A THC result of 5.00 ng/mL or more can support a Washington DUI charge without separate proof that a driver appeared impaired. But a result below 5.00 ng/mL doesn’t end the case because Washington also allows the state to allege that cannabis affected the driver.
Reading that number correctly requires examining the surrounding evidence. Led by Attorney Scott W. Lawrence, whose background includes chemistry and forensic investigation, we examine how the stop, sample collection, laboratory testing, and driving evidence fit together.
Washington Law Provides Two Paths to a Cannabis DUI
RCW 46.61.502 sets out two ways the state can try to prove a cannabis DUI involving an adult driver. Identifying the theory at issue guides the review of the evidence.
The 5.00 Ng/mL Standard
Under the concentration-based provision, the state alleges that a driver’s blood contained active THC at 5.00 ng/mL or higher within two hours after driving. The concentration itself is the legal basis for the charge, so the state doesn’t need to establish separately that the driver appeared impaired under this theory.
Active THC is the psychoactive cannabis compound used for this statutory standard. A laboratory report showing 5.00 ng/mL or more raises questions about when the driving and collection occurred and whether the result measures active THC rather than another cannabis compound.
The Affected-By Theory
The separate theory is that the driver was under the influence of or affected by cannabis or another drug while driving. The state may rely on driving behavior, officer observations, field sobriety tests, statements, a Drug Recognition Expert evaluation, and toxicology evidence.
A result below 5.00 ng/mL doesn’t establish innocence under this theory. RCW 46.61.506 allows the result to be considered with other evidence, so the question is whether the complete record supports the allegation that cannabis affected the driving.
What a THC Test Measures
A blood test provides a laboratory measurement from the time the sample was collected. It doesn’t reveal exactly when or how cannabis was consumed or directly measure how the person felt while driving.
Toxicology reports often list more than one cannabis-related compound, so the labels matter:
- Active THC: The compound used for Washington’s 5.00 ng/mL statutory threshold.
- THC-COOH: An inactive metabolite created as the body processes THC. Its presence can indicate prior cannabis exposure, but it doesn’t establish the statutory THC level.
- The reported concentration: A measurement at the time of collection, not a precise answer to when or how cannabis was consumed.
In State v. Fraser, the Washington Supreme Court upheld the 5.00 ng/mL concentration-based provision. The court also recognized an important scientific difference between THC and alcohol: no universal blood-THC level correlates with impairment in the same way Washington treats a 0.08 blood alcohol concentration.
The number therefore has a defined legal role, but it doesn’t independently answer every factual question about impairment, consumption timing, or the reliability of the testing process.
How Officers Obtain Blood Evidence
Cannabis doesn’t have a portable roadside test that measures active THC concentration. Officers generally build the initial case through observations before seeking a blood sample.
Roadside Observations & Evaluations
An officer may describe driving conduct, physical observations, statements, and performance on field sobriety tests. In some cases, the officer requests a Drug Recognition Expert evaluation, a structured assessment intended to identify signs associated with drug use or impairment.
These observations should be reviewed in context. Patrol-vehicle or body-camera video may show the instructions, testing conditions, the driver’s responses, and whether the written report matches the recording.
Consent & Warrants for Blood Collection
An officer may ask a driver to provide a blood sample voluntarily. If the driver doesn’t consent, officers may seek a warrant. The application and warrant can matter because they show what information supported the request for blood evidence.
A qualified professional then collects the sample, which should be sealed and sent for analysis, often to the Washington State Patrol Toxicology Laboratory. Under RCW 46.61.506, the analysis must use methods approved by the state toxicologist and be performed by a permit holder.
Timing Affects How the Result Is Used
The statutory two-hour period runs from the time of driving, not from an officer’s first contact with the driver. A careful timeline should account for the stop, roadside testing, transport, warrant process, medical-facility intake, and actual collection time.
When the result is 5.00 ng/mL or higher within the statutory window, the state may use the concentration-based theory. A sample collected more than two hours after driving can still be offered as evidence of a qualifying concentration during that window, but the connection between the later result and the driving time becomes a key issue.
Any active THC result above 0.00 may also be offered as evidence under an affected-by allegation. That doesn’t erase the state’s burden to prove the charge under the applicable theory. It makes the other evidence, including the driving, observations, and test administration, particularly important.
The DUI statute also provides an affirmative defense when a driver proves by a preponderance of the evidence that cannabis consumed after driving caused the concentration to reach 5.00 ng/mL or more. Because this issue depends on the specific facts, a reliable timeline and supporting records matter.
Different Rules Apply to Drivers Under 21
RCW 46.61.503 creates a separate rule for drivers under 21. A blood test within two hours of driving that shows active THC above 0.00 ng/mL but below 5.00 ng/mL can support that separate offense.
A medical marijuana authorization doesn’t exempt a driver from Washington DUI laws. The relevant questions remain the applicable statute, the reported active THC concentration, the collection timeline, and the state’s other evidence about the driving.
Records That Matter in a Forensic Review
A meaningful review begins with the evidence trail, not a conclusion based on one laboratory number. We examine whether the state’s account remains consistent from the initial stop through the reported toxicology result.
Materials worth preserving and reviewing include:
- Patrol-car, body-camera, and other video of the stop and roadside testing.
- Roadside test records and any Drug Recognition Expert evaluation.
- The warrant, consent documentation, collection records, and chain-of-custody materials.
- Laboratory and toxicology reports, testing methodology, gas chromatography results, and machine calibration records.
Attorney Lawrence is certified to administer standardized field sobriety tests and has training through the International Order of Police and the National Highway Traffic Safety Administration. In our Drug DUI defense work, we apply that training and a forensic review of the state’s evidence to identify the questions a THC result alone can’t answer.
Preserve the stop evidence, collection records, laboratory materials, and timeline before memories fade or recordings are lost. Call Snohomish Law PLLC at (425) 321-3487 to discuss the result and the evidence behind it.