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The Science Behind Marijuana Impairment Testing Is Hazy, But Evolving


Back when marijuana was illegal everywhere, testing for stoned drivers was much less complicated. Basically, if cannabinoids of any kind were found in the blood, it was grounds for a DUI. But the testing process has changed along with the types of cannabinoids now recognized by local authorities.

Science is evolving in the world of marijuana testing to help determine if someone is impaired or not, The Denver Post reports. Finding marijuana in people’s system after car crashes is now more common than years prior. Higher levels of THC are also being found in coroners’ testing.

Finding carboxy THC in a person’s blood only indicates that someone has used marijuana since it is an inactive metabolite. Looking for just tetrahydrocannabinol (THC) indicates recent use, and some experts say that it can indicate impairment.

Sarah Urfer of ChemaTox said, “Urine testing was established many years ago, and, at the time, a test was developed to look for carboxy THC since it’s what’s there in the highest amount. Nobody thought it mattered what you were looking for….Early on, scientists didn’t know for sure which of the cannabinoids were responsible for impairment. They’d measure carboxy and try to correlate it to impairment.”

This made the testing process similar to testing for alcohol, but THC is very different from alcohol and doesn’t react the same way in the body. It’s metabolized differently. Determining an acceptable level of THC intoxication has proven to be difficult as every person has a different threshold and isn’t using the same amount.

In reference to alcohol, two drinks within an hour should show a blood alcohol level around 0.05. One drink refers to a 12-ounce beer, a shot of distilled spirit measuring 1.5-ounces or a 6-oz glass of wine. When it comes to marijuana, it’s different – especially given that there are different ingestion methods.

Information from a Johns Hopkins University study from 1995 suggested that 1.75-percent THC translates to 57-nanograms-per-milliliter. Using 10-puffs of marijuana was compared to being 99ng/mL. According to the National Highway Traffic Safety Administration (NHTSA), levels showing 100 – 200 ng/mL are “routinely encountered” after recent smoking, but these levels dissipate quickly.

The NHTSA wrote Congress regarding its concerns. They’re questioning how reliable the available tests are, but there’s also an issue in determining if someone is too stoned to drive. They’re confused due to conflicting reports with some saying that marijuana use doesn’t impair driving as much as alcohol and others say it does.

NHTSA said, in a recent report, that, “Many studies, including a variety of methods, have attempted to estimate the risk of driving after use of marijuana. While useful in identifying how marijuana affects the performance of driving tasks, experimental and observational studies do not lend themselves to predicting real-world crash risk.”

Urfer said, “Testing for THC in whole blood isn’t actually that hard. Where the issue comes in is with interpretation and roadside testing.”

The number of conflicting reports are making it difficult for a scientifically effective testing process to be created for testing of potentially stoned drivers. More concerning to law enforcement is when alcohol and marijuana are both present, as they suggest the effects of both are heightened when used at the same time. For example, in 2016 in Colorado, 36-percent of drivers involved in a fatal crash tested positive for both alcohol and marijuana. Colorado has determined that 5ng/mL is enough to authorize being charged with DUI.

So, for now, regardless if you are actually too stoned to drive or not, a positive Delta-9 THC result and your location, may be considered a DUI. Arizona and Utah have zero tolerance policies, so any amount of THC in your blood will get you a conviction. Colorado lets a judge or jury decide and Washington and Montana abide by the 5ng/mL suggestion for determining marijuana intoxication.

Typically speaking, if someone has used marijuana, the effects should be completely worn off in between 1 and 4 hours depending on how much was used and which type of product was used.

Urfer said, “The public is misinterpreting the statement that you can’t tell if someone’s high because of THC level. You can’t directly correlate a number to impairment. The blood level for THC does not represent the same as alcohol does.”

THC’s effects come on virtually immediately in all methods of use except edibles (unless it is a sublingual type of edible like lozenges or hard candies), and typically start to decline within 30 minutes and, for some, the effects are completely gone in those 30 minutes.

Urfer said, “The level in the blood is dropping, but the level in the brain is not. The high is caused by the level in the brain, not in the blood. And no one has published a study that says it’s safe to drive high.”