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.

Feds question tests

NHTSA last month acknowledged these gray areas in a report to Congress that not only called into question the reliability of tests to find THC but also noted the problem with determining whether a driver is too stoned to be behind the wheel.


In fact, the NHTSA report notes that even though "research has demonstrated the potential of marijuana to impair driving-related skills," it lays out a number of other studies that show pot might not be as bad as the better-understood effects of alcohol on driving.

"Many studies, using a variety of methods, have attempted to estimate the risk of driving after use of marijuana," the NHTSA report noted. "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."

Finding THC isn't so difficult. Making any kind of universally accepted determination from the results, however, seems to freeze the legal world in its tracks.

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

Some widely cited studies have offered differing information about the impact of marijuana on driving. And not all sides agree which studies are right and which are not.

In 2012, a medical study published in Clinical Chemistry journal found "cannabis smoking increases lane weaving and impaired cognitive functions," and that certain THC concentration levels "are associated with substantial driving impairment, particularly in occasional smokers."

Then came a University of Colorado study — released in 2014, the year recreational sales of the drug were launched in the state – that found the proportion of drivers involved in