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Scientists still seek good test for marijuana-intoxicated driving

As cannabis is legalized in more and more states, determining when a person is intoxicated by the drug becomes an important legal issue in DUI law. In states where someone could be using marijuana legally for medical or even recreational reasons, it's not enough to test whether the drug is in a driver's system. Rather, for DUI, what needs to be tested is whether a driver is impaired by the drug.

In Tennessee, cannabis is not legal for any purpose. Therefore, having any detectible amount of marijuana in your system while driving is considered per se DUI.

However, medical cannabis is legal in 29 states, and recreational marijuana is legal in nine states and the District of Columbia. All of those states are trying to develop a test that can identify those who are driving while impaired but which excludes those who use the drug but don't drive impaired.

If a reliable standardized test were created for THC, marijuana's psychoactive compound, Tennessee might well adopt that test -- especially if it didn't rely on drawing blood. In 2013, the U.S. Supreme Court ruled in Missouri v. McNeely that drawing a suspect's blood generally requires a warrant, so blood tests take more time and effort than, for example, breath tests.

Unfortunately, scientists say that no single concentration of THC in a person's blood or oral fluid reliably indicates that person is impaired. Moreover, blood-THC concentrations drop rapidly and reach almost zero within about two and a half hours. It can take a little over an hour or up to four hours to obtain a blood test. That means that a person's THC concentration could drop to zero while they await testing.

Currently, many states are relying on training officers to identify signs of impairment and tailoring field sobriety tests to indicators of marijuana impairment. So far, this has also been challenging.

Scientists have noted that gender, body-fat percentage, the type of product consumed, the method of consumption used, and the amount ingested are all factors in how well people perform on field sobriety tests.

There are also challenges with people who routinely use cannabis. THC accumulates in the body and is released slowly. That can mean that a long-term daily user would test positive for marijuana even if they had quit using it more than a month before.

As of now, there is no good way to set a blood-THC limit like we've set a blood-alcohol limit for DUI. Future research may or may not be able to solve that problem. How do we protect the rights of drivers who indulge when they're not driving -- while still keeping impaired drivers off the road?

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