They’re Already Working on a Roadside Pot Test to Nail You for Driving High

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Stanford engineers hope a new saliva test will someday be the key to catching drivers who've taken too many tokes. Right now, the only way to detect marijuana in the body is through a blood test or urine sample collected in a hospital -- not exactly a convenient roadside solution.

As states continue to legalize various forms of cannabis use, law enforcement will increasingly be looking for ways to detect impairment from pot. More than 20 states and the District of Columbia already allow some sort of cannabis use, and legalization is on the ballot in five states this November.

Stanford's “potalyzer" is a handheld device that allows officers to conduct a simple field test to determine a driver's concentration of THC, the chemical in marijuana that impairs drivers. In just a few minutes, a traffic cop could swab a saliva sample from a driver's mouth and insert it into a device with magnetic biosensors that can then send the results to a smartphone or laptop. 

Why Saliva?

Although companies such as Cannabix Technologies and Hound Labs have developed breathalyzer tests for marijuana that work like those used to test for alcohol, Stanford researcher Tyler O’Brien Shultz says he’s highly skeptical that marijuana levels can be captured through air.


A THC molecule is very different from an ethanol molecule. Alcohol is volatile and evaporates quickly, so a breathalyzer test measures spirits vaporizing in your blood that are released through the lungs when you exhale.

“Because THC doesn’t evaporate,” says Stanford researcher Tyler O’Brien Shultz, “It is nearly impossible to test marijuana through breath detection, so it makes it sense to look at a different medium.”

The saliva test employs the same magnetic nanotechnology Stanford researchers have used in cancer screenings. In this case they're measuring the THC concentration in someone’s spit rather than looking for cancer cells.

There is a not a standardized “safe” THC level for driving, although the range in academic studies is generally between two and 25 nanograms (or billionths of a gram) per milliliter of blood. The potalyzer can detect concentrations of THC in the range of 0 to 50 nanograms per milliliter of saliva. The device obviously won’t be useful until states set a limit for officers to test against.

Current System Lags

Right now traffic cops must rely on visual observation to determine whether someone is high. Officers look for red eyes or a bulging neck vein, or they’ll ask a driver to close their eyes and guess when 30 seconds has passed.

If the driver appears impaired, the officer will take them into custody. At the police station, a drug recognition expert will  assess the driver to determine whether a blood test and a trip to a hospital is necessary. The process can take about two-and-a-half hours from the time a driver is stopped until their blood is drawn. Shultz says that's way too long.

“THC will likely fall below the safe limit designated by most states within three to four hours of consuming the drug,” says Schultz. “In other words, if someone is high when you pull them over, they won’t be high when you draw their blood.”

Urinalysis is also not ideal for assessing impairment, because tests pick up marijuana that is  latent in the body from use that could be days old. That is not a problem with Stanford's device.

The potalyzer's potential extends far beyond weed. The device can measure any small molecule, so a test could also look for morphine, heroin or cocaine. The biosensor chip has 80 sensors capable of testing for multiple substances. But it will be at least a few years before the saliva test will be deployed by police. Schultz says the device needs further field tests, human trials and regulatory approval before it goes on the market.