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New Saliva Test for THC Could Be More Reliable Than Cannabis Breathalyzers

By Alexander Beadle

Published: Apr 06, 2020   
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For drivers suspected to be operating under the influence of alcohol, breathalyzers provide a quick non-invasive way to determine sobriety and impairment.

As cannabis legalization gains steam throughout North America, law enforcement agencies are grappling with the fact that there is no similar way to assess if drivers are impaired from cannabis use.

Now, presenting their creation at the virtual American Chemical Society Spring 2020 National Meeting & Exposition, researchers at the University of Texas at Dallas have unveiled a rapid response, field-deployable biosensor that can detect and quantify concentrations of the intoxicating tetrahydrocannabinol (THC) in human saliva.

The need for a THC test

Currently, the only reliable ways of testing the level of THC present in person’s body is to take a blood test. Blood tests are invasive, time-consuming, and many law enforcement officers don’t have the necessary skills needed to carry out the test at the roadside.

To address the issue, research groups from across the country have developed ‘cannabis breathalyzers’ to measure THC levels in a driver’s breath. One prototype, developed last year at the University of Pittsburgh, uses carbon nanotubes, 100,000 times smaller than a human hair, to “detect THC at levels comparable to or better than mass spectrometry.”

But, according to the new study’s lead author, Shalini Prasad, a professor of bioengineering at the University of Texas at Dallas, the actual THC levels present on the breath are so low that such tests risk producing error-prone data that requires extensive processing to filter out the effects of other compounds. By comparison, she says, the THC content of saliva is a lot more closely correlated with the levels seen in the blood, making saliva a more promising medium to test.

“People have the perception that driving after smoking marijuana is safer than driving drunk, but both substances can have similar effects, such as slowed reaction time, diminished alertness and reduced self-awareness,” said Prasad in a press statement. “This is an emerging field, but preliminary clinical reports suggest that anywhere above 1 to 15 nanograms of THC per milliliter of blood is considered a level of impairment.”

One extensive review of 60 studies found that marijuana affected all areas relevant to safe driving, including psycho-motor skills, continued attention, visual function, and reaction time.

Building a THC biosensor

The test developed by Prasad and her colleagues consists of two engineered THC sensor strips and an electronic reader.

The sensor strips contain two electrodes which are coated in an antibody that binds to THC, allowing the cannabinoid to be separated quickly and effectively from the other compounds present in saliva, ready for quantification.

To use the test, a drop of saliva is introduced to the sensor strip and inserted into the electronic reader, which applies a voltage across the sensor. With THC attached to the antibody coating, the electrical current measured across the sensor changes, as a result of the polarization that occurs between the antibody and THC surfaces. The electronic reader is able to measure this change in current and use it to calculate the concentration of THC in the saliva.

“We used the antibody so that we could really only look at the needle in the haystack,” Prasad explained. “This is the first demonstration of a prototype device that can report both low and high concentrations of THC in a noninvasive, highly sensitive and specific manner.”

Tests carried out by the researchers using human saliva spiked with THC indicated that the biosensor was accurate at detecting and determining THC levels ranging between 100 picograms per milliliter to 100 nanogram per milliliter, which well covered the ranges where THC concentration would indicate impairment.

What is next for the test?

Recreational cannabis use is illegal in Texas, where the research team is based, and so the team hasn’t yet been able to analyze saliva samples from people who have actually smoked cannabis or used cannabis products. However, Prasad says that there are researchers from other states with legal adult-use cannabis policies, and local law enforcement agencies, who have expressed interest in collaborating in order to test the biosensor further.

Prasad also listed a number of other applications where the biosensor might be of interest outside of law enforcement. For example, she says there has been interest from the medical cannabis community and from various lifestyle companies that want to find a way to help people manage their cannabis consumption. In addition, she says, lawmakers and regulatory groups may be interested in using data generated by the device to further develop laws surrounding cannabis use. 

Alexander Beadle

Science Writer

Alexander Beadle has been working as a freelance science writer since 2017 and has covered the cannabis industry for Analytical Cannabis since 2018. He has also written for our sister publication, Technology Networks, and the cannabis industry consultant firm Prohibition Partners, among others. Alexander holds a Master's in Materials Chemistry from the University of St. Andrews, where he won a Chemistry Purdie scholarship, and conducted research into zeolite crystal growth mechanisms and the action of single-molecule transistors.


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