By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist
The question: How accurate is the breathalyzer in Texas?
The short answer: Less accurate than the prosecution will tell the jury. Texas uses the Intoxilyzer 9000, an infrared breath testing instrument that produces a single number presented in court as a blood alcohol concentration. That number is an indirect measurement built on physiological assumptions that do not fit every person, influenced by medical conditions the machine cannot see, and subject to procedural errors that happen more often than the State likes to admit. A breath test result is evidence. It is not a verdict.
Here is the longer answer: what the Intoxilyzer 9000 actually measures, where the errors come from, and how a forensic defense takes the number apart.
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The Device: The Intoxilyzer 9000
The breath testing instrument in use across Texas is the Intoxilyzer 9000, manufactured by CMI, Inc. It replaced the older Intoxilyzer 5000 as the official breath alcohol instrument under the Texas Department of Public Safety Breath Alcohol Program. Every county that uses breath evidence in a DWI prosecution is using the 9000 including Dallas, Tarrant, Collin, Denton, McLennan, Rockwall, and every other county where our firm practices. NOTE: Whether or not to have a breath testing program is up to each individual police agency. Some agencies got rid of their breath testing programs and only offer blood testing as an option to a person arrested for DWI. This is not a county by county decision.
The 9000 is not a new invention. It is an iterative refinement of a decades-old measurement technology, with all the strengths and limitations of that technology. Understanding those limitations is the first step in understanding what a breath test result actually means.
How the Intoxilyzer 9000 Actually Works
The 9000 measures alcohol indirectly, using infrared (IR) spectroscopy. When you blow into the instrument, a sample of deep-lung (alveolar) air is captured in a chamber. Infrared light is passed through the chamber, and the instrument measures how much of that light is absorbed at specific wavelengths associated with ethanol. The more absorption, the more ethanol in the breath sample. The instrument then converts that breath alcohol measurement into an estimated blood alcohol concentration.
That conversion is where the trouble starts. The machine does not measure blood alcohol. It measures breath alcohol and then applies a fixed mathematical assumption to estimate what your blood alcohol must be.
The Partition Ratio Problem
The Intoxilyzer 9000 assumes that, for every unit of alcohol in your breath, there are 2,100 units of alcohol in your blood. That 2,100:1 figure is called the partition ratio, and it is the single biggest source of forensic error in breath testing.
Real partition ratios vary from roughly 1,100:1 to 3,400:1 across the population. They vary by sex, by body temperature, by hematocrit, by breathing pattern, by the phase of alcohol absorption the person is in, and by dozens of other physiological variables. A person with a true partition ratio of 1,500:1 who is blowing a 0.09 on the 9000 actually has a blood alcohol concentration well below 0.08 because the machine is assuming a ratio that does not match that person’s physiology.
The legal fiction of a fixed 2,100:1 ratio makes prosecution easier. It does not make it accurate. Scientific literature on partition ratio variability has existed for decades, and it is fair game on cross-examination of any technical supervisor who tries to defend the number.
Mouth Alcohol and the 15-Minute Observation Period
The Intoxilyzer 9000 is designed to measure alcohol from the deep lungs. It has no way to distinguish deep-lung alcohol from alcohol sitting in the mouth or upper airway. Mouth alcohol (trapped in dental work, regurgitated from the stomach, or left over from a recent drink) produces an artificially elevated reading because mouth alcohol is far more concentrated than alveolar air.
Texas DPS protocol addresses this with a 15-minute continuous observation period. Before the breath test is administered, the operator is required to observe the subject continuously for 15 minutes to ensure there has been no belching, regurgitation, vomiting, chewing, smoking, eating, or drinking. If any of those events occur, the 15-minute clock restarts. NOTE: In reality, the observation period has morphed into an “in the presence of” period. Instead of watching the test subject for 15 minutes the breath test operator is usually typing information into the instrument and not looking at the person at all.
Video review in Texas DWI cases routinely shows observation periods that were not continuous, not 15 minutes long, or that missed events the operator should have caught. When the observation period breaks down, the reading that follows is not forensically defensible.
Medical Conditions That Can Skew a Reading
Several medical conditions can inflate a breath test result through mechanisms the machine cannot detect:
- Gastroesophageal reflux disease (GERD) and acid reflux. Stomach contents, including unabsorbed alcohol, can be pushed back up into the esophagus and mouth during the test, producing a mouth-alcohol effect the observation period was meant to prevent.
- Diabetes and diabetic ketoacidosis. Elevated ketones (particularly acetone) produce an absorption signature that older breath instruments can misread. The 9000 uses multiple infrared wavelengths to try to filter this out, but elevated ketone states can still affect the analysis in some cases.
- Low-carbohydrate and ketogenic diets. Sustained ketosis from dieting raises breath acetone concentrations. In certain conditions, this can produce spurious readings or alter the instrument’s breath-sample verification.
- Dental work, dentures, and bridges. Residual alcohol can be trapped in dental appliances and released during the breath sample, again producing a mouth-alcohol effect.
- Respiratory conditions. Asthma, COPD, and other conditions affecting lung function can alter breathing pattern and sample composition, which influences the reading.
None of these conditions appear anywhere on the offense report. They surface only when a defense attorney asks the right questions of the client and subpoenas the medical records to support the defense.
Breath Temperature, Breathing Pattern, and Physiology
The Intoxilyzer 9000 assumes a breath sample temperature of 34 degrees Celsius. Every degree of variation from that assumption changes the reading by roughly six to seven percent. A person running a fever, a person who has just exercised oe a person whose breath is naturally warmer than average will each register higher than they should.
How the subject blows into the instrument also matters. Prolonged exhalation forces increasingly deep alveolar air into the sample, and alveolar air has a higher alcohol concentration than upper-airway air. Officers often coach subjects to blow harder and longer to get a satisfactory sample. That coaching can push the reading higher than a shorter, more representative exhalation would have produced.
Finally, where the subject is in the absorption, peak, and elimination curve at the moment of the test matters enormously. A person still in the absorption phase (whose alcohol is not yet fully distributed between blood and tissue) can produce a breath test reading that does not accurately reflect actual impairment at the time of driving. This is the basis for the rising-BAC defense, which requires retrograde extrapolation analysis. NOTE: Deandra Grant wrote her Master’s Thesis on Retrograde Extrapolation and it was later published twice in Counterpoint: The Journal of Science and the Law.
Calibration, Maintenance, and the Human Element
The Intoxilyzer 9000 is a precision instrument, and like every precision instrument it requires calibration, maintenance, and inspection. Under DPS Breath Alcohol Program rules, each instrument must be inspected and certified on a regular schedule by a Breath Test Technical Supervisor. Every inspection generates a paper trail:
- Maintenance records documenting repairs, replaced parts, and service events.
- Calibration records showing the reference standards used and the results obtained.
- Technical supervisor credentials documenting the qualifications of the person certifying the instrument.
- Subject test records showing every test run on the machine, including tests that were invalidated, retried, or produced unusual readings.
All of these records are discoverable. All of them are subject to challenge. Maintenance gaps, reference standard problems, technical supervisor qualification issues, and patterns of unusual readings on a specific instrument can all be developed on cross-examination.
Measurement Uncertainty: The Math the State Does Not Emphasize
Every forensic measurement has uncertainty. A mass measured on a scale is not exactly that mass. It is that mass plus or minus the uncertainty of the measurement. A blood alcohol concentration measured by a breath instrument is no different.
Manufacturer and laboratory specifications for breath testing instruments typically acknowledge a measurement uncertainty of roughly plus or minus 0.01, and often more, once real-world conditions are accounted for. That uncertainty is almost never reported to the jury. A reading of 0.08 is presented as a number certain and not as a number with a margin that could reasonably range from 0.07 to 0.09 or wider.
In scientific reporting, a measurement without its uncertainty is an incomplete measurement. In forensic science, a result reported without its uncertainty overstates the strength of the evidence. That is not a technicality. It is foundational measurement science, and it applies to every breath test result introduced in a Texas DWI trial.
Case Results
What This Means for Your Case
The accuracy of the Intoxilyzer 9000 is not a single question. It is a set of questions that have to be asked for each specific case:
- Was the 15-minute observation period actually observed? Video review tells the real story, not the report.
- Does the defendant have a medical condition that could affect the reading? GERD, diabetes, respiratory conditions, dental work (all require medical record review).
- What was the defendant’s breath temperature? Fever, illness, and recent physical exertion all matter.
- Was the defendant in the absorption phase at the time of the test? Timing of the last drink relative to the test is the foundation of the rising-BAC defense.
- What do the instrument’s records show? Calibration, maintenance, reference standards, and subject test history are all discoverable.
- What is the true measurement uncertainty of this specific reading? The single number on the ticket does not tell the whole forensic story.
A DWI defense that ignores any of these questions is leaving forensic evidence on the table. A defense that works through all of them can shift the scientific ground under the prosecution’s case.
The Bottom Line
The Intoxilyzer 9000 is a piece of scientific equipment, not an oracle. It produces estimates, not truths. Its output depends on assumptions that do not fit every human body, procedures that are not always followed, and records that do not always hold up under scrutiny. A breath test reading is the beginning of the forensic conversation in a Texas DWI case but not the end of it. If you have been arrested based on a breath test, you are entitled to a defense that takes the machine apart at the level of the science.
Breath Test Defense at Deandra Grant Law
Deandra Grant Law defends DWI and intoxication-offense cases across North and Central Texas including Dallas, Fort Worth, Plano, McKinney, Frisco, Allen, Lewisville, Denton, Rockwall, and Waco. Our team includes an ACS-CHAL Forensic Lawyer-Scientist with a Master of Science in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology which the credentials required to cross-examine a Breath Test Technical Supervisor on partition ratio, measurement uncertainty, and instrument performance. Deandra spends an entire chapter covering breath alcohol analysis in her book The Texas DWI Manual.
If a breath test is being used against you, call Deandra Grant Law at (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation. If you took a breath test and lost your driving privileges, you still have a 15-day ALR deadline running from the date of service of the notice of suspension. Do not wait.
Have a DWI question you want answered in this series? Submit it at texasdwisite.com! You might see it featured in a future Ask Deandra post.
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