How Do You Test for Marijuana Impairment?

How Do You Test

Colorado legalized marijuana and those over 21 years of age may purchase it from retailers. In this segment, we discuss how the testing is handled to determine driver impairment. As with alcohol, Colorado has a measurable level (limit) in place, which is five nanograms of tetrahydrocannabinol (THC)—per milliliter of whole blood. In 2016, approximately 30% of traffic fatalities involved an impaired driver, which translated into 196 deaths. Roughly 17% of those arrested for operating a vehicle under the influence involved marijuana. Surprisingly, 55% of marijuana users do not think marijuana affects motorist safety. Colorado law enforcement is actively adding drug recognition experts (DRE) to enhance detection.

Challenges in Testing

Traditional testing for marijuana, such as for employment, has been through urine samples; however, when doing so to enforce impaired driving, there is more complexity. Research and development is rapidly underway, as a need exists to find a solution that is accurate, affordable and efficient. In contrast to alcohol, marijuana lacks consensus on what defines impairment and is being debated. The Denver Post found that drivers fatally injured in auto accidents are increasingly testing positive for marijuana usage.


Tetrahydrocannabinal (THC) serves as an indicator of marijuana usage. Sarah Urfer, of the ChemaTox lab in Boulder, says her organization contracts with many Colorado agencies for marijuana screening. She explained that THC is different than alcohol in how it acts in the body and it is challenging to determine current impairment. Unlike the .08 blood alcohol level threshold for driving under the influence of alcohol, the measurement of THC has been debated for determining marijuana impairment.

Reliability in Determining Impairment

The National Highway Traffic Safety Administration (NHTSA) says levels detected in the 100-200ng/mL range are common shortly after usage, yet falls quickly. Levels of concentration seem heightened when users ingest through vaporizers or edibles. The NHTSA report to Congress suggested the reliability of current testing methods is mediocre in truly measuring and determining whether a driver is under the influence. Sarah Urfer says blood testing is fairly accurate; however, current roadside testing is unpredictable. John Jackson, Police Chief in Greenwood Village, says testing is still inadequate and reminds us that DUI alcohol testing took over 25 years to develop. Measuring levels of THC in the blood provides a likelihood of impairment, yet is subject to a judge or jury’s decision. Accurate methods of field testing for marijuana impairment are still developing.

Roadside Testing Solution Needed

Kristi Kelly of Colorado’s Marijuana Industry Group agrees that an accurate means of determining impairment is needed. NHTSA used the word “artificial” in describing the current THC tests for impairment. The drug’s “high” is based on the concentration in the brain, rather than levels in the blood. THC blood levels are significantly reduced when the test is finally administered, as these levels peak immediately following usage. A saliva swab method is being tested by the State Patrol, which could provide an “on the scene” solution.

CDOT Initiative

The Colorado Department of Transportation had partnered with Lyft and the Marijuana Industry Group for a “320” promotion offering discounted rides as an alternative to “high driving.” CDOT reminds Colorado drivers that marijuana adversely affects your scope of vision, response time, and depth perception during vehicle operation. They encourage cannabis users to abide by the law and make plans for a sober ride ahead of time.

With Colorado’s marijuana legalization, drivers may have a misconception that getting behind the wheel while under the influence is not a crime or a safety concern. Independent scientific studies show that using marijuana and driving poses a safety risk similar to drunk or distracted driving. A clear challenge exists in developing a solution for field testing (roadway conducted) that is consistent and less invasive than drawing blood. The state’s current threshold for defining impairment is five nanograms of THC—per milliliter of whole blood; however, critics insist this is not a truly accurate means of determining if an individual is currently impaired. One certainty is that drivers should act responsibly to prevent injuries, fatalities and property damage by not driving while impaired.

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