The term "drunk driving" implies that the driver was actually impaired by alcohol. But while alcohol or drug impairment can lead to a DUI conviction, drivers can also be convicted of driving under the influence for having a prohibited amount of drugs or alcohol in their body. A drunk or drugged driving charge based on blood alcohol concentration (BAC) or the quantity of drugs in your system is known as a "per se" DUI.
All states have per se DUI laws that apply to the level of alcohol in a driver's blood. These laws generally prohibit driving with a BAC of .08% or more. (Get an estimate of how many drinks it takes.) However, a handful of states also have per se drug DUI laws. Per se drug DUI laws typically single out a few of the more common drugs (like marijuana, cocaine, and methamphetamine) and specify the prohibited amount. Some states even make it a per se drug DUI to operate a vehicle with any amount of certain drugs in your system.
A per se DUI is based solely on the alcohol or drug content in the driver's body. In other words, a per se conviction doesn't require proof that the driver was actually impaired by the substances ingested.
Per se DUI charges seem fairly cut and dried: You're either above the legal limit or you're not. However, there are still a few defenses that can work.
Illegally obtained evidence. Blood tests are one of the most common methods of measuring BAC. However, police generally need a warrant before they can require you to take a blood test. If police required you to submit to testing without first obtaining a warrant, you might be able to get the results of the test thrown out as illegally seized evidence.
Challenging the accuracy of the BAC test. In some cases, it's possible to challenge the accuracy of BAC testing results. For instance, breathalyzer results can be unreliable if the officer doesn't follow proper procedures or the machine isn't maintained or calibrated correctly. And blood samples that aren't stored properly can ferment, making the final measurement artificially high.
Rising-blood-alcohol defense. After drinking alcohol, your BAC doesn't stand still. While the alcohol is absorbing into your body, your BAC rises. And at some point later, once the alcohol is metabolized, your BAC begins to fall. With the rising-blood-alcohol defense, a defendant argues that in the time between being pulled over and when the BAC measurement was taken, his or her BAC rose above the legal limit. Usually, the rising-blood-alcohol defense requires the testimony of an expert who knows the science behind alcohol metabolization. And your chances of success are generally better when your BAC was close to the .08% limit.
The facts of each case are different. So, if you've been arrested for driving under the influence, it's best to talk to an experienced DUI attorney. A qualified DUI lawyer can review the facts of your case and let you know if you have any possible defenses.