Defending Against a DUI: Challenging DUI Chemical Tests

By , Attorney University of San Francisco School of Law
Updated 8/11/2020

In all states, an adult who drives with a blood alcohol content (BAC) level of .08% or above is guilty of a DUI or DWI. (The limit in Utah is now .05%.) For drivers under the age of 21, almost all states set the limit at .01% or .02%.

Police officers often administer chemical tests to measure the BAC in your breath, blood, urine, or saliva. The results of these tests are usually a big part of the prosecution's case. So, successfully challenging the validity of these tests, can mean the difference between winning and losing a DUI case.

Implied Consent Advisements

In many states, if you refuse to take a chemical test, your license may be automatically suspended. However, the laws of some states require police to advise you of the refusal consequences prior to requesting that you take a DUI chemical test. When the arresting officer fails to give you the mandatory advisement, the results of the test might not be admissible in court or in a license suspension hearing.

Similarly, a few states require police officers to offer a choice between tests (usually, between breath or blood). If officers fail to comply with the law, this could affect the admissibility of test results.

Improper Procedures Used for DUI Chemical Tests

States usually have requirements as to how chemical tests must be administered. Depending on the type of test, the requirements might be related to how the machines are calibrated and maintained or medical protocol to follow.

If the officer or person actually performing the testing did not comply with any of the legal requirements, the results might not be admissible against the driver.

Challenging the Accuracy of BAC Test Results

A wide range of factors can affect chemical tests, rendering the test results inaccurate. Challenging the accuracy of chemical BAC tests requires technical and legal expertise. And a successful defense related to the accuracy of DUI test results often involves the testimony of an expert witness. Here are a few of the more common examples of how DUI lawyers attack the reliability of BAC chemical tests.

Breath-Test Observation Periods and Residual Alcohol

Breathalyzers are designed to measure the amount of alcohol present in a person's lungs. So, to obtain a proper measurement, it's imperative that the suspect not have any residual alcohol in his or her mouth. Otherwise, the breathalyzer measurement can be artificially high. To guard against this problem, police are supposed to observe the suspect for a certain period of time (ten minutes or so) prior to giving the breath test. If the police fail to follow this procedure, there's no way of knowing whether the breath-test measurement is accurate.

Breathalyzer Error Due to Improper Maintenance

Many breathalyzer machines are fairly accurate if maintained properly. However, these machines—even under optimal conditions—have an inherent margin of error. And when breathalyzers aren't properly maintained, the possible error is increased. So, defense attorneys can sometimes cast doubt on the reliability of breath-test results if law enforcement hasn't kept the machine in proper working order.

Testing During the Absorption Phase

It takes time—about 45 minutes to three hours—for alcohol to become fully absorbed into a person's body. So, if you drink right before getting in your car, the alcohol from your drinks might not be causing impairment while you're driving, even if a chemical test administered later shows a BAC over the legal limit (your BAC will rise between the time you were driving and the time that the test was administered—often an hour or more after the police stop you).

Blood Test Fermentation

Blood tests are usually the most reliable of the various ways of measuring drugs and alcohol in a person's system. However, if blood samples aren't handled and preserved properly, they can ferment. When fermentation occurs, the amount of alcohol in the sample increases, resulting in an inaccurately high measurement of alcohol.

Getting Legal Help

Mounting many of these DUI defenses may require the expertise of a lawyer. If you are accused of a felony DUI or DWI, you should talk to an attorney—be sure to find one experienced with handling DUIs or DWIs.

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