatively simple one; alcohol is highly water soluble and spends much of its time in the body within the watery blood.  Peak blood concentration of alcohol coincides with peak impairment.  While BAC level represents a robustly-researched accurate measurement of the alcohol impairment of a driver, the presence of THC in a driver’s body has not been shown to be a predictable measure of cannabis impairment.   

The disconnect between THC blood concentration and impairment results from the nature of THC, a lipophilic or fat-soluble substance.   THC travels via the bloodstream to the brain and then processed through the endo-cannabinoid receptor system.  The body pulls the THC out of the bloodstream and deposits it in the fatty tissue of the body, including the brain, which is composed largely of fat.  The THC slowly leaches from fatty tissue as a metabolite to the body’s excretory system, resulting in elimination primarily through urine, sweat, the breath or the blood.  Peak blood concentration of THC occurs prior to peak impairment.  In short, unlike the very simple drug, alcohol, blood concentrations of THC and its metabolites fail to establish impairment at a particular time necessitating other proof or testimony demonstrating impairment from THC.  The proof of THC impairment often takes the form of witness testimony establishing driving and driver behavior, results of standardized field sobriety tests, and drug recognition evaluations. 

Unlike alcohol, the estimation of impairment from THC remains difficult.   High concentrations of THC reach the blood and brain shortly after smoking starts, causing impairment, but blood concentration decreases quickly after smoking stops, since THC quickly leaves the blood to be distributed into fatty tissue.

The THC then can remain in the fatty tissue for a fairly long time, but small amounts will slowly leach out of fatty tissue and back into the blood, causing a continual low blood concentration long after ingestion.  Because THC in the blood can result from both recent as well as past use, impairment cannot be inferred from blood levels.  Early studies show maximal driving impairment twenty to forty minutes after smoking, and the risk of driving impairment may decrease after 2.5 hours, at least in those who smoke 18mg Delta 9-THC or less (the dose often used experimentally to duplicate a single joint), but, unlike alcohol, no specific THC blood level correlates with impairment.   Thus, no BAC for THC!  Lacking a correlative BAC to determine impairment, the impact of cannabis use on the critical skills necessary to drive remains best detected from the physiological symptoms of impairment.

The method of THC consumption also demonstrates the inadequacy of blood THC levels in determining impairment.  Smoking THC products results in quick absorption into the blood through the lungs, and thus a more direct route to the brain.  For edibles that contain THC, peak blood levels occur around three hours after ingestion.  Because THC products consumed through oral ingestion must be absorbed through the digestive system, edible THC takes longer to travel through the body, to the blood and eventually to the brain.  Thus, ingesting THC via edibles may result in increased consumption because of the delayed effect of use as the THC passes through the digestive system and then into the blood and onto the brain.  An edible high generally lasts much longer than smoking or vaping, anywhere from six to eight hours.   But, unlike alcohol, blood levels of THC fail to equate to a specific level of impairment.

Exiting a bar, the odor of alcohol, or an admission of drinking, fails to demonstrate impairment.  Similarly, evidence of possession of marijuana or its consumption alone and without more remain insufficient to prove impairment.   Support for the conclusion of impairment may be found in driving behavior and in physiological observations.   Clues of impairment resulting from driver behavior may be found in four general areas – maintaining lane and speed, braking issues, vigilance, and judgment.  Clues of impairment resulting from suspect behavior include difficulty with motor vehicle controls, trouble exiting vehicle, challenges with retrieving and/or location documents, repeating questions/comments, changing answers, swaying or unsteadiness, odor of alcohol or drugs, glassy or red eyes, flushed face, unsteady gait, slurring, poor coordination, and slowed reactions.   SFSTs (walk and turn, one-leg stand, and horizontal gaze nystagmus (HGN)) may generate additional clues of impairment, as can the observations from a DRE who utilizes a nationally-standardized protocol for identifying drug intoxication that identifies seven different categories of drugs and the physical symptoms associated with each.

While no one test or measure currently exists to demonstrate driver impairment resulting from THC, a combination of bodily fluid testing and observations of driver behavior serve as potential evidence of the influence of THC on driving behavior.