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In fighting a DUI case, it may be necessary to seek the opinion of an expert witness. Much of the evidence that makes up a DUI prosecution is highly scientific in nature and may require an understanding of complicated biological and physiological processes. In challenging the prosecution’s case, it may be necessary to enlist the services of an expert witness to prepare a report or even to testify at a hearing or at trial.
There are many ways in which an expert witness may be helpful to a defendant who has been charged with driving under the influence. In some cases, the defendant may have a valid “rising blood alcohol” defense. This defense relies on an understanding of how the human body absorbs alcohol once consumed. After a person drinks alcohol, his or her measurable blood alcohol concentration will gradually increase. Once peak absorption is reached, the BAC will level off and then slowly decrease as the body eliminates alcohol. If a person has just consumed alcohol and is in the “rising” phase of absorption, he or she would be expected to have a higher BAC when tested following an arrest than at the time of driving.
In these cases, it may be necessary for an expert to testify regarding “retrograde extrapolation,” which is the process of determining a person’s BAC at an earlier point in time based on a test result obtained after driving has occurred. If the expert can show why a driver’s BAC was actually under 0.08 percent at the time of driving, the defendant would have a strong defense if charged with driving with a BAC of 0.08 percent or higher. The expert could testify both at the defendant’s criminal trial as well as at his or her DMV administrative hearing, where the chief concern is whether or not the driver had a BAC of 0.08 percent or higher at the time of driving.
If the driver selected the blood test, he or she may want to have the blood sample retested and then have an expert witness testify regarding the results of the retest. This is often done in driving under the influence of drugs cases. The law enforcement agency is required to preserve a portion of the blood sample and make it available to the defense in order to have independent testing conducted. The retest may show that the drugs detected were at levels so low that they would have no intoxicating effect. In many cases, the law enforcement testing will only reveal whether or not a blood sample tested negative or positive for a certain drug. Further testing can determine how much of a drug was in the defendant’s blood. The expert could testify about the retest and can explain what the results mean in regards to the defendant’s level of impairment.
Many expert witnesses formerly worked for law enforcement agencies and can testify regarding whether or not proper procedures were followed. In many cases, the testimony of an expert will have a critical impact on a DUI case.