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DWI Breath Tests in Texas — How They Work and How to Challenge Them

Breath test evidence is the foundation of thousands of DWI prosecutions in Texas every year. The Intoxilyzer 9000 is the breath testing instrument used by Texas law enforcement which replaced its predecessor, the Intoxilyzer 5000. It produces a number and prosecutors present that number to the jury as if it were a definitive measurement of the defendant’s blood alcohol concentration at the time of driving.

It is not. Breath testing is an indirect, estimated measurement of blood alcohol concentration based on assumptions that are not true for every person in every situation. At Deandra Grant Law, our forensic training gives us the scientific foundation to expose the limitations and errors in breath test evidence. Deandra has been trained as both a Maintenance Technician and Operator of the Intoxilyzer 5000 which gives her insight into breath testing that most defense attorneys do not possess.

An entire chapter in Deandra’s book, The Texas DWI Manual, is devoted to breath alcohol analysis.

How Breath Testing Works

Breath testing instruments like the Intoxilyzer 5000 and 9000 measure (in theory) the amount of ethanol in a sample of deep lung air (alveolar air) and use that measurement to estimate the concentration of alcohol in the blood. The instrument uses infrared spectroscopy: it passes infrared light through the breath sample, and ethanol molecules absorb specific wavelengths of that light. The amount of absorption is used to calculate the breath alcohol concentration, which is then converted to a blood alcohol estimate using a fixed mathematical ratio.

Why Breath Tests Are Less Reliable Than Prosecutors Claim

The Partition Ratio Problem

The Intoxilyzer 9000 uses a fixed partition ratio of 2100:1 to convert breath alcohol to blood alcohol. This ratio assumes that 2,100 milliliters of deep lung air contains the same amount of alcohol as 1 milliliter of blood. This assumption is a population average. It is not accurate for every individual.

Scientific research has shown that actual partition ratios vary from approximately 1,100:1 to 3,400:1 among individuals. For a person with a lower partition ratio (for example, 1,500:1), the Intoxilyzer 9000 will overestimate their blood alcohol concentration. This means a person whose actual BAC is below 0.08 can produce a breath test result above 0.08 simply because their body chemistry differs from the assumed average.

Mouth Alcohol Contamination

The Intoxilyzer 9000 is designed to measure alcohol from deep lung air. But if residual alcohol is present in the mouth, throat, or esophagus, it can contaminate the breath sample and produce an artificially elevated result. Sources of mouth alcohol include:

• Recent belching, burping, or acid reflux (GERD) that brings stomach contents into the mouth or esophagus
• Dentures, dental work, or oral piercings that can trap alcohol-containing substances
• Recent use of mouthwash, breath sprays, or cough medications containing alcohol
• Recent vomiting

Texas law requires a 15-minute observation period before a breath test to ensure the subject does not burp, vomit, or introduce anything into their mouth. If the observation period was not properly conducted, or if the subject experienced silent reflux (GERD) during the period, the test result may be invalid.

Medical Conditions That Produce False Results

Several medical conditions can cause the Intoxilyzer 9000 to produce inaccurate results:

• Gastroesophageal reflux disease (GERD): Stomach acid and alcohol vapors can travel into the esophagus and mouth, contaminating the breath sample
• Diabetes and ketoacidosis: Diabetic individuals may produce acetone and other ketone bodies that the Intoxilyzer 9000 can misidentify as ethanol, producing a falsely elevated reading
• Chronic obstructive pulmonary disease (COPD) and asthma: Respiratory conditions that affect the gas exchange in the lungs can alter the concentration of alcohol in exhaled breath
• Hypoglycemia: Low blood sugar conditions can produce breath compounds that interfere with testing

Instrument Calibration and Maintenance

The Intoxilyzer 9000 must be properly calibrated, maintained, and quality-checked according to the Texas Breath Alcohol Testing Program protocols established by the Texas Forensic Science Commission. We review maintenance records, calibration logs, and quality assurance data to determine whether the instrument was functioning properly at the time of the test.

Operator Error

The law enforcement officer administering the breath test must be certified and must follow specific protocols. Failures in the observation period, improper operation of the instrument, and failure to document the testing procedure can all compromise the reliability of the result.

The Intoxilyzer 9000: Texas’s Breath Testing Instrument

Texas uses the CMI Intoxilyzer 9000 as its approved breath testing instrument. The Intoxilyzer 9000 replaced the earlier Intoxilyzer 5000 series and uses dual-wavelength infrared spectroscopy. While the manufacturer claims improved reliability over earlier models, the fundamental limitations of breath testing such as partition ratio variability, mouth alcohol contamination and medical condition interference apply to the Intoxilyzer 9000 just as they applied to earlier instruments.

Our forensic training includes specific instruction on the Intoxilyzer 9000’s technology, calibration requirements, known error sources, and the Texas Breath Alcohol Testing Program protocols. This allows us to challenge breath test results with a level of technical specificity that most defense attorneys cannot achieve.

Defend Your DWI Case with Science — Contact Deandra Grant Law

If you are facing DWI charges in Texas, contact Deandra Grant Law for a free, confidential consultation. Our attorneys have the forensic training to challenge the prosecution’s evidence at a level most defense attorneys cannot match. Lead attorney Deandra Grant has defended thousands of DWI cases over 30+ years.

Call (214) 225-7117 or schedule an appointment online at texasdwisite.com.