How do officers determine marijuana impairment?
Washington’s legalization of cannabis did not change the fact that driving under the influence of marijuana is a crime. What did change is how officers determine impairment. Unlike alcohol, there is no roadside device that instantly measures THC levels, so investigations rely on a combination of observation, standardized tests, and—when necessary—a blood draw authorized by a search warrant.
From Observation to Probable Cause
When a driver is stopped—whether for speeding, drifting across lanes, or another traffic violation—officers first look for signs of impairment. This may include:
- Slowed reaction time or delayed responses
- Bloodshot eyes or the odor of cannabis
- Difficulty following instructions or maintaining balance
If these observations raise suspicion, the officer can begin a DUI investigation. For marijuana cases, that process differs slightly from alcohol enforcement.
No Instant THC Test
There is currently no portable roadside test that measures active THC levels. Breathalyzers only detect alcohol. That means officers must rely on field sobriety tests and their training to assess whether a driver appears impaired. Some departments also use Drug Recognition Experts (DREs)—specially trained officers who evaluate impairment through a 12-step protocol including eye examinations, divided-attention tests, and vital-sign checks.
Implied Consent and Blood Draws
Under Washington’s Implied Consent Law (RCW 46.20.308), drivers automatically consent to a breath test for alcohol. But that same rule does not extend to marijuana. Officers can ask for a voluntary blood sample if they suspect THC impairment, but a driver has the right to refuse. If the officer still believes there is probable cause, they can request a search warrant—usually obtained electronically or by phone.
Once a judge signs the warrant, the blood draw becomes mandatory. Refusing a court-ordered draw can lead to additional penalties or forced compliance. You cannot refuse a blood draw in this instance.
How THC Blood Tests Work
When a blood sample is taken, a toxicology lab analyzes it for active THC (the psychoactive component) and THC-COOH (an inactive metabolite). Only active THC counts toward the legal standard.
- A result of 5 nanograms per milliliter (ng/mL) or higher within two hours of driving is a per se DUI under RCW 46.61.502.
- Even below 5 ng/mL, prosecutors can still pursue charges if other evidence shows impairment.
Search Warrants Replaced “Blood Warnings”
Older DUI procedures used “Implied Consent Warnings for Blood,” allowing warrantless draws in limited circumstances. Washington courts ruled those practices unconstitutional, leading to the current warrant-based system. Officers now rely on probable cause and judicial approval rather than automatic consent.
What Happens After the Blood Test
Once blood results are returned, prosecutors combine the toxicology report with the officer’s observations, field test results, and DRE evaluations. Together, these determine whether to file DUI charges. The lab process can take months, which is why marijuana DUI cases often move slower than alcohol cases. The prosecution has 2 years to file charges and recently has been filing cases 23 months after the initial arrest. Some courts (like Puyallup) will file charges without blood results and proceed if they believe they can prove a DUI without blood results.
Key Takeaways
- There is no roadside THC test—officers depend on training, field tests, and DRE evaluations.
- Blood draws require a search warrant, unless you consent voluntarily.
- Refusing a warranted blood draw can lead to additional charges.
- THC results over 5 ng/mL meet Washington’s per se DUI threshold.
- Marijuana DUIs rely heavily on officer observation and lab confirmation.