Recent press is focusing on new hardware for police roadside breath testing for marijuana. Similar in use to a standard PBT, or portable breath tester for alcohol, the new devices identify and quantify THC from a breath sample.

Current available options are primarily urine testing and blood testing. Urine testing is useless for driving and other impairment questions. Urine testing quantifies THC COOH, a psycho-inactive cannabinoid. THC is the primary psycho-active cannabinoid. Blood testing reveals the quantity of this potentially impairing chemical.

Other than quite low amounts, active THC in blood is rapidly converted, or metabolized into its non-psychoactive cousin, carboxy THC, or THC COOH. While arguably an improvement over urine testing since the THC level depletes rapidly after ingestion by smoke or vapors, that benefit must be kept in context. What it measures is a quantity of THC in blood. What it does not measure is impairment or lack of impairment. While higher levels of blood-THC suggest more recent ingestion, impairment is not revealed.

Alcohol has been shown quite consistently to have a similar impairing impact on different drinkers with comparable blood alcohol levels. That permits generally reliable measurements of blood alcohol to generally indicate lack of impairment or impairment. More scientifically, the experiments comparing BAC, or blood alcohol content and behavior consistently show a “normal” or “bell shaped curve” Poisson distribution. That means that most people fall within a fixed range with a decided peak in the middle, when comparing impairment with BAC.

That is decidedly NOT true of THC levels in blood. When the numbers are compared with the impairing impact of cannabis, the result is NOT a Poisson distribution. There is no decided peak in the middle. There is no neat graph that is suggested. The blood level of active THC simply does not suggest a level of impairment.

So measuring a driver’s breath might lead to a suspicion that ingestion had been recent, it certainly does not provide relevant or useful information about impairment or lack of impairment.

CAUTION: mixing alcohol and cannabis is especially risky for drivers. The amount of each does not predict the amount of impairment. They work together, making the whole greater than the sum of the parts. This is working synergistically.

For more detail on the relationship of THC COOH and THC, please see the summary.