Are you high? The science of testing for marijuana impairment is hazy, and evolving – The Denver Post

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

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