A Justice Department research says states may need to “get away from that idea” that marijuana impairment can be tested based on the concentration of THC in a person’s system.
Frances Scott, a physical scientist at the National Institute of Justice (NIJ) Office of Investigative and Forensic Sciences under DOJ, discussed the challenges of cannabis impairment testing in an episode of the Justice Today podcast that was posted late last month.
Scott questioned the efficacy of setting “per se” THC limits for driving that some states have enacted, making it so a person can be charged with driving while impaired based on the concentration of cannabis components in their system. Ultimately, there may not be a way to assess impairment from THC like we do for alcohol, she said.
One complication, Scott said, is that “if you have chronic users versus infrequent users, they have very different concentrations correlated to different effects.”
“So the same effect level, if you will, will be correlated with a very different concentration of THC in the blood of a chronic user versus an infrequent user,” she said.
That issue was also examined in a recent federally funded study that identified two different methods of more accurately testing for recent THC use that accounts for the fact that metabolites of the cannabinoid can stay present in a person’s system for weeks or months after consumption.
“The problem is we’ve funded this research that pretty conclusively shows that the THC concentration in the blood is not particularly well-correlated with impairment for driving,” Scott said. “One of the outstanding questions is trying to figure out, is there a good proxy, a good metric, that we can use?”
Hellbound and Down 
I honestly don’t trust them. They just want cops to make up for the DUI shortfall.
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