There is no reliable roadside test to measure marijuana impairment. But police are trying to make one.
Testing for THC
In states like Colorado, there is a THC blood test that law enforcement uses to show “presumed” impairment. The test seeks delta-9-tetrahydrocannibol, also known as THC, in the blood. If a person has more than five nanograms of THC, under Colorado law a court or jury can infer that they are impaired. However, there is no hard and fast number proving a person is actually impaired. This is because of the nature of THC, which stays in the body for a long time. Someone who is a habitual user of marijuana can still have five nanograms of THC in their blood several hours after smoking.
One study involved 30 habitual smokers staying in a research facility for a month and smoking marijuana. In some instances, smokers were seen to have THC in their blood a month after smoking. Clearly, if police are serious about marijuana impairment while driving, a new approach is needed.
Some scientists have recently turned to breath testing as an alternative. Companies like Cannabix Technologies and Hound Labs are working on a breathalyzer or marijuana. However, these devices are difficult to make, since THC degrades quickly in breath and appears in very small amounts. An accurate breath test, then, would likely need to seek out a wide combination of chemicals besides THC to prove impairment.