Medical cannabis user Michelle Gray spent hours at a Halifax police station proving she wasn't high — but she still had her car impounded and her licence suspended for a week.

Now the Halifax woman, who uses cannabis for multiple sclerosis, plans to challenge the legality of roadside saliva tests that critics say are unreliable in determining a person's sobriety.

"I would never opt to be in this position, but I am in this position," Gray told As It Happens host Carol Off Wednesday.

"If I can make change for, you know, especially medical users, regular daily users that are cautious with their usage of cannabis, I don't want them in the same position that I was in."

Tested positive, then tested negative

Gray was pulled over at a routine RCMP check stop on Jan. 4 when the officer noticed the smell of cannabis in her car.

She says she explained to the officer that she was a medical user, but that it had been nearly seven hours since she last consumed pot.

The officer administered a roadside saliva test, and Gray tested positive for THC, the psychoactive ingredient in cannabis. THC is stored in the fat cells and can remain detectable in a person's body for as long a month after usage, according to the Mayo Clinic.

"I was stunned, shocked. And it's kind of like everything just started spinning slowly," Gray said. "And the officer then informed me because I failed that test, they had to place me under arrest."

Drager DrugTest 5000 (Drager)

Gray was then taken to a police station so that another officer could perform what's called an extensive sobriety test — a 12-step process designed to detect impairment.

The officer walked her through various exercises to test her speech and balance, she said, both of which can be negatively impacted by MS.

"If I had have been in a flare-up when this situation happened, I would have instantly failed that extensive sobriety test," she said.

But Gray passed and was released without charge.

Still, her licence was suspended for seven days because she failed the roadside test, and she had to pay $253 to get her car back after it was towed.

The province's Motor Vehicle Act allows police to issue suspensions for drivers who fail a roadside screening test.

"It's a weird provision that they've had in there for a number of years involving alcohol and now they've changed it recently to include testing for marijuana as well, even though the roadside test for marijuana cannot make any determinations to whether somebody is impaired or not," Nova Scotia lawyer Tom Singleton, who is not connected to Gray's case, told As It Happens.

The Nova Scotia RCMP said a positive roadside test may result in a "maximum" 24-hour licence suspension.

"In this case, the driver was incorrectly issued a seven-day suspension," the police force said in an emailed statement.

"As a followup, we have contacted the individual involved to acknowledge our error."

Tests approved by federal government

Now Gray has teamed up with lawyers at Acumen Law to challenge both the the legality of the Drager DrugTest 5000 roadside test and Nova Scotia's roadside suspension provision.

The Vancouver law firm has been looking for cases to challenge the tests since they were approved for use by the federal Department of Justice in August.

"The attorney general of Canada approves roadside drug screeners only for Criminal Code investigative purposes," Jutice Department spokesperson Ian McLeod said in an emailed statement.

"Provinces and territories may choose to use federally approved drug screeners for purposes within their exclusive jurisdiction, including the enforcement of provincial highway traffic laws."

Several cities across Canada, including Ottawa, have decided against using the device due to the questions being raised about the machine's accuracy, especially in cold weather.

A study published in the Journal of Analytical Toxicology, based on the use of the device in Norway, raised concerns about a significant number of false-positive and false-negative results.

The machines test for THC, but they do not measure impairment.

"There is no correlation between the level that you're at, the active THC in your body, and impairment," Cpl. Lisa Croteau, a spokesperson for the RCMP in Nova Scotia, told Global News.

Written by Sheena Goodyear with files from The Fifth Estate and The National. Interview with Michelle Gray produced by Ashley Mak.