I’ve posted in the past on the difficulties law enforcement faces in detecting impairment from marijuana while driving — both subjectively (symptoms, field sobriety tests and the officer’s opinion) and objectively (analysis of blood or other bodily substances). See, for example, Identifying and Proving DUI Marijuana ("Stoned Driving"), Can Breathalyzers Measure Marijuana?, New Efforts to Push Roadside DUI Marijuana Test and San Diego Begins Using Mouth Swabs to Detect Drugged Drivers. There is even disagreement among scientists as to how much marijuana must be ingested to become impaired, and how the metabolism (absorption and elimination) of marijuana functions in any individual — for example, how long the active metabolites remain in the blood. See How Much Does It Take to Impair Driving? and New Study: Minimal Impairment From Marijuana.
The following excerpts from a segment of a recent public radio presentation does an excellent job of laying out the difficulties in detecting marijuana impairment and measuring levels of active THC (delta-9-tetrahydrocannabinol) in the blood.
Scientists Still Seek a Reliable DUI Test for Marijuana
July 30, 2017. NPR – Despite the increasingly legal use of cannabis in many states, cops still don’t have the equivalent of a reliable alcohol breathalyzer or blood test — a chemically based way of estimating what the drug is doing in the brain. Though a blood test exists that can detect some of marijuana’s components, there is no widely accepted, standardized amount in the breath or blood that gives police or courts or anyone else a good sense of who is impaired…
A number of scientists nationally are working hard to create just such a chemical test and standard — something to replace the behavioral indicators that cops have to base their judgments on now…
Turns out it can be a lot harder to chemically determine from a blood or breath test that someone is high than to determine from such a test that they’re drunk.
Ethanol, the chemical in alcoholic drinks that dulls thinking and reflexes is small and dissolves in water. Because humans are mostly water, it gets distributed fairly quickly and easily throughout the body and is usually cleared within a matter of hours. But THC, the main chemical in cannabis that produces some of the same symptoms, dissolves in fat. That means the length of time it lingers in the body can differ from person to person even more than alcohol — influenced by things like gender, amount of body fat, frequency of use, and the method and type of cannabis product consumed.
In one study, researchers had 30 frequent marijuana users stay at a research facility for a month without any access to drugs of any sort and repeatedly tested their blood for evidence of cannabis.
"And it shocked everyone, including ourselves, that we could measure, in some of these individuals, THC in the blood for 30 days," says Marilyn Huestis, a toxicologist with the University of Maryland School of Medicine who recently retired from leading a lab at the National Institute on Drug Abuse.
The participants’ bodies had built up stores of THC that were continuing to slowly leech out, even though they had abstained from using marijuana for a full month. In some of those who regularly smoked large amounts of pot, researchers could measure blood THC above the 5-nanogram level for several days after they had stopped smoking.
Conversely, another study showed that people who weren’t regular consumers could smoke a joint right in front of researchers and yet show no evidence of cannabis in their blood.
So, in addition to being invasive and cumbersome, the blood test can be misleading and a poor indicator of whatever is happening in the brain…
The NPR segment went on to discuss the difficulties police officers have in judging whether a person who has consumed marijuana was impaired. After law enforcement training seminars involving volunteers who had smoked different amounts of marijuana, the program concluded:
Right now, these officer’s opinions loom large. If they decide you’re driving high, you’re going to jail. But at the end of the day, they’re just making educated guesses. Two different officers could watch the same person doing the same sobriety test and make different decisions on whether to arrest. In previous courses, officers had decided that a volunteer was impaired when in fact the volunteer hadn’t smoked at all.
So, just like the THC blood test, the judgments officers make can also yield false positives and negatives….
An increasing number of states are simply throwing up their hands and, in effect, deciding that actual impairment is not necessary: the crime is in driving with an arbitrary amount of THC in the blood — even if there is no actual impairment at all.
This follows what the federal government imposed on the states a few years ago: a new crime of driving with 0.08% blood-alcohol, to overcome the difficulties of having to prove the driver was actually impaired — despite the proven fact that many people are not impaired at that level or higher. In alcohol cases, however, it is at least possible to measure alcohol levels, and roughly determine absorption and elimination times.
But changing the crime of driving while impaired by marijuana to one of having an arbitrary amount in the system makes arrest and conviction much easier for police and prosecutors, right? And isn’t that the important thing?