As the landscape of marijuana legality evolves, drug testing has shifted from merely detecting the presence of cannabis in the body, typically through a urine sample, to more intricate methods aimed at determining the precise quantity of marijuana in one’s system, primarily through blood tests. This transition is not without its complexities and controversies, as the active ingredient in marijuana, delta-9-tetrahydrocannabinol (THC), does not manifest consistently in all users, unlike alcohol. To shed light on this issue, we sought insights from the director of a chemical testing laboratory in Minnesota. Here’s what we’ve learned:

1. Legal Limits for Stoned Driving: Colorado has set a legal driving limit of 5 nanograms of THC per milliliter of blood. This threshold has sparked debates, with some arguing it’s too low for chronic marijuana users. In contrast, others insist it’s too high, suggesting that even one ng/ml could imply impairment. However, based on in-depth European studies, the lab director found that five ng/ml is a reasonable threshold. THC affects reaction times and depth perception and may induce paranoia, making driving under its influence just as risky as driving drunk.
2. Chronic Users and THC Levels: Regular marijuana users may have 5 nanograms of THC in their system persistently, as THC takes longer to dissipate in their bodies. Colorado addresses this by allowing a permissible inference in DUI cases, meaning that the 5 ng/ml limit is a presumption, not a definitive confirmation, of stoned driving. Medical experts may argue that individuals with 7 or 10 nanograms of THC are entirely sober, especially in medical marijuana users.
3. Time Sensitivity in Testing: Unlike alcohol, where authorities have a two-hour limit to test blood-alcohol concentration, Colorado has not set a time frame for measuring psychoactive THC. THC’s parent compound has a relatively short lifespan, dropping from 100 nanograms to 5 nanograms in about five hours, particularly in first-time users. To accurately determine impairment, drawing a blood sample soon after marijuana consumption is essential.
4. Fat Solubility of THC: THC is fat-soluble, leaving the body slowly. This characteristic explains why THC takes a while to become undetectable in urine. Urine tests are not accurate indicators of sobriety, as they do not reflect current impairment levels. Unlike alcohol, which is water-soluble and can be metabolized quickly, THC creates a depot in the body, continually releasing the drug even after consumption has ceased.
5. Different Consumption Methods: Blood tests do not differentiate between smoking, ingesting, or vaporizing marijuana. However, smoking results in faster impairment due to the direct absorption of THC in the lungs, while edibles are metabolized in the liver before entering the bloodstream. Different strains of marijuana also vary in potency, affecting how quickly the five ng/ml limit is reached.
6. The Precision of Blood Testing: Currently, a blood test offers the most precise measure of active THC levels. While formal testing procedures are outlined in DUI laws, advancements are underway to develop more efficient methods. A marijuana breathalyzer is in development, but it faces challenges, as THC is not released from the lungs in the same way as alcohol. Saliva testing has limitations due to potential contamination and leftover THC from previous use. Hair testing can detect past THC use but is less suitable for quantifying present THC levels.