Most people treat the number as the case. A machine produced a figure, the figure was above 0.08, and that appears to be the end of the discussion.
It is not. Texas uses two very different forms of chemical testing, they measure different things, and each has its own distinct catalogue of failure modes. Knowing which one you are facing tells you where the case is actually fought.
What Each Test Measures
A breath test does not measure the alcohol in your blood. It measures alcohol in deep lung air and converts that figure into an estimated blood alcohol concentration using a fixed assumed ratio of roughly 2100:1. The instrument approved for statewide use in Texas is the Intoxilyzer 9000.
That conversion is where the first problem lives. The 2100:1 figure is a population average, not a personal measurement. Real blood-to-breath ratios vary meaningfully between individuals — and for someone whose true ratio sits below the assumed figure, the instrument reports a number higher than their actual blood alcohol concentration. Nobody measures your personal ratio before the test.
A blood test measures alcohol concentration directly, typically by gas chromatography, on a sample drawn into tubes containing a preservative and an anticoagulant. It is the more direct measurement — but it introduces a collection chain, a laboratory, and a delay that a breath test does not have.
The Two Compared
| Breath | Blood | |
| What it measures | Alcohol in deep lung air, converted by an assumed ratio | Alcohol concentration in the sample itself |
| When it happens | Shortly after arrest, at the station | Often hours later, at a hospital or by a warrant |
| Sample kept | No — the breath is gone | Yes — the sample can be retested |
| Main vulnerability | The assumed ratio, physiology, and machine upkeep | Collection, preservation, chain of custody, and timing |
Where Breath Tests Break Down
- The assumed ratio. Individual variation in blood-to-breath ratio can push a reading above a person's true concentration.
- Reflux and belching. Alcohol carried up from the stomach contaminates the deep-lung sample the instrument is supposed to be reading. This is why the observation period before the test exists — and why a lapse in it matters.
- The observation period. Texas breath testing regulations require a continuous observation period before the specimen is taken. If the operator was completing paperwork, escorting another arrestee, or otherwise not observing, that requirement was not met.
- Body temperature. An elevated temperature raises the amount of alcohol carried in breath, and nobody takes your temperature before a breath test.
- Breathing pattern. How hard and how long you blow affects the result, and the instruction given is often "keep blowing until I tell you to stop."
- Certain metabolic states. Ketones associated with uncontrolled diabetes or low-carbohydrate dieting can be read as alcohol by some instruments.
- Maintenance and certification. Instrument maintenance records, calibration checks, reference sample results, and the operator's certification status are all documented — and are all discoverable.
Where Blood Tests Break Down
- Fermentation in the tube. If the preservative is inadequate, the tube is expired, or the sample is stored improperly, micro-organisms can generate ethanol after the draw — producing alcohol that was never in the person.
- The swab. An alcohol-based skin preparation at the draw site can contaminate the sample.
- Who drew it, and how. Qualification of the person taking the specimen and the conditions of the draw are both open to examination.
- Chain of custody. Every transfer between the draw site, storage, and the laboratory should be documented. Gaps are common and consequential.
- Laboratory practice. Instrument calibration, analyst qualifications, quality control runs, and any corrective-action history at the lab are all fair ground.
The Timing Problem
A blood draw frequently happens two or three hours after the driving. The State must prove your alcohol concentration at the time you were operating the vehicle, not at the time a nurse filled a tube. Bridging that gap requires retrograde extrapolation — calculating backwards from the later result.
Texas has a governing authority on when that calculation is reliable. In Mata v. State, 46 S.W.3d 902 (Tex. Crim. App. 2001), the Court of Criminal Appeals set out what courts must weigh: the length of time between the offense and the test, the number of tests and the interval between them, and the extent to which the expert knew the individual characteristics of the defendant — things like weight, drinking pattern, and what and when the person last ate.
In practice the State's expert often knows very few of those variables. Where the extrapolation rests on assumptions rather than known facts, its reliability is squarely in issue.
Refusal, Warrants, and Your Licence
Chapter 724 of the Transportation Code sets out implied consent. Refusing carries an administrative consequence on a separate track from the criminal case:
- Refusal — under § 724.035, a 180-day suspension, rising to two years where there is a prior alcohol-related enforcement contact within the preceding ten years.
- Failure (testing at or above the limit) — under Chapter 524, a 90-day suspension for a first occurrence and one year for a subsequent one.
Two things people get wrong here. First, the administrative case has its own clock — a hearing must be requested within 15 days of the notice of suspension, or the suspension takes effect automatically. That deadline runs while you are still absorbing the arrest.
Second, refusing does not mean no sample is taken. It means the State needs a warrant. As covered in our article on the 12-step DRE evaluation, State v. Villarreal, 475 S.W.3d 784 (Tex. Crim. App. 2014), holds that a warrantless, non-consensual blood draw under the implied consent and mandatory-blood-draw provisions violates the Fourth Amendment absent a warrant or an applicable exception. Whether a valid warrant supported the draw in your case is a threshold question.
The Second Sample Almost Nobody Requests
This is the most underused provision in Texas DWI practice, and it is time-limited.
Under Transportation Code § 724.019, a person who submits to a specimen at an officer's request may — on request, and within a reasonable time not exceeding two hours after arrest — have a physician, qualified technician, chemist, or registered professional nurse of their own choosing take an additional blood specimen for analysis. The officer must allow a reasonable opportunity to contact that person.
An independent sample, taken close in time, is the single most effective answer to a disputed State result. The two-hour window means it is available only to people who know about it at the moment it matters — which is very few.
What This Means for Your Case
Both test types are challengeable, and the challenge is documentary. Maintenance and calibration records, operator certification, the observation-period log, the blood warrant, the collection kit, the chain-of-custody paperwork, and the laboratory's quality control all exist — and are obtained by asking for them, promptly.
For how the body processes alcohol over time, see our article on how long alcohol stays on your breath. If you are facing a DWI in Collin or Denton County, our DWI defense lawyers can request the full testing file and the ALR hearing before those deadlines pass. Contact us for a free consultation.
Frequently Asked Questions
Is a blood test more accurate than a breath test in Texas?
A blood test measures alcohol concentration directly, while a breath test estimates it from deep lung air using an assumed ratio of roughly 2100:1. That makes blood the more direct measurement, but it introduces its own problems — a collection chain, laboratory handling, and a delay of hours that requires retrograde extrapolation to relate the result back to the time of driving.
Can you request your own blood test after a Texas DWI arrest?
Yes. Under Transportation Code § 724.019, a person who submits to a specimen at an officer's request may ask to have a physician, qualified technician, chemist, or registered professional nurse of their own choosing take an additional blood specimen. The request must be made within a reasonable time not exceeding two hours after arrest, and the officer must allow a reasonable opportunity to contact that person.
What happens to your license if you refuse a breath test in Texas?
Refusal triggers an administrative suspension under Section 724.035 — 180 days for a first refusal, or two years where there has been a prior alcohol-related enforcement contact within the previous ten years. Testing at or above the limit carries a 90-day suspension for a first occurrence under Chapter 524. A hearing must be requested within 15 days of the notice or the suspension takes effect automatically.
What is retrograde extrapolation in a DWI case?
It is the calculation used to estimate what a person's alcohol concentration was at the time of driving, working backwards from a sample taken hours later. In Mata v. State, 46 S.W.3d 902 (Tex. Crim. App. 2001), the Texas Court of Criminal Appeals identified what determines reliability: the time between the offense and the test, the number of tests and the interval between them, and how much the expert actually knew about the individual's characteristics.
Can police take blood without a warrant in Texas if you refuse?
Generally no. State v. Villarreal, 475 S.W.3d 784 (Tex. Crim. App. 2014), holds that a warrantless, non-consensual blood draw taken under the implied consent and mandatory-blood-draw provisions violates the Fourth Amendment without a warrant or an applicable exception. Refusal still carries the separate administrative licence suspension.