A New Technique for Detecting Cannabis Impairment may change the Game for Cannabis and Driving

The quest for an accurate breath-based biomarker similar to breathalyzers for alcohol have stretched on for almost a decade, with plenty of money and prototypes proposed but not much to show for it. In fact, a study at the University of Sydney in 2021 found that THC concentrations in blood, which in six American states are used to determine cannabis impairment, are only weakly associated with elevated risk behind the wheel. This makes the recent study published by scientists at Massachusetts General Hospital (MGH) a potential game changer. Rather than examine THC limits, the research uses light-based imaging within the brain to detect impairment. And from what their paper in Neuropsychopharmacology has revealed, the analytical prowess of their procedure improves not only upon breath-based technologies but other forms of cannabis impairment detection used on the roads today.

To do this, MGH researchers had to answer a question that for some reason hasn’t been really asked during this entire period: how do you determine actual impairment from cannabis use, and at what point does it create a problem for someone behind the wheel. With alcohol, it’s relatively simple. It’s one chemical that is being broken down in one’s body immediately after consumption, and there will be impairment for as long as it’s present. Most notably, it’s also expelled through breath, which is why breathalyzers work as well as they do on the road. However, as anyone who’s ever taken a drug test knows, cannabis metabolites can linger within a person’s body long after the high wears off and it’s safe to drive. Yet as the MGH paper observes, cannabis impairment doubles the risk of a motor vehicle accident. It’s one of the reasons the AAA opposes broader legalization. If America is ever going to have legal cannabis nationwide, this question needs to have a better answer than what we have now.

The MGH researchers believe they’ve found the culprit, and it isn’t THC exposure. Rather, it’s higher levels of oxygenated hemoglobin found in various parts of the brain, scanned by a technology named functional near-infrared spectroscopy (fNIRS). Previous research by the MGH team and elsewhere has noted perfusion arising from THC in prefrontal areas of the brain that can be measured by fNIRS. As Dr. A. Eden Evins told the Boston Globe, “Essentially, the [impaired] brain becomes more active but less efficient at processing, so the body gives it priority and sends more oxygen.”

These levels of impairment were matched with two clinical raters of impairment, one self-reports from the test subjects and the other the assessment of blinded clinicians to come up with a baseline of impairment which they then programmed into an algorithm. The program predicted impairment 76% of the time, and only found false positives in 10% of the subjects tested. This is a marked improvement over the assessment of Drug Recognition Experts, a discipline within law enforcement which assesses drug impairment. Their assessments were also tested and compared with fNIRS within the paper, and they came in second in accuracy at 67.8%, with a false positive rating of 35.4%. This discovery comes hot on the heels of the completion of a three-year study conducted by researchers at UC-San Diego that Hytiva® wrote about in September. This study, like the MGH study, assessed regular cannabis consumers who were evaluated before and after smoking joints containing 5.9% and 13.4% THC as well as placebo on a driving simulator. The test found about 50% of the 180 people tested had diminished performance. Most notably, they found that subjective assessments of their ability to drive, which usually kicked in at the 90-minute mark, did not track accurately to their performance on the driving simulator. However, the significance of that impairment differed in various users. And the UCSD researchers noted, once again, that THC levels made no difference in impairment. In fact, those who smoked the 5.9% joint had a HIGHER concentration of THC in their bloodstream than those who smoked the 13.4%.

So the MGH study comes at an opportune moment, when even Drug Recognition Experts can fall prey to cognitive or social biases. However, it’s early days for the technology proposed by MGH. While fNIRS is non-invasive and far more portable outside of lab settings than its MRI cousin, it will still take more testing to improve upon its already impressive numbers, and a usable tool will have to be built upon this technology that law enforcement can use on the road. Nonetheless, it’s still good to hear that slowly but surely, cannabis DUI detection is moving away from the Stoned Ages of THC levels and breathalyzers towards measuring what actually makes people dangerous on the road, and testing for it accordingly.