What Are the Types of THC Measured in a Washington DUI Test?
When Washington law enforcement orders a blood draw after a suspected marijuana DUI, toxicology labs don’t simply look for “marijuana” — they can analyze specific compounds that reveal whether a driver was recently impaired or merely has residual cannabis in their system. They can also test for other drugs if requested by law enforcement.
The two most important forms are active THC (Delta-9-THC) and its metabolized form (THC-COOH), but several related cannabinoids — such as Delta-8-THC, Delta-10-THC, and THC-A — also play background roles worth understanding.
Active vs. Metabolized THC
Active THC (Delta-9-THC)
This is the psychoactive chemical responsible for the “high.” It appears in blood for only a few hours after use and directly affects coordination and reaction time. Under RCW 46.61.502, drivers with 5 nanograms per milliliter (ng/mL) or more of active THC within two hours of driving are presumed impaired — a per se DUI.
Metabolized THC (THC-COOH)
After your body breaks down Delta-9-THC, what remains is THC-COOH, an inactive metabolite that lingers in fat tissue and releases slowly over time. It does not cause impairment and is not part of Washington’s legal threshold, but its presence can show a history of use — especially for daily or heavy consumers.
Other Forms of THC and How Washington Handles Them
Delta-8-THC
A hemp-derived variant that produces a milder intoxicating effect. It is chemically distinct from Delta-9 but similar enough to cause impairment. Washington classifies Delta-8 under its controlled-substance analog statute, yet there is no numeric DUI limit for it. If an officer suspects impairment after Delta-8 use, prosecutors must rely on observed behavior and field sobriety tests, not lab numbers.
Delta-10-THC
Another synthetic cannabinoid similar to Delta-8, also without a defined impairment threshold. Washington toxicology labs do not routinely test for Delta-10 in DUI cases, though the compound could still appear on broader forensic panels.
THC-A (Tetrahydrocannabinolic Acid)
A non-psychoactive precursor to Delta-9-THC found in raw cannabis. When heated — through smoking or cooking — THC-A “decarboxylates” into active Delta-9. Blood tests may list small amounts of THC-A, but it has no bearing on impairment or DUI prosecution.
What the State Actually Measures
The Washington State Toxicology Laboratory focuses on:
- Delta-9-THC (active) — counts toward the 5 ng/mL per se limit
- THC-COOH (inactive) — evidence of prior use only
Other cannabinoids may appear, but only active Delta-9-THC determines whether a driver exceeds the legal standard for marijuana DUI.
Key Takeaways
- Washington’s 5 ng/mL DUI limit applies only to active Delta-9-THC.
- Delta-8 and Delta-10 have no statutory threshold but can still lead to DUI charges if impairment is proven through officer observation.
- THC-A and THC-COOH are non-impairing and serve only as indicators of prior use.
- Frequent users can test positive for THC-COOH long after the high wears off — a distinction critical in DUI defense.