If your DUI case comes to trial, you can be all but certain that the prosecutor will introduce evidence of your BAC while you were driving. This is a fairly neat trick because, of course, no chemical test was given to you when you were behind the wheel. The test you took that led to the BAC results to be used against you was taken at the police station quite some time later; perhaps an hour or two after you actually drove. How then, can that BAC level be accurate? The short answer is it may not be.

Statutory Presumption

Under California law, VC 23152(b) states that it is a rebuttable presumption that the person had 0.08 percent blood alcohol level, if a chemical test was performed within three hours after driving, which resulted in a 0.08 blood alcohol level or more. Even though this presumption exists, the prosecutor will typically call an expert witness, such as a crime lab toxicologist, to explain retrograde extrapolation, the term used to describe the theory whereby test results taken at a later time can be used to determine your BAC from an earlier time.

Guilty Until Proven Innocent?

When a presumption arises, the legal effect is to shift the burden of proof to the other part. In a DUI case, therefore, you are essentially presumed to be guilty and must prove otherwise. Fortunately, the presumption is rebuttable, which means your Orange County DUI lawyer can potentially create doubt as to the accuracy of the results.

Assumptions Made

Although there are scientific principles involved in the measurement of the absorption and elimination of alcohol, the efficacy of retrograde extrapolation relies on two assumptions:

• When you were driving, the alcohol you consumed had been fully absorbed into your bloodstream, and between the time you drove and when the test was administered you were in the elimination phase.
• Your individual elimination rate is known.

There are many factors that impact the absorption and elimination of alcohol, but among the most variable are the type and quantity of alcohol consumed, whether the alcohol was consumed on an empty stomach, or if not, how much food was eaten, and how long after drinking you were pulled over by the police.

Additionally, most Orange County forensic labs use the same elimination rate for every suspect, and the simple fact is that while elimination rates typically fall within an average range, they are unique for every individual.