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January 16, 2019 in General Posted by |

Driving High: Why Marijuana Testing is Unreliable

The current state of chemical testing used to determine whether a driver has used marijuana and is impaired is not reliable for the following reasons:

– The test does not indicate when marijuana was consumed;
– The test does not indicate the quantity of marijuana consumed; and
– There is presently no consensus on how much marijuana leads to impaired driving. 

The primary psychoactive ingredient in marijuana is THC (delta-9-tetrahydrocannabinol). This is what makes users feel “stoned” or “high”. It can also produce alterations in motor behavior, perception, cognition, and memory.

The smoking of marijuana produces a very rapid high with the most intense blood concentration of THC occurring very soon upon it being smoked. Typical marijuana smokers experience a high that lasts approximately two hours. After three to five hours, most behavioral and physiological effects return to their normal levels.

Edible consumption of marijuana produces a more delayed and lower peak THC level. 

Types of Testing

Blood Tests

Blood tests can produce false positives because THC is fat-soluble.  This means that some of it gets stored in the body’s fatty tissues.  As a result, it can be detected by chemical tests used in DUI marijuana investigations well after a suspect is no longer high.

With occasional marijuana use, THC is detectable in the blood for up to 12 hours after consumption.  However, with chronic marijuana use, THC can show up in the blood for up to two days.

As a result, false positives for marijuana impairment in DUI marijuana blood tests are common, specifically when people are no longer high.

Urine Tests

Urine tests for driving under the influence of marijuana cases are even less reliable than blood tests.  They do not directly detect for the presence of THC, but instead for inactive metabolites such as THC carboxylic acid (THC-COOH). 

These inactive metabolites can be detected in urine long after marijuana has ceased to have any impairing effects.  Research shows that in some instances, they can be detected up to four weeks in chronic users. 
Because inactive metabolites do not themselves cause impairment, a positive urine test does not indicate that one was driving “under the influence” of marijuana.  It merely indicates that cannabis has been consumed at some point in the recent past. 

Newer strains of Cannabidiol (CBD) may be particularly susceptible to false positives. CBD is a non-psychoactive substance in marijuana and is often used medicinally by consumers who do not want to feel high but are using the substance for issues such as pain or inflammation.

Saliva Tests

Studies have shown that saliva sampling is quick, non-invasive, and can pinpoint the latest use to a matter of hours.
Some California counties – including Los Angeles, Bakersfield, Sacramento and Fullerton – have begun using these oral swabs on a test basis. At present, an oral swab is a voluntary pre-arrest test: like a road-side Breathalyzer or a field sobriety test.

This may soon change, as the Los Angeles City Attorney’s Office has said that it is hoping to test this methods’ legality as proof of DUI marijuana in California cases.

This is why prosecutors in DUI marijuana cases cannot rely on chemical tests as “smoking guns.”

A good DUI defense lawyer will make this clear to the prosecutor and, if the DUI of marijuana case goes to trial: to the judge or jury as well.

A positive marijuana test alone does not prove that a person was too high to drive safely. Even medical experts cannot agree as to what impact marijuana has on driving. There is no expert consensus as to how much marijuana is sufficient to impair one’s ability to drive (and thus make one guilty of driving under the influence of marijuana).

A defense attorney who understands this current data can often successfully argue that a “stoned” driver is not necessarily an impaired one.

In addition to chemical test results, evidence of DUI marijuana may include:
– Driving pattern
– Statements made to the police
– Physical appearance
– Performance on Field Sobriety Tests (FSTs)
– The presence of marijuana or paraphernalia in the vehicle car or on a person
– Evidence that a person is addicted to marijuana

Signs of marijuana use that the officer might look for in deciding whether to make an arrest for DUI of marijuana include:
– Dilated pupils
– Rapid heart rate
– Rapid breathing
– The odor of marijuana emanating from the body
– Red eyes
– Dry “cotton” mouth
– Slowed reaction time

The law previously declared it illegal to drive while impaired by marijuana or any other drug.  However, with the legalization of marijuana in several States, marijuana DUI law has entered the spotlight. 

Based upon today’s testing methods, the prosecution of suspects in marijuana DUI cases is fraught with unreliability and the danger of misleading results.  For the criminal defense practitioner, there is ample opportunity to discredit the prosecution’s case and challenge the admissibility of said evidence. 

As scientists develop more new detection tests, it remains to be seen whether these methodologies will be subject to the same level of criticism.


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