A "per se" DUI (driving under the influence) charge is based on a driver having a certain amount of drugs or alcohol in his or her blood. All states have DUI laws that prohibit driving with a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah). And a number of states—including Ohio, Washington, and Georgia—also have laws making it illegal to operate a vehicle with a certain concentration of drugs in your system.
Though it's also a crime to drive while impaired by drugs or alcohol, per se DUI laws don't require proof that the driver was actually affected by the drugs or alcohol ingested. To get a per se DUI conviction, the prosecution just needs to show the motorist was driving with the prohibited concentration of drugs or alcohol in his or her body. And, with drug DUI laws in some states—like Oklahoma, Pennsylvania, and Illinois—any amount of a controlled substance in your system can lead to a conviction.
Implied consent laws generally require all drivers lawfully arrested for driving under the influence to take a BAC test (typically, a blood or breath test) if asked to do so by law enforcement. So, in most DUI cases, the prosecution will have test results showing the amount of drugs or alcohol in the driver's system. Generally, test results are crucial evidence for proving a per se DUI charge.
Sometimes BAC test results make it easy for the prosecution to prove a DUI charge. But in other cases, defense attorneys are able to poke holes in the prosecution's evidence by finding problems with the testing equipment or procedures. The "rising-blood-alcohol defense," for example, is often used by defense attorneys to challenge BAC evidence.
Every case is different. So, the best way to find out where you stand is to talk to an experienced DUI attorney.