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  • Writer's pictureBrad Thornton

San Antonio DWI Attorney: Understanding the Science Behind Intoxilyzer Tests and Their Accuracy

Updated: Jan 30, 2023


Breath Alcohol testing, also known as intoxilyzer tests, are a commonly used tool by law enforcement to determine a person's blood alcohol concentration (BAC) and, as a result, whether they are driving under the influence (DUI) or driving while intoxicated (DWI). However, while intoxilyzer tests are widely used, they are not always accurate and can be challenged in court. In this post, we will take a closer look at the science behind intoxilyzer tests, the factors that can affect their accuracy and how an expert witness can assist the defense attorney in a breath testing case.


The most widely used type of breath testing instrument in Texas is the Intoxilyzer 9000, which uses infrared spectroscopy to measure the amount of alcohol in a person's breath. When a person inhales, a small sample of breath is taken and then passed through an infrared light beam. The light is absorbed differently by the alcohol in the breath, and this difference is used to determine the person's BAC.


However, there are several factors that can affect the accuracy of intoxilyzer tests, including:

  • Calibration: Intoxilyzer machines need to be regularly calibrated to ensure accurate results. If a machine is not properly calibrated, it may give false readings.

  • Mouth alcohol: If a person has recently consumed alcohol or has mouthwash, cough syrup or other substances that contains alcohol, it can affect the breath test results, leading to a higher BAC reading.

  • Medical conditions: Some medical conditions such as acid reflux, diabetes and asthma can affect the results of a breath test, leading to a higher BAC reading.

  • Operator error: A test needs to be given by a properly trained operator. Also an operator needs to use the device properly and follow the given protocol for the test.

To counter these factors, an expert witness can play an important role in a breath testing case. An expert witness can evaluate the breath testing device and the administration of the test and can help to identify any potential sources of error.


When it comes to DWI cases, it is important to have an attorney who is highly trained in DWI investigation and is able to effectively challenge the prosecution's evidence. Brad Thornton, a former prosecutor and Board Certified Criminal Defense Attorney.


As a prosecutor, he was trained in Field Sobriety Tests, Drug Recognition Testing, and Accident and Crash Reconstruction and has the experience and knowledge necessary to build a strong defense for those charged with DWI. With his unique background and skills, he can help to challenge the accuracy of the intoxilyzer test, and fight to achieve the best possible outcome for his clients.

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Attorney Brad Thornton is a highly experienced and Board-Certified criminal defense lawyer. With his background as a former Chief Prosecutor, he has a unique understanding of the criminal justice system and is able to provide comprehensive and effective representation to his clients. He is also deeply committed to ensuring that all individuals receive fair treatment in court, regardless of their background or circumstances.

Brad Thornton is dedicated to helping his clients achieve the best possible outcome for their case, whether it is in San Antonio or elsewhere in South Texas. He recognizes the stress and anxiety that can come with being accused of a crime, and approaches his clients with compassion while keeping them informed at every step of the process. His knowledge and experience make him a strong advocate for his clients.