All states have two types of DUI charges. A person can be convicted of a “per se” DUI based on driving with a certain amount of drugs or alcohol in his or her system. And a person can be convicted of a DUI for driving while actually impaired by drugs or alcohol.
To get a per se DUI conviction, the prosecution generally just needs to show the driver had a Blood Alcohol Concentration (BAC) of .08% or more while at the wheel. Some states also have per se drug DUI laws that apply the same concept to controlled substances. For instance, a state might make it a per se drug DUI to drive with .02 milligrams or more of cocaine per liter of blood in your system.
An impairment DUI charge is based on how the driver was affected by drugs or alcohol ingested. States have different standards for what counts as “under the influence” or “intoxicated.” Some states define an impairment DUI as driving while “substantially impaired” by drugs or alcohol, while other states only require proof that the substances ingested had “some effect” on the driver.