Are You High? US Lawmakers, Police, Prosecutors Grapple With How to Define Marijuana Impairment

Article by David Migoya, The Denver Post

Are you high? The science of testing for marijuana impairment is hazy, and evolving Lawmakers, police, prosecutors grapple with how to define marijuana impairment. District 3 police officer Michael Davis, left, talks with Corporal David Trujillo Denver Police Traffic Operations Bureau DUI/DRE as he begins an investigation of a one car accident where the driver is suspected to be under the influence at the Denver Health Medical Center

There was a time when marijuana was illegal everywhere and testing for it was as easy as could be.

It didn’t matter the level of cannabinoids found in a person’s body. If it was there, they were breaking the law.

It’s different now.

The tests have changed from depositing a urine sample into a cup to drawing blood or offering oral fluids. Also different is the particular type of cannabinoid — the chemical compound that reacts in the brain — detected by any of those tests.

The evolving science of testing for marijuana, and the lack of consensus over how to measure impairment, is a defining feature of the drug. It separates marijuana from alcohol and creates challenges for lawmakers, police and prosecutors, not to mention users.

The issue is critical as the state moves forward in determining how to handle driving under the influence of pot. A Denver Post investigation found that the numbers of drivers in fatal crashes testing positive for marijuana — though not necessarily high — is rising sharply, and coroners are finding higher levels of potency in their tests.

The cannabinoid most widely tested for in the past – known as carboxy THC – is actually an inactive metabolite that only indicates prior marijuana use, sometimes as long as a month ago. In time, other metabolites of THC — short for tetrahydrocannabinal, the psychoactive ingredient in marijuana — were found to be better indicators of recent use and, some say, impairment.

“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,” said Sarah Urfer, president and owner of ChemaTox, a Boulder lab that handles DUI screening for about three quarters of the law enforcement agencies in Colorado. “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.”

But THC is not the same as alcohol. It reacts differently in the body, it metabolizes differently and its impairing impact is different. Unlike the 0.08 blood-alcohol level that’s widely accepted as indicative of drunken driving, establishing a credible level for THC has been elusive.

It is generally accepted that two standard drinks — about 1/2 ounce of alcohol —  in an hour will raise someone’s blood-alcohol level to 0.05, approaching the legal limit. One drink is a 12-ounce beer, a 1.5-ounce shot of distilled spirit, or a 6-ounce glass of wine.

For pot, the differences are striking since it depends on the manner ingested — smoked, edible, concentrate — and how much. A Johns Hopkins University study from 1995 found that four puffs of smokable marijuana with 1.75 percent THC content translates to 57 nanograms per milliliter, and 10 puffs as much as 99 ng/mL. The National Highway Traffic Safety Administration says levels of 100-200 ng/mL are “routinely encountered” after smoking but quickly dissipate.

Concentrate levels for vapor ingestion are typically higher, as well as for edibles, although the rate of distribution into the blood varies considerably.

Read full article here.

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