Why It's Impossible to Test for Impairment by THC in Blood, Urine, or Saliva

Why It's Impossible to Test for Impairment by THC in Blood, Urine, or Saliva

Although the majority of patients and adults in legal cannabis states have no employment protections for off-the-job use, a small handful of states and localities have proposed or instituted some form of protections. At the heart of the issue is the inability of the approximately $7 billion drug testing industry to develop a test that can accurately measure impairment by THC in blood, urine, or saliva.

With alcohol as a frame of reference, it may seem like a foregone conclusion that THC impairment can be measured through a human sample. Because of how cannabis can linger in the body for up to a couple weeks, however, off-the-job use of cannabis could produce a dirty test and result in loss of one’s job if it is a condition of employment, even for medical users and in states where adult use is legal. Opponents to protections suggest they would support them when THC impairment tests become available.

But are such tests even possible? Scientists, doctors, and researchers familiar with the issue are suggesting it really isn’t possible to test impairment by THC in the body, nor are drug screens necessarily an adequate measure of impairment for anything. They are pushing for impairment testing that they say best promotes safety and personal responsibility through measuring reaction times, balance, and other true indicators of actual impairment. They suggest THC testing is not and will never be an accurate way to measure someone’s impairment from any substance.

Testing THC

Not only do doctors, researchers, and scientists not support the concept of measuring impairment through THC content, some go even further to suggest it is discriminatory and rooted in protection of the lucrative drug testing industry.

“[Cannabis impairment testing] has no validity in the workplace, it is a total waste of resources, and discriminatory to continue using it there,” said David G. Ostrow, MD, PhD, LFAPA.

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Dr. David Bearman, a physician, historian, substance abuse and cannabis expert, added that a person can have significant levels of THC in their system, pass a drug test, and not be considered impaired if they are using Marinol, a Schedule III FDA-approved medicine made of isolated and synthesized THC.

“Random drug testing is not cost effective, is bad for morale and rarely, if ever, detects the most common drug of abuse: alcohol,” said Bearman. “The mere presence of THC or its metabolites does not tell if the employee is impaired… trying to control what your employees do on the weekends is so 19th century.”

All physicians approached for comment for this article agreed THC content in human samples is in an incorrect approach. Dr. Jahan Marcu agrees. Marcu has studied cannabis, the industry, and worked to educate legislators on the science of cannabis use for over 15 years, and recently founded the International Research Center on Cannabis and Mental Health.

“[Testing for THC] is the wrong approach,” Marcu said. “There is no similar test for any other drug except alcohol… You can’t do a breathalyzer or blood test to determine cannabis impairment much like you can’t do it for muscle relaxants, opiates, fentanyl, or any other drug,” Marcu said. We don’t have to spend a bunch of money to invent something that is not going to work because how do we implement that and how do we train law enforcement officials when it simply won’t work?”

Marcu suggests that in certain higher-risk industries, like those where employees are operating heavy machinery, there absolutely should already be effective impairment testing to prevent accidents and injuries.

“If the rule is you can’t be impaired or intoxicated at work while doing this job, then you need to use a test that catches all impairment or intoxication,” he said. “The established impairment tests currently being used by law enforcement and at work sites are effective at determining impairment. They don’t need to create something new. There are things that are already working in their state that they can use to determine impairment. The drug testing industry can still make their money. If there is an accident or issue, impairment tests are issued followed by drug screening.”

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Russ Phifer is the executive director of the National Registry of Certified Chemists. He and Dr. Marcu have been “researching this topic separately and together for about four years now” and have both reached this same conclusion.

“Standard Field Sobriety Testing (SFST) has been utilized for many years to determine driving impairment for alcohol; why should cannabis be any different, if the issue is impairment, not blood or urine concentration of a metabolite? As any toxicologist can tell you, the dose response relationship, and, hence, impairment level, varies considerably from individual to individual, based on factors such as general health, stress level, frequency of use, genetics, body weight, and other factors. Any effort to determine impairment using analytical methods, whether a breathalyzer, swipe test, or a blood or urine test, cannot possibly determine or confirm impairment at the time of concern,” Phifer said. “Further, when the component tests of the SFST battery are combined, conclusions are accurate in 91 percent of cases, overall, and in 94 percent of cases if explanations for some of the false positives are accepted. Drug tests generally produce false-positive results in 5 percent to 10 percent of cases and false negatives in 10 to 15 percent of cases.”

He adds that analytic methods, at best, “are a poor way to determine impairment,” and points to a 2010 Princeton study by the National Workrights institute that concluded that “the available information indicates that impairment testing is not just a better answer on paper, but in practice, as well. Employers who have used impairment testing consistently found that it reduced accidents and was accepted by employees. Moreover, these employers consistently found that it was superior to urine testing in achieving both of these objectives."


Dr. Sunil Aggarwal, is a well-known cannabis research and physician in Washington who has also studied this issue extensively. He suggests a new cell-phone app-enabled system called DRUID could be a better measure of impairment because it is a “validated application that can be run on a mobile device that can measure one’s reaction time, speed, coordination, balance, and other psychomotor variables and compare it to an individual’s baseline value.”

DRUID, or “DRiving Under the Influence of Drugs,” was created and designed by Dr. Michael Milburn, a now-retired professor of psychology at University of Massachusetts at Boston. He earned his PhD at Harvard before teaching psychology at UMass for 40 years, where he specialized in research methods, measurement, and statistics.

“The coming worldwide legalization of cannabis is sort of highlighting the issue that we need a way to measure impairment, and actually what I am calling for is a paradigm shift in how we think about impairment and testing and law enforcement and workplace safety and stuff like that. We need to really focus on behavior,” said Milburn. “DRUID determines impairment from any source through eye coordination, balance, and reaction time.”

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Individuals can download the app for both Apple and Android phones. User start by setting their baseline: taking the reaction time tests and standing on one foot at a time completely sober. Users then use the app before they get in the car or perform a task by running through the exact same tests, which are similar to the standard field sobriety tests already being used by law enforcement, except potentially more accurate because the measurement is calibrated to the individual. DRUID will issue a score, and if it determines the user is impaired, it warns them that they should not drive or operate heavy machinery.

Milburn had the idea to develop DRUID during the 2016 legalization campaign in Massachusetts. He was reading an op-ed in the Boston Globe authored by the governor, attorney general, and mayor of Boston urging voters not to approve legalization for a variety of reasons, specifically highlighting the issue of cannabis impairment on the roads and in the workplace.

“It was just really, I thought, dishonest,” he said. “I thought, ‘I bet I could figure out how to do that.’”

Milburn has stated in interviews that he didn’t develop DRUID so law enforcement could arrest people, but so that users could test themselves before deciding to get behind the wheel or go to work. When asked why that was an important approach, he responded, “Why was trying to promote responsibility of cannabis use important?”

“If my app persuades impaired people not to drive and it saves their lives, the lives of people in the car with them and those on the road around them, then this project is really tapping into something important emotionally for me...Promoting responsibility is about saving lives, and that is really what it has been about for me,” Milburn said.

Milburn is blunt in his desire to end drug testing as a form of employment discrimination for cannabis users.

“DRUID can disrupt the drug testing industry. Drug testing for cannabis doesn’t measure impairment...Using a workplace safety version of DRUID could really disrupt the drug testing industry and actually promote safety,” Milburn said. “Studies have shown that random drug testing doesn’t necessarily promote safety or reduce drug use, people figure a way around it; fake urine and so on... It’s a $7 billion-a-year industry. There is so much money involved with that, which is appalling giving the lack of accuracy and efficacy for that industry.”