Accuracy of pot impairment tests questioned ahead of legalization
OTTAWA -- Blood tests for measuring THC impairment are less accurate than those measuring alcohol in impaired drivers, experts say, raising questions about how police will enforce stiff new penalties when the government legalizes marijuana.
The government says it would introduce new penalties for driving while impaired by marijuana, ranging from a fine to up to 10 years in prison for the most serious cases. The penalties are to be based on the nanograms of THC found per millilitre of blood, also known as per se limits.
While roadside testing would likely be conducted through a cheek swab, the level of impairment would be confirmed with either an evaluation by a trained officer or a blood test, according to a background document provided by the Justice Department.
But it's unclear whether blood tests to measure the level of THC impairment, the main psychoactive ingredient in cannabis, are accurate.
"A THC blood test is completely inadequate to determine impairment. The best we can do given the biology of the problem is a cheek swab which would measure use in the last few hours," said Mark Kleiman, a professor of public policy and author of Marijuana legalization: What everyone needs to know.
THC is fat soluble, so it can enter fat cells and re-emerge later, Kleiman said in an interview with CTV News.
"So you could be positive for THC in blood days after the last time you used, and at a point where you're not subjectively impaired at all," he said.
"If somebody smokes a lot and then stops for two days, they're going to be at a fairly high level and it's not even actually predictable."
A May, 2016 report by the American Automobile Association Foundation for Traffic Safety found blood testing for cannabis isn't the same as it is for alcohol.
"Legislators and scientists struggle with the question of determining an appropriate science-based limit for cannabis use by drivers," the report said.
"Unlike in the case of alcohol, where substantial experimental and epidemiological evidence was available to guide and support the setting of an evidence-based per se limit, the relevant research data relating to cannabis is limited and the findings are often inconclusive."
The study also found roadside evaluations like walking in a straight line and standing on one leg showed minimal differences based on the level of THC in a subject's blood.
"There was no correlation between blood THC concentration and scores on the individual indicators, and performance on the indicators could not reliably assign a subject to the high or low blood THC categories," the report said.
Jeff Walker, chief strategy officer at the Canadian Automobile Association (CAA), said the blood impairment level will likely be one of a range of evaluations used to determine impairment.
"It's going to be those behavioral things where police officers identify you've got a problem at the side of the road, and/or the drug recognition experts assess you when you get back to the police station assess you," Walker said.
"Those things are going to be just as important as things like how much THC is in your system to judge whether or not you're impaired."
The cheek swabs, Walker said, only measure the presence of THC -- not the amount of it.
"In the end it's going to be this combination of having it in your system, people noticing you've been impaired or behaviorally not right, essentially, either on the road or back at the police station. It'll be a bunch of these variables that'll be part of the conviction process."