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.
For a fuller picture of how these cases are defended, see our marijuana DUI defense page.
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.
Sources
If you need help applying this to your specific facts, contact our office.