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How to Detect if You’re Driving Under the Influence of Marijuana?

By Craig Guillot

smoking_potAs more states move to legalize recreational marijuana, regulators and authorities around the nation are grappling with how to better measure if motorists are too high to drive.

Research from the University of Iowa shows means of testing for marijuana impairment could potentially let impaired drivers off the hook while penalizing others who haven’t consumed marijuana in weeks.

Twenty-nine states and the D.C. allow marijuana to be used legally in one form or another. Recreational marijuana use is legal in California, Massachusetts, Maine and Nevada.

Alabama and Mississippi both have laws permitting marijuana for medical use, but only for severe epileptic conditions.

States with legalized marijuana have recently a rise in marijuana-impaired driving. Information from the Colorado State Patrol revealed that the DUI/D (Driving Under the Influence of Drugs) citations related to marijuana increased from 12.2 percent in 2014 to 14.5 percent in 2015.

A CDOT survey found that 21 percent of respondents who used marijuana in the past year had driven a vehicle after consuming it within the past month. Those who drove within two hours after using marijuana did so 17 times per month on average. The survey noted that many drivers aren’t fully aware of the consequences of driving high.

“Our research indicates that unfortunately a lot of marijuana users are driving high and many believe they will not get a DUI for doing so,” says Amy Ford DCOT spokesperson.

Data on the topic, however, is still sparse.

What’s too high to drive?

Authorities are cracking down but there isn’t yet a scientifically-based national consensus on how to determine if a person is “too high to drive.” In Colorado and Washington, a person can be charged with DUI if a blood test reveals THC concentrations of 5ng/ml or higher. In Oregon, authorities can use a urine test and the officer’s observation to determine if the driver is impaired.

Dan Estes, impaired driving manager for the Oregon Department of Transportation, says the testing “isn’t perfect” because a urine test only shows the person consumed marijuana in the past and doesn’t prove that they’re currently under the influence. According to a report by the National Drug Court Institute, carboxy-THC can be detected in urine for up to 30 days.

Timothy Brown, associate research scientist at the University of Iowa’s National Advanced Driving Simulator, co-authored a study on the use of marijuana impairment on driving. He says THC is processed differently than alcohol and can stay in the body for much longer periods of time. This means blood, urine, and even hair tests don’t produce accurate results to determine impairment.

“If someone smoked a joint 8 hours ago and still has residual THC in their system, is that still having an effect and still crossing the blood/brain barrier? We really don’t know and it’s difficult to say when that effect in the brain is present,” Brown says.

Brown says what they do know is that THC concentrations spike high in the blood immediately after dosing. With alcohol, Brown says effects in the brain are “very well tied to” blood alcohol content that can be easily measured with years of research indicating that a .08 blood alcohol content makes a person too impaired to drive.

Brown says a chronic user of marijuana could have a concentration of “THC that’s always going to be there” even if they are not under the influence at the moment. He says the presence of THC does not prove impairment nor is there a scientific consensus on a level of THC that would indicate a person is too high to drive.

“It’s really a challenge. There are many additional questions you need to consider when looking at chronic users because there could always be [that level of THC] in the blood,” he says.

Paul Armentano, deputy director of the National Organization for the Reform of Marijuana Laws, says law enforcement should focus on testing performance measures “rather than relying on the detection of compounds that are not consistently associated with behavioral impairment.”

Armentano advocates things like MyCanary, a mobile phone based human performance app which tests memory, balance, reaction time, divided attention, and time perception.

Will police implement marijuana testing?

Brown says for a “complicated drug” like THC oral fluid samples can be a more precise and accurate way to test. While there is no such testing method on the market, such products are in development. Kathy Stitzlein, a biomedical engineering PhD student at the University of Akron, is a co-developer of Cannibuster, a microfluid device that uses noninvasive saliva testing to detect THC levels in a matter of minutes.

Stitzlein says they are currently testing of an alpha prototype and hope to soon start beta testing a device in the field.

“We’re looking at various, non-invasive ways to test bodily fluids at the roadside and are currently testing with saliva which can offer more current values,” says Stitzlein.

British Columbia-based Cannibax Technologies is also developing a THC breathalyzer that is similar to an alcohol breathalyzer and could detect impairment in minutes. Cannibax says the system would detect recent marijuana use within a two hour time period at time of testing.

Editor’s Note: This is an updated version of an article published on April 11, 2016.