Breath Testing Not A Reliable Indicator Of Recent Marijuana Use, Federal Study Finds

A new federally funded study illustrates the difficulty of developing a breathalyzer-like device for cannabis: Even when using carefully collected samples and laboratory analysis, researchers found THC levels were too inconsistent to tell whether someone had smoked marijuana recently.

The findings, from researchers at the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, “do not support the idea that detecting THC in breath as a single measurement could reliably indicate recent cannabis use,” authors said.

“A lot more research is needed to show that a cannabis breathalyzer can produce useful results,” said co-author Kavita Jeerage, a NIST materials research engineer. “A breathalyzer test can have a huge impact on a person’s life, so people should have confidence that the results are accurate.”

To conduct the study—which was published in the Journal of Breath Research on Monday—researchers took breath and blood samples from 18 participants in Colorado who smoked the same kind of retail marijuana, which contained roughly 25 percent THC. Samples were collected in what a NIST press release describes as “a comfortably appointed white van that would park conveniently outside participants’ homes” both 15 minutes before and an hour after people smoked marijuana.

Analysis was done in a lab, not in the field, NIST said. A representative emphasized in an email to Marijuana Moment that the agency “is not working on developing a roadside breathalyzer device” but instead seeking to better understand “the basics of how to accurately measure THC and related compounds in breath.”

Of 14 participants who provided breath samples both before and after smoking marijuana, eight—a slim majority—“showed the anticipated increase in THC after cannabis use,” authors wrote. The rest of the results were noisy. “THC was not detected in three post-use breath extracts and the remainder of post-use extracts were similar to or lower than baseline extracts,” the researchers wrote.

“These observations,” the report continues, “suggest that reproducible breath aerosol collection remains an ongoing challenge.”

Blood measurements, meanwhile—as measured by THC concentrations in plasma—seemed to be a far more reliable indicator of recent cannabis use, the study found. “THC in breath at 1 h post-use was not necessarily higher than baseline, even when THC in blood indicated compliance with the protocol and at least a five-fold increase immediately post-use,” it says.

Tara Lovestead, a NIST supervisory chemical engineer and one of the report’s co-authors, said the team expected to see higher THC concentrations in breath samples collected after participants smoked marijuana. Instead, measurements spanned a similar range both before and after use.

“In many cases, we would not have been able to tell whether the person smoked within the last hour based on the concentration of THC in their breath,” Lovestead said.

NIST acknowledged that the small scale of the study means the results “do not carry statistical weight.” But agency and the report’s authors said the findings highlight the need for further study.

“One key question that we cannot yet answer is whether breath measurements can be used to distinguish between a person who uses cannabis regularly but hasn’t done so lately, and someone who consumed an hour ago,” Lovestead said. “Having a reproducible protocol for breath measurements will help us and other researchers answer that question.”

While the current study only involved smoking marijuana as a method of delivery, lead author Kavita Jeerage told Marijuana Moment that looking at other forms of consumption is “definitely important” and that NIST is currently conducting separate research that includes both smoking and vaping—and uses a different breath sampling device.

“We hope to expand these studies in the future,” she said. “Studying different modes of cannabis use and different breath sampling devices will enable us to determine how THC and other cannabis compounds enter the breath and are carried in breath.”

Investigating different modes of use “could make distinguishing recent use from past use more elusive, but if we can understand the underlying science, it will provide knowledge to improve devices for this purpose,” Jeerage said.

As more states have legalized cannabis, scientists and entrepreneurs have scrambled to develop field sobriety tests for THC, with some pursuing Breathalyzer-inspired devices. One California company, Oakland-based Hound Labs, said in 2015 that it expected widespread rollout of its device to law enforcement by the end of the following year. But so far no cannabis field sobriety test has seen widespread adoption.

NIST, an agency of the U.S. Department of Commerce, issued a notice last year seeking a contractor to study “how the concentration of cannabis compounds on a person’s breath changes in the hours after using marijuana.”

The current study was funded a grant from the Department of Justice’s National Institute of Justice, which has awarded the research team an additional $1.5 million to continue and expand the study. NIST says the next installment will involve 40 or more participants and more than a thousand breath samples, which “should give the results more statistical heft.”

Many policymakers see field sobriety tests as an important step in combatting impaired driving. The House Appropriations Committee report said in June of last year that members remained concerned about people driving under the influence of substances. It urged regulators to continue “efforts to ensure stakeholders can identify drug-impaired driving and enforce the law.”

A month earlier, Rep. Earl Blumenauer (D-OR) urged the Department of Transportation to reform its cannabis testing policies in light of the difficulty of determining recent use. He cited agency data showing that tens of thousands of truckers and other commercial drivers are penalized for using cannabis and noted that there’s no way to tell whether that use was days or weeks ahead of when they were tested. The congressman told Transportation Secretary Pete Buttigieg that his department “should rapidly reform requirements for testing drivers and returning them to service, as well as develop an accurate test for impairment.”

Data on highway safety and cannabis legalization is highly contentious, in large part because of the difficulty of accurately measuring the amount of cannabis in a person’s system as well as its impact on driving ability.

A study published in 2019, for example, concluded that those who drive at the legal THC limit—which is typically between two to five nanograms of THC per milliliter of blood—were not statistically more likely to be involved in an accident compared to people who haven’t used marijuana. But other studies have indicated possible increases in highway road deaths following adult-use legalization.

Yet other research has suggested that impaired driving actually goes down after legalization, at least according to self-reported surveys. A report published last year by the research nonprofit RTI International found that people were less likely to drive within three hours of consuming cannabis in states where some form of cannabis was legal. Other research found that auto-insurance premiums decreased in states that legalized medical marijuana.

Experts and advocates have emphasized that evidence isn’t clear on the relationship between THC concentrations in blood and impairment.

The Congressional Research Service in 2019 determined that while “marijuana consumption can affect a person’s response times and motor performance…studies of the impact of marijuana consumption on a driver’s risk of being involved in a crash have produced conflicting results, with some studies finding little or no increased risk of a crash from marijuana usage.”

Another study found that smoking CBD-rich marijuana had “no significant impact” on driving ability, despite the fact that all study participants exceeded the per se limit for THC in their blood.

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