If an officer believes you’re driving under the influence of anything, they will conduct a field sobriety test and may consult with a drug recognition expert. If officers establish probable cause, they will bring you to a precinct and ask your permission to draw your blood for testing. If officers have reason to believe you’re under the influence of something, they can get a warrant for a blood draw from a judge. If you’re in a serious accident, then a blood draw will be mandatory. (Source: Seattle Police Department)
The legal limit is five nanograms of THC per millimeter of blood.