Lawmakers are considering a renewed effort to test impaired drivers for illegally driving under the influence of marijuana.
The Montana Highway Patrol said at a Thursday hearing on the bill there is a significant increase in the number of people driving under the influence of the drug.
The state crime lab already tests blood samples in drunk driving and other crimes for levels of THC - an ingredient in marijuana. But currently there is no state law defining an illegal threshold.
County attorneys and police argue a standard is needed just as with alcohol to keep the roadways safe.
But marijuana advocates argue the test is unreliable and measures agents that don't cause intoxication but remain in the blood stream long after impairment.
Scientists with the state crime lab testified today before members of the House, in a effort to explain the challenges associated with testing for THC.
Sarah Braseth, a forensic toxicologist for the state says there are many factors to consider.
"Typically the higher the THC level in the blood, the more recent the drug use was, so you can kind of narrow it down, So if you see levels about ten nanograms, then this person may have just used the drug and it increases the chance they are impaired," says Braseth.
House Bill 168 proposes a 5 nanogram per milliliter limit, but experts say the target range for impairment can vary based on time elapsed from use and biological details of a suspected user.