Police Still In Need Of A Reliable DUI Test For Marijuana

By Kelly Burch 08/02/17

Many police officers rely on judgment to evaluate whether a driver is under the influence of pot, which is not always foolproof.

police officer pulling over a motorist.

In an RV in Colorado, Sharica Clark was getting high for the most unusual reason. 

"We're going to willfully smell like pot around a bunch of cops,” Clark told NPR. It was all in the name of helping the officers, however. Clark and three others had volunteered to get high and let Colorado state patrol officers decide whether they were too impaired to operate a vehicle. 

“The whole point of this class is to get the officers to make correct decisions," said Chris Halsor, an attorney who was running the training session. Because there is no way to objectively measure impairment from marijuana, officers need "confidence that they're making the right arrest decision and confidence that they're letting people go who really aren't impaired."

As marijuana use becomes legal in more of the country scientists and authorities are discovering just how complicated it is to develop a reliable test to evaluate a driver’s level of impairment from using marijuana. 

Now, many police officers rely on their own judgment to evaluate whether a driver is under the influence of marijuana. But scientists say that is not always accurate. 

"We like to know the human error and the limitations of the human opinion,” Tara Lovestead, a chemical engineer at the National Institute of Standards and Technology in Boulder, Colo., who is working on setting standards for what a marijuana detection test might require.

While a breathalyzer gives an accurate depiction of alcohol impairment, a breath test for marijuana is much more complicated and would have to screen for a variety of chemicals, not just marijuana’s main active ingredient, THC. 

"One thing to look for would be metabolites — something that comes out of the breath that shows it actually went through your system," Lovestead said. 

For now the most accurate way to determine intoxication level is through blood testing. 

States like Colorado have established limits: If a person has more than 5 nanograms of delta-9-THC per milliliter of blood, a court or jury can infer that they are impaired, according to Colorado law. Although the legal system has decided that level indicates impairment, Lovestead says that there is no scientific evidence to back that up. 

"We just don't know whether or not that means they're still intoxicated, or impaired or not," she said. "There's no quantitative measure that could stand up in a court of law.”

That’s because research shows that blood tests are fraught with complications. 

THC, the chemical in marijuana, gets dissolved in fat it can last much longer in someone's body than alcohol, which is dissolved in water. To understand just how long, one group of researchers kept heavy marijuana users in a facility for a month. The study participants didn’t have any access to marijuana or any other drugs, but some users still screened positive for THC when the month was up. 

"And it shocked everyone, including ourselves, that we could measure, in some of these individuals, THC in the blood for 30 days," says Marilyn Huestis, a toxicologist with the University of Maryland School of Medicine who recently retired from leading a lab at the National Institute on Drug Abuse.

Researchers realized that the marijuana users’ bodies had built up stores of THC, which slowly dispersed. Some users measure over Colorado’s impairment point for days after they stopped using.

Another study showed that it’s easier to detect marijuana in people who use the drug regularly, but that people who rarely used the drug could smoke immediately before a blood test and not show evidence of marijuana use.

In the meantime, officers like those in the training program are left making judgment calls, which they may disagree on. 

"It's too subjective," Lovestead said.

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Kelly Burch writes about addiction and mental health issues, particularly as they affect families. Follow her on TwitterFacebook, and LinkedIn.