Generally, having a high blood alcohol concentration (BAC) is bad news for a DUI defendant. In every state, it's illegal—a "per se" DUI—to drive with a BAC of .08% or more. However, there are several ways of challenging BAC test results. One of the more common ways of fighting a DUI involving a high BAC is with the "rise-blood-alcohol defense."
When blood alcohol testing reveals a high BAC, it doesn't necessarily mean the motorist was over the limit while driving. Blood alcohol concentration is constantly changing: At any given time, BAC is either rising or falling. And there's always at least some delay between when a motorist is actually driving and BAC testing occurs. So, motorists with high BACs can sometimes rely on the rising-blood-alcohol defense to beat a DUI charge.
Here's how the rising-blood-alcohol argument typically goes: The driver's BAC was below the legal limit while driving but rose above the limit during the delay between when the driver was stopped by police and testing occurred. But could this actually happen with a person's BAC? In general, the answer is yes.
Everyone metabolizes alcohol at different rates. For example, the body of a smaller person would normally process alcohol more quickly than a larger person. But, regardless of body type, it takes time for alcohol to reach a person's bloodstream. So, there's a window of time between when a person ingests alcohol and when that alcohol actually increases the person's BAC. With the rising-blood-alcohol defense, the motorist tries to convince the jury that his or her driving occurred in the window before the BAC spike happened.
State DUI laws vary. Some states have largely eliminated the rising BAC defense by defining a per se DUI as having a BAC of .08% or more within a certain time (typically, two or three hours) of driving. For drivers in these states, the rising BAC defense generally isn't an option.
Normally, establishing the rising-blood-alcohol defense requires testimony from an expert. In other words, the science of how alcohol metabolism works needs to be explained to the jury by a witness who's knowledgeable about the process. This can sometimes be accomplished through cross-examination of a prosecution witness. But oftentimes, the defense will call their own expert to testify on the subject.