Texas DWI Tests and Evidence: Breath, Blood, and Field Sobriety

Why DWI evidence is more fragile than it looks

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.

The firm’s edge here is real. Managing Partner Deandra Grant holds the ACS-CHAL Forensic Lawyer-Scientist designation, a Master’s Degree in Pharmaceutical Science, and a Graduate Certificate in Forensic Toxicology. She was trained as an operator and maintenance technician of the Intoxilyzer, teaches gas chromatography at Axion Labs in Chicago, and authors the Texas DWI Manual. Partner Douglas E. Huff holds the same ACS-CHAL designation. These challenges are made at the level of the chemistry, not just the legal argument.

The legal BAC limit in Texas

The legal limit is a blood or breath alcohol concentration of 0.08 for most drivers. It is 0.04 for commercial drivers and any detectable amount for drivers under 21. But a number is only one way the state can prove intoxication. It can also try to prove you lost the normal use of your mental or physical faculties, even below 0.08. See the legal BAC limit.

Field sobriety 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 rates the prosecution cites (about 88 percent for HGN, 79 percent for the walk-and-turn, and 83 percent for the one-leg stand) only hold when the test is given exactly to protocol, to a person from the studied population. On the roadside, that rarely happens. Ordinary factors like nerves, footwear, the road surface, weight, age, and medical conditions can cause a sober person to fail. NHTSA’s own field studies underscore the problem: in the 1998 San Diego validation study, false positives were roughly six times more common than false negatives. See the field sobriety tests, the HGN eye test, and the walk-and-turn test.

The breath test (Intoxilyzer 9000)

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. Common problems include mouth alcohol during the 15-minute observation period, the assumed 2,100:1 partition ratio (real ratios range from about 1,100:1 to 3,400:1), calibration and maintenance gaps, interferents the device reads as alcohol, and operator error. Medical conditions matter too, GERD can carry alcohol from the stomach into the mouth and inflate a reading. See the DWI breath test.

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. Texas forensic labs analyze blood using headspace gas chromatography with a flame ionization detector (GC-FID). Blood cases break down at collection (an alcohol swab, the wrong tube, an unqualified draw), the tube and preservative (too little preservative allows in vitro fermentation), coelution in the analysis, hospital-vs-forensic conversion, chain-of-custody gaps, and lab and analyst error. A blood draw also has to be lawful in the first place; under Missouri v. McNeely, police usually need a warrant, consent, or a genuine emergency. See the DWI blood test and the blood search warrant.

The science of alcohol in the body

A test result is a snapshot, taken sometime after you were driving. What your BAC was at the wheel can be very different from what the machine measured later. Two concepts often decide these cases: absorption and elimination (a BAC can still be rising at the time of testing, so your level while driving may have been below the limit), and retrograde extrapolation (the state’s attempt to work backward from the test using assumptions that are frequently wrong). These are exactly the issues a forensic-trained lawyer can turn into reasonable doubt.

Refusing a breath or blood test

You can refuse testing in Texas, but refusal has consequences under the implied consent law, including an automatic license suspension and the chance that police seek a warrant for your blood. Whether refusing helped or hurt depends on the facts of your case. See refusing a DWI test and implied consent.

How a lawyer challenges DWI test results

Strong DWI defense attacks the evidence at its source. That can mean obtaining the machine’s maintenance and calibration records, the lab’s data and analyst notes, and the video of the field tests, then showing where the procedure or the science failed. See challenging the breath test, challenging the blood test, and the DWI defense framework.

Frequently Asked Questions

What is the legal alcohol limit in Texas?

A BAC of 0.08 for most drivers, 0.04 for commercial drivers, and any detectable amount for drivers under 21.

Do I have to take field sobriety tests in Texas?

No. Field sobriety tests are voluntary, and they are designed in a way that is easy to fail even when sober.

How accurate is the Intoxilyzer breath test?

Its results can be affected by calibration, maintenance, operator error, mouth alcohol, and conditions like GERD, so the number is far from infallible.

Can a DWI blood test be wrong?

Yes. Blood results can be undermined by collection, the wrong tube, storage, chain-of-custody gaps, and lab analysis errors.

What is retrograde extrapolation?

It is the state’s attempt to estimate your BAC at the time of driving from a later test, using assumptions that are often unreliable.

Should I refuse a breath or blood test?

Refusal carries a license suspension and police may seek a warrant, so whether it helps depends on your situation.

Does Texas require two breath samples?

Yes. A valid Texas breath test requires two separate samples that must agree within 0.02 of each other. Disagreement between them is a basis to challenge the result.

Is the Evidence Against You Actually Reliable?

Breath and blood results are challenged every day. Get a free, confidential review of the testing in your case with a forensic-trained Texas DWI defense lawyer. Deandra Grant Law serves Dallas, Fort Worth, North Texas, and Waco. Call (214) 225-7117.

Explore Tests & Evidence Topics

• Legal BAC Limit — What 0.08 means, and the other way the state proves intoxication.
• Field Sobriety Tests — The three standardized tests and why they are easy to fail.
• HGN Eye Test — How the eye test works and why nystagmus is not proof of drinking.
• Walk-and-Turn Test — The clues, the strict rules, and the conditions that cause failure.
• DWI Breath Test — How the Intoxilyzer 9000 works and the many ways it can be wrong.
• DWI Blood Test — Collection, storage, lab analysis, and the errors that undermine results.
• Refusing a DWI Test — Your right to refuse and what it costs.