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Impairment Not Accurately Predicted By THC Levels: Federal Study

THC-based testing is not an accurate way to measure impairment, according to a new federal study.


You know all those THC tests that supposedly monitor levels of driver impairment and thus increase public safety? The tests suck at reliably doing what they are supposed to do.

The study found amount of THC in a person’s system after consuming Cannabis isn’t an accurate predictor of impairment.

The National Institute of Justice (NIJ) funded the research. It involved 20 people who either ate or vaporized cannabis with varying levels of THC. Sobriety and cognitive tests were administered to the participants.

The groups getting doses higher than 5 milligrams of THC were all “negatively impacted.” They experienced observable psychomotor impairment. But the RTI International researchers found something unexpected. “THC levels in biofluids were not reliable indicators of marijuana intoxication for their study participants.”

‘Per se’ impairment limits questioned

That raises questions about “per se” impairment laws that are in place in several states. These laws bar people from driving, presuming impairment if more than a certain amount of THC is in their blood.

“These important findings come as no surprise,” NORML Deputy Director Paul Armentano said. “Despite a handful of states imposing per se THC thresholds as part of their traffic safety laws, there exists no science demonstrating that these arbitrary limits are reliable predictors of either recent cannabis exposure or impairment.”

The study, originally published last year, was promoted by NIJ in a tweet on Thursday. It also found that balancing, walking and turning “were not sensitive to cannabis intoxication for any of the study participants.”

Participants’ blood, urine and oral fluid were collected during the testing. It was sent to forensic laboratories.

“Results from the toxicology tests showed that the levels of all three targeted cannabis components (THC, cannabidiol, and cannabinol) in blood, urine, and oral fluid did not correlate with cognitive or psychomotor impairment measures for oral or vaporized cannabis administration,” NIJ said.

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