How Much THC Does it Take to Make a DUI?

Italian Market tragedy highlights problems with marijuana blood test.

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Last March, Max Drosi, 28, of Miami, blew a red light on 8th and Washington in the Italian Market, plowing his rented Chevy Impala into an oncoming SUV and sending the vehicle careening into a family of pedestrians on the sidewalk outside Captain Jessie G’s. The crash seriously injured a two-year-old and his mother Lacky Xiong, 26, in addition to killing 11-year-old Samantha Ortanez-Nguyen of Sicklerville.

Drosi has already admitted to being distracted by his car’s GPS device (which was giving him directions to Geno’s) at the time, but his pot-based DUI is a different story. Facing charges that include aggravated assault, homicide by vehicle, and DUI, Drosi maintains that he was not under the influence of marijuana during the accident, saying he had not smoked any pot since the night prior to the crash. Two Philly cops, pathologist Richard Cohn and a blood test performed shortly after the incident, however, say otherwise.

With Drosi’s two prior marijuana possession charges, in 2004 and 2007—along with a 2002 charge for aggravated assault—it certainly is not impossible that he was stoned during the accident. If he was, then he absolutely should be charged with a DUI—there is, on that charge, no defense for his actions. People, of course, should not drive intoxicated. However, as any marijuana activist can tell you, blood tests for cannabis intoxication aren’t exactly accurate or meaningful in determining impairment at the scene due to the way THC behaves in the body.

As with booze, Pennsylvania currently operates on a “per se” limit to determine intoxication in blood tests for THC, with that defined limit being one nanogram of active THC per milliliter of blood (down from 5ng/ml in 2011). The trouble is, beyond that rule being essentially zero-tolerance, there’s very little direct correlation between the nanogram amount of THC found and behavioral impairment at the time of arrest, unlike with alcohol and blood alcohol content—especially at 5 ng/ml or less.

Marijuana, not surprisingly, does not function physiologically in the same way as alcohol, and to test for intoxication from the two drugs in similar ways makes little scientific sense. Alcohol is water-soluble and metabolized by the body relatively quickly, meaning that tests measuring the amount of alcohol in the blood, like the infamous Breathalyzer, can be correlated to actual impairment. Cannabis’s main metabolite, on the other hand, is lipid soluble, meaning that the body redistributes THC in fat cells, metabolizing them more slowly.

Due to THC’s much slower metabolization rate, levels of the chemical will remain higher in the body for longer after use—and that’s without impairment. If smoked, marijuana’s psychoactive effects tend to wear off in about two to four hours (about eight if eaten), but its byproducts remain in the body for hours or days afterward, especially in consistent, heavy smokers. Take, for example, the case of Denver Westword medical marijuana critic William Breathes, who took a blood test while unimpaired some 15 hours after last medicating. Breathes, who was declared sober by a doctor for the test, ended up with about 13.5ng/ml worth of THC in his body after 15 hours of abstention, nearly three times the legal amount proposed by Washington’s current I-502 marijuana legalization bill. In 2009, a similar study of 25 subjects found that six participants managed to test positive for THC after a week of abstaining from cannabis, the highest being 3 ng/ml.

Aspects like tolerance, frequency of use and method of ingestion all make determining the relationship between nanogram amount and impairment difficult, and most advocates say a higher amount is necessary if nanogram amount is to be used at all to determine intoxication (one noted drug researcher recommends 15-30ng/ml). Variations in testing methods that produce different total results—as much as a 4.5 nanogram difference—obscure accurate standardized results as well.

With unfit testing that attempts to equate nanograms-per-milliliter with BAC, the risk of creating false positives increases significantly and creates an unfair bias against marijuana users. Reaction from medical cannabis states has, not surprisingly, been strong, with Colorado recently shooting down a bill for the third time that would institute the 5ng/ml per se limit and essentially criminalize every medical marijuana patient at the state level. Washington’s own legalization bill has caused a rift in the cannabis community over the inclusion of similar limits as well, with NORML joining on as a supporter despite that measure.

Nationally, this is not a question of whether it is safer to drive under the influence of marijuana over alcohol—after all, the practice shouldn’t be accepted on the whole for obvious reasons. It is, however, a question of whether our law enforcement should prosecute a significant and growing sector of the American population with results from a test that is neither meaningful nor scientifically valid in what it attempts to determine. People like Max Drosi, whether or not he was actually high when he killed that 11-year-old (which we might never actually know), cloud public judgment on the issue by providing an extreme and terrible example that easily provides an argument for the “pot is bad” camp. It’s just a shame that this example had to be so tragic.