Why marijuana blood tests fall short of the alcohol standard in determining intoxication

CLEVELAND, Ohio — When it comes to enforcing drunken driving laws, police have a battery of tools at their disposal. Field sobriety tests, along with breath, blood, and urine tests for blood alcohol concentrations are well established indicators of a driver’s degree of intoxication.

However, in the case of marijuana, the drug’s unique metabolism makes the question of intoxication – on the road or in the workplace – more complicated.

That’s because tests for drug levels in the blood only tell us whether someone used marijuana in the last several days or even weeks, not whether they are under the influence at the time.

Earlier marijuana use still causes positive blood tests because the psychoactive component of cannabis – tetrahydrocannabinol or THC – sticks around the body in fat cells and is released back into circulation long after the intoxicating effects on the brain have gone away.

The limitations of drug testing have come up as Ohioans vote on a Nov. 7 ballot proposal – Issue 2 – that would legalize recreational marijuana use in Ohio.

Here’s what we know.

What a drug test for cannabis does and doesn’t measure

Tests for cannabis don’t actually measure THC, the component of cannabis responsible for the feeling of being “high.” Shortly after THC reaches the bloodstream it is rapidly taken up in the fat cells of the brain and body, and little remains in circulation to measure.

Instead, as a proxy for THC, a urine test for cannabis measures a product of its metabolism – THC-COOH.

As the THC bound up in fat trickles back into the blood, it passes through the liver, where it is first modified into OH-THC, which is psychoactive. Then it is modified into THC-COOH, which is not psychoactive. These metabolites are returned to circulation, then filtered by the kidneys into the urine.

THC-COOH is the primary chemical found in the urine when THC is metabolized and, as a result, the presence of this chemical in the urine indicates that cannabis has been ingested.

But while THC-COOH tells us that a person is a cannabis user, it doesn’t provide any information about when, how, or how much was consumed, or if the person is currently intoxicated.

There are a number of factors, including a person’s weight, age, gender, overall health and bodyfat composition – along with the frequency and amount of cannabis they use. These determine how long it takes for the THC to be fully metabolized and leave the body.

There is no simple formula for how long a person will test positive for cannabis, or a threshold above which THC-COOH can be used to determine intoxication.

Instead, a combination of other indicators of intoxication and a subsequent positive test for cannabis are typically used as evidence of cannabis intoxication.

The legal implications of testing

Regardless of whether cannabis becomes legal for recreational use, testing positive for marijuana can still have life-altering implications, making it important to understand how long a person might test positive after use, and whether they might be considered impaired.

Ohio law does not prohibit employers from hiring, disciplining, or terminating the employment of persons using medical marijuana. This means that employers can still enforce drug-free workplace policies that effectively prohibit employees from consuming marijuana, recreationally or medically, and a positive test, regardless of impairment could get you fired.

In addition, an employee who sustains injury in the workplace due to marijuana intoxication will not receive employee compensation.

And finally, operating a vehicle while under the influence of drugs or alcohol – including marijuana will remain as it has always been: against the law. If an officer determines that you are impaired while driving, and you later test positive for alcohol, cannabis, or any other drug, this could increase the chances that you could face jail time, fines, or have your license suspended or revoked.

How long will I test positive? How long am I ‘high’?

The length of time a person tests positive after cannabis use varies widely.

Frequent use increases the amount of THC stored in the fat tissue of the body, causing an individual to test positive longer. Higher bodyfat content can also increase the length of time a test will be returned as positive. Older persons will test positive longer than younger persons, and sedentary people will test positive longer than athletes.

The general rule is that an individual will test positive for several days to a week after single cannabis use. However, frequent users could test positive for three weeks to a month following their last use.

That said, a positive test, as stated earlier, is not the same as intoxication. The length of time a person is intoxicated from marijuana use depends a lot on how much they consumed and how they consumed it.

Studies have shown that most of all of the psychoactive effects have dissipated four hours after smoking or vaping, and 12 hours after consuming edible cannabis.

The difference in the high experienced after cannabis taken as an edible vs. cannabis that was inhaled through smoking or vaping, are due to the fact that inhaled drugs enter circulation almost immediately via the lungs and arrive in the brain shortly after.

By contrast, drugs that are eaten first take time to move through the digestive tract, and then are metabolized by the liver before they ever make it into the blood.

For this reason, edibles require higher doses of THC because most of it is eliminated in the body’s waste. And OH-THC, the psychoactive metabolite produced by the liver, plays a more prominent role in the effects.

Implications for driving

While a change in the Ohio law would mean officers would no longer make arrests for possession of cannabis, drivers who are pulled over and suspected of being under the influence of alcohol or drugs will be evaluated the same as they are currently, said Lt. Nathan Dennis, spokesman for the Ohio State Highway Patrol.

“Regardless of whether marijuana becomes legal or not in Ohio, we’ve been arresting people impaired on marijuana for years; so it’s not really going to change anything in that aspect for us,” said Dennis.

“The roadside testing that is done is the same whether we are looking at potential drug use, potential alcohol use, or both.”

What that means for Ohioans is that drivers who are stopped and suspected of operating a vehicle under the influence of drugs or alcohol will first be checked for a variety of indicators of drug or alcohol use. This includes changes in the eyes like dilated pupils, redness, or altered eye movements, and given a field sobriety test.

But drivers who test positive for marijuana won’t necessarily be charged with crime, said Dennis, because officers recognize that not everyone who tests positive is impaired.

“It’s gonna stick around in your system for a while, and that’s where for us it comes back to field sobriety testing,” said Dennis. “Is that person currently impaired? Is that drug currently psychoactive in their system? That’s what we’re looking for.”

Although Dennis says he’s confident that the combination of field sobriety and laboratory testing they do works to identify and arrest someone who is driving while impaired on cannabis or other drugs, not everyone agrees.

Several recent research papers have found that field sobriety testing does not always reliably identify cannabis intoxication. And that means that cannabis users may need to take extra precautions.

In short, that could mean avoiding getting behind the wheel.

The take home message, said Dennis, is that if you’re going to use marijuana and there’s any chance you are impaired, don’t drive, because “it’s not going to end well.”