Updated April 29, 2016
After you drink alcohol, your body begins to process it. Your blood alcohol concentration (BAC) rises as the alcohol is absorbed into your body, then falls as the alcohol dissipates or burns off. This change in your BAC over time can come into play in a DUI (driving under the influence) trial. At trial, the prosecutor tries to prove that you were over the legal limit, and the defense attorney attempts to at least cast doubt on the prosecutor’s case.
This article discusses how prosecutors and defense attorneys use the phenomena of rising and falling BAC in DUI cases. (For more general information, see What Does the Prosecution Have to Prove in a DUI Case?)
DUI Stops and Measuring BAC
After arresting a driver on suspicion of DUI, police typically measure BAC using a breath or blood test. (The law presumes that drivers have consented to DUI testing.) Drivers with BACs that are over the legal limit—.08% or higher is illegal for drivers over 21 years old—can be convicted of a per se DUI. In a per se DUI prosecution, the focus is on the driver’s BAC—not whether the driver’s ability to drive safely was actually impaired.
(Be aware that all states also have laws against driving while impaired, sometimes called “impairment DUI” or “DUI-less-safe.” These laws apply even where there’s no chemical test showing BAC. To find out more about the difference between the two types of DUI, see What’s the Difference Between Per Se and Impairment DUIs?)
Whatever the rule in your state, at a per se DUI trial, the prosecution will try to prove that your BAC was over the limit during the relevant time period. A good defense attorney, however, will be looking for ways to poke holes in the prosecution’s case.
Rising BAC Defense
In states where the prosecution has to prove your BAC was over the limit at the time of driving, defense attorneys often use a “rising BAC” defense at trial. The theory of this defense is that you were under the legal limit when you drove, but, because the alcohol was still being absorbed into your system, your BAC rose to an illegal level by the time you were tested.
To establish this defense, an attorney might bring in an expert witness who can look at your measured BAC and “extrapolate” what your approximate BAC was at the time of driving. An expert testifying that your BAC was likely below the legal limit while you were driving could improve your chances of beating the charge.
Everyone metabolizes alcohol at different rates. For example, women typically absorb alcohol more quickly than men, and alcohol ordinarily leaves a larger person’s body at a faster rate than it would for someone smaller. Experts often rely on these factors—gender and body size—and a number of other circumstances to extrapolate a driver’s BAC. These other circumstances might include:
- the number and type of drinks the driver had
- how the driver consumed the drinks (shots or sips)
- when and what the driver last ate
- the passage of time (between consumption and driving, consumption and testing, and driving and testing), and
- any medical conditions the driver had that affect liver function.
By looking at factors like these in combination with known average absorption and dissipation rates, an expert can estimate the driver’s BAC at the time of driving.
Falling BAC From the Prosecution’s Perspective
If, when tested, your BAC was below but near the legal limit, the prosecutor might argue to the jury that your BAC was actually higher when you drove but it had fallen by the time you were tested. (The falling blood alcohol phenomena is sometimes called alcohol “elimination.”) An expert witness for the prosecution might use some of the factors mentioned above to extrapolate that your BAC at the time of driving was higher than when it was measured by police. The jury could find you guilty of a per se DUI even though your BAC was below the legal limit when you were tested.
Talk to a DUI Attorney
DUI laws differ by state, and the facts of every case are different. It’s always a good idea to talk to an experienced DUI attorney if you’ve been arrested for or charged with DUI. You can speak to an attorney about DUI expert witnesses and whether the facts of your case might give you a viable defense.