The Evidence in a Texas DWI Case
Most Texas DWI cases are built on three kinds of evidence, and each is presented as science. Field sobriety tests at the roadside, where the officer counts defined clues rather than forming an impression. A breath test on the Intoxilyzer 9000, which measures breath and converts it to a blood figure using an assumed ratio. And a blood test analyzed by gas chromatography in a laboratory. Each has documented conditions under which it produces an unreliable result, and each is attacked through its own record: the video and the protocol, the instrument's calibration and maintenance logs, or the tube, the chain of custody, and the chromatogram.
This is the largest section on the site, and that is the point. Twenty-two pages, roughly one for every distinct piece of evidence the State can bring to a Texas DWI trial, written by a lawyer who holds the ACS-CHAL Forensic Lawyer-Scientist designation and has taught the field sobriety tests as a certified instructor. Most DWI pages describe the tests. These take them apart.
The Three Kinds of DWI Evidence
Most Texas DWI cases are built on three kinds of evidence: field sobriety tests at the roadside, a breath test on the Intoxilyzer 9000, or a blood test analyzed in a lab. Each is presented as science, and each has documented ways it produces unreliable or wrong results.
A breath number or a lab report looks objective, so juries tend to trust it. But every method the state uses rests on assumptions, equipment, and human steps that can fail. The defense that wins a DWI usually does not argue that the science is fake. It shows, point by point, where this test, on this night, with this machine or this sample, did not produce a reliable result.
One thing to carry into every page below. A number is not the only way the State can prove its case. Texas defines intoxication two ways, and losing the normal use of your mental or physical faculties is a complete route to a conviction with no test at all. That is why the roadside evidence matters even in a case with a lab report, and why a result under the limit does not end a case.
This page routes. Each subject below has its own page, and the detail lives there.
The Roadside Tests
Field sobriety tests are the roadside exercises an officer uses to build probable cause. They are voluntary in Texas, and you can decline them. There are three standardized tests recognized by the National Highway Traffic Safety Administration: the horizontal gaze nystagmus (eye) test, the walk-and-turn test, and the one-leg stand test.
These tests are scored on clues, and the rules for giving and grading them are strict. The accuracy figures the prosecution cites hold only when a test is given exactly to protocol, to a person from the studied population. On the roadside that rarely happens, and ordinary factors like nerves, footwear, the road surface, weight, age, and medical conditions can cause a sober person to fail.
- Field Sobriety Tests. The three standardized tests, how they are scored, and why administration decides everything.
- The HGN Eye Test. What nystagmus is, and why its presence is not proof of drinking.
- The Walk-and-Turn Test. The clues, the strict instructions, and the conditions that produce failure.
- The One-Leg Stand Test. The divided-attention task and what it actually measures.
- Who Fails Field Sobriety Tests While Sober. The physical, medical, and environmental factors that cause sober people to fail.
- Medical Conditions Defense. The conditions that mimic impairment across both roadside and chemical testing.
- A Critical History of the SFSTs. Where the three-test battery came from, and what the validation studies actually measured.
The Breath Test
Texas measures breath alcohol with the Intoxilyzer 9000, which uses infrared light to estimate alcohol and then converts that reading to a BAC using an assumed ratio between breath and blood. That conversion, and the machine itself, introduce error.
Breath cases break down in a small number of well-documented places: the observation period before the test, mouth alcohol reaching the sample from somewhere other than deep lung air, the assumed breath-to-blood ratio applied to a person whose actual ratio is different, calibration and maintenance gaps, substances the device reads as alcohol, and operator error.
- The Breath Test. How the test is administered in Texas and what the number actually represents.
- Intoxilyzer 9000 Problems. The instrument itself, its records, and where it can be wrong.
- Mouth Alcohol and the Breath Test. The observation period, GERD, dental work, and how a mouth source inflates a reading.
- Partition Ratio Variability. The assumed conversion between breath and blood, and the range real people fall in.
- Medical Conditions That Cause False Breath Tests. GERD, diabetes, ketosis, fever, medications, and workplace solvent exposure.
- A Horse Vet Blew a 0.087 Without Drinking. What a controlled study of ethanol vapor exposure showed about the observation period.
The Blood Test
A blood test is often treated as the gold standard, but it has its own long list of failure points, from the needle to the lab.
Blood cases are attacked in sequence, and the sequence is the useful thing to understand. There is the draw itself, including who performed it and what was used to prepare the skin. There is the tube, its additives, and whether the sample was preserved well enough to keep it from changing. There is storage and transport. There is the instrument and the analytical method. And there is the paperwork that ties the vial in the lab to the person in the car.
- The Blood Test. What happens from the draw to the report, in order, including the tube, the chain of custody, and the records to demand at each step.
- In Vitro Fermentation. How a sample can generate alcohol after it leaves the body.
- How the GC-FID Works. The instrument Texas forensic labs use, explained, including coelution and what the chromatogram shows.
- Hospital Blood vs Legal Blood. Why a hospital number and a forensic number are not the same measurement.
- Why Lab Accreditation Is Not Enough. What an accreditation certificate evaluates, what it does not, and three accredited labs that failed anyway.
The Number Itself
A test result is a measurement taken at one moment. The offense is about a different moment, the one when you were driving. Everything in this group lives in the gap between the two.
- The Legal BAC Limit. What the limit is for different drivers, and the second route to a conviction that does not use a number at all.
- The Rising BAC Defense. When alcohol is still being absorbed, the level at the wheel can be lower than the level at the machine.
- Retrograde Extrapolation. The State’s attempt to calculate backward from the test to the time of driving, and the assumptions it requires.
Drug Cases
A drug case is not a breath case with a different substance. There is no per se limit for most drugs, the relationship between a blood concentration and actual impairment is far weaker than it is for alcohol, and the officer’s observations carry correspondingly more weight.
- Driving Under the Influence of Drugs. How Texas charges a drug DWI and what the State has to prove.
- Drug Recognition Experts. The multi-step evaluation protocol and how it is audited step by step.
- Roadside Cannabis Testing. The oral fluid and breath devices, and why detection is not impairment.
- The Tolerance Problem in Drug DWI. Why a blood concentration does not translate to impairment.
Who Reads This Evidence for the Defense
The pages above are not a reading list. They are a description of work that has to be done by someone qualified to do it, and that is the part where DWI defense actually separates.
Challenging a breath instrument means reading its maintenance and calibration records and knowing what a failed reference check looks like. Challenging a blood result means reading the chromatograms and the analyst’s bench notes, not just the one-page report. Challenging a field sobriety test means knowing the officer’s own training curriculum well enough to score the video against it. Those are three different literacies, and none of them is a legal argument.
Deandra Grant Law has two ACS-CHAL Forensic Lawyer-Scientists on staff, a credential built around chemistry and forensic testing rather than courtroom procedure. The firm’s DWI practice also carries field sobriety test instructor training and breath instrument operator and maintenance training, which is what lets the analysis start at the level of the science rather than the level of the paperwork.
What those credentials mean and what to ask any firm about them is on DWI lawyer credentials.
Every Page in This Section
Five shelves, one per kind of evidence. The instrument, the sample, the roadside exercises, the drug cases where there is no number at all, and the science that connects a number to the moment you were driving.
The Breath Test
The Blood Test
The Roadside Tests
Drug Cases
The Science of the Number
The science on this page is the raw material. The moves that turn it into a suppressed result, an excluded expert or a dismissal live in Defenses.
What This Evidence Feeds
Deandra M. Grant, JD, GC, MS, ACS-CHAL Forensic Lawyer-Scientist
She holds a Master of Science in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology, both from the University of Florida. She is the author of The Texas DWI Manual and has defended Texas DWI cases since 1994.
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