Everything You Need to Know About Marijuana Blood Tests


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Over the past few years, quite a few states have legalized marijuana for medical and recreational use. With this legalization, police officers needed a way to test drivers who appeared to be too stoned to drive.

This, in turn, has made the marijuana blood test the go-to way to check for impairment during a traffic stop.

Roadside tests and demonstrated impairment still play a role and assessment by a Drug Recognition Expert is now common. But most police departments attempt to verify these results with a blood test. If the allegedly impaired driver has more than the state’s per se limit (often 1 to 5 ng/mL of THC), a DUI conviction usually becomes much more likely.

However, according to a study by the AAA Foundation for Traffic Safety, this type of legal limit is “arbitrary and unsupported by science.” For now, though, it is the law in quite a few states.

How does THC blood testing work?

THC blood testing is an attempt to make prosecuting marijuana-impaired driving as clear-cut as drunk driving. The police pull over a driver for a traffic offense, observe signs of intoxication or impairment, conduct some field sobriety tests and ask them to submit a blood sample. Just as in a drunk driving arrest, the driver can refuse the blood test. However, this leads to an automatic administrative license revocation and harsh penalties.

When the driver agrees to the blood draw, a nurse, paramedic or other trained person draws the sample. If that sample shows more than the per se limit (or any amount of THC in zero-tolerance states), the police have a strong case against the driver.

This type of conviction leads to similar penalties as a traditional DUI, including license suspension, fines and the possibility of jail time.

Why are marijuana blood tests questioned by many users and even experts?

A May 2016 AAA Foundation study found it takes an average of 165 minutes from first contact with police until blood is drawn for a test. The study also shows THC blood levels dropped an average of 5 ng/mL during the first two hours after use in study participants.

However, unlike alcohol, the metabolism of THC is not predictable, and therefore, there is no true way to predict what someone’s blood level was two hours and 45 minutes prior.

At the same time, five nanograms of active THC is an incredibly low threshold for a regular user, and many may maintain this level in their blood no matter when they last smoked, vaped or otherwise consumed marijuana. This does not affect their ability to drive many hours after their last use, and they demonstrate no problems with the field tests.

Still, if the police test their blood, they can face serious consequences.

For now, the court system believes these marijuana blood tests are the most accurate and trusted method for police to prove impairment, and they rely on them heavily to prosecute allegedly impaired drivers.

Has the per se limit changed cannabis-impaired driving statistics?

Yes. Since states began passing per se limits on THC, a number of media outlets have reported a spike in cannabis-impaired driving and fatality accidents involving marijuana use.

Take Washington State, for example. According to NPR, the Washington State toxicology lab saw a drastic increase in how many blood samples tested positive for marijuana in the first three years after legalization—from 19 percent to 33 percent.

However, there is an even larger jump when you look at only active THC, which has not yet been in the system long enough for metabolization. Before legalization, there was active THC in about half of all blood samples tested for marijuana after a fatality accident. In 2015, the Washington Traffic Safety Commission told NPR they found active THC is about 85 percent of all tested samples after a deadly accident.

Clearly, these statistics show something changed. Whether that is because more people are using marijuana before driving is not clear, though. Many police departments have trained more officers to better identify signs of drug use or impairment, as well. The increase could simply be because police are better at deciding which samples require testing.

What should I do if I fail a marijuana blood test in a state with a per se limit?

Your best option to minimize the effects a failed marijuana blood test could have on your life is to call a knowledgeable marijuana DUI attorney in your state as soon as possible after your arrest.

There are a number of ways you can fight this type of charge, and a lawyer can look at your individual situation and determine the best way to build a strong defense. Just make sure you choose an attorney familiar with your state’s per se impairment limit, and the issues surrounding marijuana blood tests.

Barbara Bowden is an attorney and owner of The Law Offices of Barbara A.Bowden.

RELATED: Drug Testing 101
You can keep up with all of HIGH TIMES’ marijuana news right here.

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