BAC & Breath Test Challenges San Antonio

BAC Evidence in a Bexar County DWI — What the Number Actually Proves and What It Doesn’t

Texas law defines intoxication in two ways under Penal Code §49.01: either the loss of normal use of your mental or physical faculties, or a blood alcohol concentration of 0.08 or more. When the prosecution has a breath or blood test result above 0.08, it typically leads with that number. But a BAC number from an Intoxilyzer 9000 or a laboratory blood test result is not self-proving evidence of guilt. It is a machine output or a lab result — and like any machine output or lab result, it is only as reliable as the process that produced it.

The Intoxilyzer 9000 is the breath testing instrument used by SAPD and BCSO for DWI breath tests in Bexar County. The machine must be calibrated and maintained on a documented schedule. The specific machine used in your test has a maintenance log that is obtainable through open records. The officer who administered the test must be certified to operate the Intoxilyzer 9000, and that certification has a renewal requirement. The required 15-minute observation period before the test must be documented. Any deviation from the testing protocol — or any maintenance gap in the records — supports a challenge to the reliability of the result.

Blood draws in Bexar County DWI cases are typically processed through the Bexar County Medical Examiner’s toxicology laboratory. The blood draw must be performed by a qualified technician. The sample must be properly preserved and stored. The chain of custody from the venipuncture to the lab bench must be documented and unbroken. The analyst must be properly certified and the testing methodology must be validated. Every link in that chain is obtainable and challengeable. Call 210-692-4913 today.

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Two Ways Texas Proves DWI — Two Ways We Fight Both

The prosecution can pursue both intoxication theories simultaneously. We challenge both from the first day.

Theory	What the Prosecution Uses	Our Challenge
Per Se BAC ≥0.08Penal Code §49.01(2)(B)	Intoxilyzer 9000 result or laboratory blood test	Machine maintenance, calibration, operator cert, 15-min observation, retrograde extrapolation
Loss of Normal UsePenal Code §49.01(2)(A)	Officer observations, SFST results, dashcam video	SFST protocol violations, medical conditions, fatigue, medications, environmental conditions
BAC ≥0.15 EnhancementPenal Code §49.04(d)	Blood or breath result ≥0.15 at time of test	Same machine/lab challenges + retrograde extrapolation to time of driving
Retrograde ExtrapolationExpert testimony — contested science	State expert estimates BAC at time of driving from test time	Absorption rate variables, food consumption, individual metabolism, expert cross-examination

BAC breath test challenge San Antonio DWI — Intoxilyzer 9000 maintenance records and blood draw chain of custody Bexar County

Breath Tests vs. Blood Draws — How Each Is Challenged in Bexar County

Intoxilyzer 9000 breath tests. The Intoxilyzer 9000 operates by measuring the infrared absorbance of alcohol in a breath sample. The machine applies a partition ratio of 2,100:1 to convert breath alcohol to blood alcohol concentration — meaning it assumes the alcohol concentration in 2,100 mL of alveolar breath equals the alcohol concentration in 1 mL of blood. This ratio is a population average. For individuals with a partition ratio higher than 2,100:1 — which is common — the machine overstates the actual BAC. We challenge the partition ratio assumption, the machine’s maintenance and calibration records, the officer’s certification to operate it, and the 15-minute observation period documentation on every Bexar County Intoxilyzer 9000 case.

Blood draw warrants. SAPD and BCSO officers routinely obtain blood draw warrants under Texas Transportation Code §724.017 when defendants refuse the breath test or when the officer elects a blood draw for other reasons. The blood draw must be performed by a qualified phlebotomist or medical professional. The sample must be properly preserved in a tube with the correct anticoagulant-to-preservative ratio. It must be stored under refrigeration with a documented chain of custody from collection through the Bexar County Medical Examiner’s toxicology lab. We request the complete lab file — not just the summary result — and examine every link in the chain.

BAC DWI defense attorneys San Antonio — Mark Hull and Allison Tisdale challenging breath test and blood draw evidence in Bexar County

How We Challenge BAC Evidence in Bexar County DWI Cases

Technical BAC defense requires knowing what records to request, what to look for, and how to present the challenge in the Cadena-Reeves Justice Center. These are the six things we do on every case.

01

Request the complete Intoxilyzer 9000 maintenance file

We request the maintenance and calibration log for the specific instrument used in your test, the operator certification records for the officer who administered the test, and the instrument’s error log. A maintenance gap, a failed calibration, or an officer whose certification lapsed all support suppression of the result.

02

Verify the 15-minute observation period was documented

Texas DPS regulations require the operator to observe the defendant for a continuous 15-minute period before administering the breath test to ensure no mouth alcohol contamination occurs. We verify whether this observation period was properly documented in the officer’s notes and on the Intoxilyzer printout.

03

Challenge the partition ratio and retrograde extrapolation

If the prosecution uses expert testimony to link the test-time BAC to the drive-time BAC through retrograde extrapolation, we examine the assumptions: the absorption phase calculation, the individual’s elimination rate, food consumption, and the specific physiological variables that affect the 2,100:1 partition ratio assumption.

04

Request the complete blood draw lab file

For blood draw cases, we request the complete Bexar County ME toxicology file: the collection documentation, the phlebotomist’s qualifications, the tube type and proper anticoagulant-preservative ratio, the refrigeration and storage logs, the chain of custody from collection to analysis, the analyst’s certification, and the instrument calibration records for the gas chromatograph used in the analysis.

05

Examine for fermentation and contamination

Improperly preserved blood samples can ferment in the tube after collection, producing additional ethanol that was not present at the time of the blood draw. If the sodium fluoride content is insufficient or the sample was stored at elevated temperature, the measured BAC at the lab may be higher than the actual BAC at the time of collection.

06

Present the challenge before DA Gonzales’s DWI unit

Allison Tisdale prosecuted DWI cases before joining the defense. She knows how this office evaluates its Intoxilyzer and blood draw evidence, where the documentation is most likely to have gaps, and how to present a documented BAC challenge that moves a case toward dismissal.

BAC Legal Limits in Texas — What Each Threshold Means for Your Case

Different BAC levels trigger different charges and consequences. All are challengeable on the same technical grounds.

BAC Level	Legal Significance	Charge / Consequence
Any detectable amountUnder 21 — DUI, not DWI	Zero tolerance for drivers under 21	Class C DUI • License suspension • Education requirement
0.08+Penal Code §49.01(2)(B)	Per se intoxication — no further proof required	Class B DWI • 72 hrs – 180 days • $2,000
0.15+Penal Code §49.04(d)	Enhanced Class A misdemeanor	Class A DWI • Up to 1 yr • $4,000 • Mandatory IID
Any level (2nd offense)Penal Code §49.09(a)	Prior DWI conviction elevates the charge	Class A misd. min. • Mandatory 30-day jail • IID
Any level (3rd offense)Penal Code §49.09(b)	Two prior DWI convictions	3rd Degree Felony • 2–10 yrs TDCJ • $10,000
RefusalTexas Transportation Code §724	ALR suspension 180 days (first refusal)	No per se BAC • Prosecution uses impairment theory + refusal as consciousness of guilt

BAC Defense in Bexar County — Technical Knowledge, Local Courts

Mark Hull has 20+ years of Texas criminal defense experience and has challenged breath test and blood draw evidence in Bexar County DWI cases for over 20 years. BAC challenges require specific technical knowledge: Intoxilyzer 9000 maintenance records, partition ratio science, and the specific documentation requirements for the Bexar County ME’s blood testing process. That technical depth comes from years of practice in these specific courts against these specific machines and labs.

Allison Tisdale prosecuted DWI cases including BAC-reliant prosecutions as a Texas state prosecutor before joining the defense. She knows how DA Joe Gonzales’s DWI unit evaluates the strength of its BAC evidence and where the documentation is most likely to have problems. We carry a 5.0 rating across 363 Google reviews and Over 930 dismissals or rejected cases since 2021.

◆ Trial-Tested Criminal Defense

20+ years of Texas criminal defense experience. Bexar, Travis, Williamson, and Hays county courts. Mark Hull.

◆ Former State Prosecutors on Staff

Mark Hull and Allison Tisdale both prosecuted DWI cases. They know how DA Joe Gonzales’s office builds Bexar County DWI prosecutions — and where they fall apart.

◆ 20+ Years in Bexar County Courts

Regular appearances in the Cadena-Reeves Justice Center — Bexar County Courts at Law and District Courts. We know the DWI unit prosecutors and how the docket moves.

◆ Over 930 dismissals or rejected cases since 2021

Bexar County, Hays County, Travis County, and surrounding Central Texas courts.

Over 930dismissal or rejected cases since 2021

5.0Google Rating (363 Reviews)

20+Years in Bexar County Courts

BAC & Breath Test Questions — San Antonio & Bexar County

Common questions about breathalyzer results, blood tests, and BAC evidence in Bexar County DWI cases.

Can a breath test result be challenged in Bexar County?+

Yes. A breath test result from an Intoxilyzer 9000 can be challenged on several independent grounds: the machine’s maintenance and calibration records may show documentation gaps; the operator may not have been currently certified to administer the test; the required 15-minute continuous observation period may not have been properly documented; the specific machine’s error log may show reliability issues; or the partition ratio assumption underlying the conversion of breath alcohol to blood alcohol concentration may not apply to your individual physiology. We request the complete maintenance file on every Bexar County breath test case.

Can a blood draw result be challenged in Bexar County?+

Yes. Blood draw results in Bexar County DWI cases processed through the Bexar County Medical Examiner’s toxicology laboratory can be challenged on chain of custody grounds (any documentation gap from collection to analysis), preservation grounds (improper anticoagulant-to-preservative ratio or inadequate refrigeration that can cause in vitro fermentation), analyst certification grounds, and gas chromatograph calibration grounds. We request the complete lab file — not just the summary result — on every blood draw case.

What is retrograde extrapolation and how does it affect my case?+

Retrograde extrapolation is a method prosecutors use to estimate what your BAC was at the time you were driving based on a test administered later — sometimes hours after the traffic stop. An expert calculates your probable BAC at the time of driving by working backward through your estimated elimination rate. We challenge retrograde extrapolation by examining the assumptions: the absorption phase (were you still absorbing alcohol when tested?), your individual elimination rate (which varies significantly), food consumption, and any other physiological variables that affect the calculation. Retrograde extrapolation is contested science that is specifically challengeable.

What happens to my case if I refused the breath test?+

A refusal has two consequences: an ALR suspension of 180 days on a first refusal rather than 90 days for a first failure, and the refusal can be introduced as consciousness-of-guilt evidence in the criminal trial. However, SAPD and BCSO routinely obtain blood draw warrants under §724.017 when defendants refuse, meaning blood evidence is often collected anyway. A refusal does not make prosecution impossible — it changes the evidence type and shifts the ALR timeline. The Fourth Amendment analysis on the stop, the SFST administration challenge, and the blood draw chain of custody challenge are all still fully available.

Does it matter which Intoxilyzer 9000 was used?+

Yes. Every Intoxilyzer 9000 is a specific numbered instrument with its own maintenance history, calibration log, and error log. A machine that has had calibration failures, that was recently serviced for a specific malfunction, or that has a documented history of giving readings outside the acceptable reference range is a machine whose results are specifically questionable. We identify the specific instrument used in your test through the arrest documentation and request that machine’s complete maintenance file.

What is the partition ratio and why does it matter?+

The Intoxilyzer 9000 converts a breath alcohol measurement to a blood alcohol concentration estimate using a ratio of 2,100:1 — meaning the machine assumes 2,100 milliliters of alveolar breath contains the same amount of alcohol as 1 milliliter of blood. This ratio is a population average derived from research. Individual partition ratios vary significantly — studies show a range from approximately 1,700:1 to 2,900:1 across the population. For an individual whose actual partition ratio is higher than 2,100:1, the machine will report a BAC higher than the person actually has. We challenge the partition ratio assumption in cases where the reported result is close to a legal threshold.

DWI Defense Resources — San Antonio & Bexar County

Related pages covering DWI defense in Bexar County courts.

San Antonio DWI Lawyer

DWI defense hub — all charge levels, ALR process, and defense strategy for Bexar County.

BAC & Breath Test Challenges

Intoxilyzer 9000, blood draw chain of custody, and partition ratio challenges in Bexar County.

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First offense in San Antonio — no deferred adjudication, permanent record, full defense.

DWI Conviction Consequences

Full penalty breakdown for every DWI level — license, record, employment, and CDL.

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Zero tolerance DUI — any detectable alcohol is enough. UTSA and UISD area cases.

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IH-35 / IH-10 drug stop challenges and Fourth Amendment defense in Bexar County.

Expungement San Antonio

Clearing a dismissed DWI permanently under Texas Chapter 55.

San Antonio Criminal Defense

Full criminal defense hub for all charge types in Bexar County courts.

What Our Clients Say

5.0 stars • 363 Google reviews from clients across Bexar County, Hays County, Travis County, and Central Texas.

RL

Ronnicka Lopez

★★★★★

Google

“After two years, the case was dismissed and dropped. They made this whole ordeal easy, kept me in the loop, told me exactly what to expect. Mrs. Allison is such an amazing and sweet lady — the whole team. I see exactly why they received multiple awards for best law firm in the state.”

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Blake Shires

★★★★★

Google

“My case was dismissed. Mr. Hull was patient, attentive, and consistently communicative from start to finish. He explained every step and made sure I felt supported throughout. I highly recommend Mark Hull and The Hull Firm.”

SH

Shawn Hallman

★★★★★

Google

“These attorneys were responsive, intelligent, and relentless. I appreciated how they always had a plan and stayed one step ahead. If you’re facing criminal charges, you want them on your side.”

210-692-4913

BAC & Breath Test Challenges — San Antonio DWI Defense

BAC Evidence in a Bexar County DWI — What the Number Actually Proves and What It Doesn’t

Two Ways Texas Proves DWI — Two Ways We Fight Both

Breath Tests vs. Blood Draws — How Each Is Challenged in Bexar County

How We Challenge BAC Evidence in Bexar County DWI Cases

BAC Legal Limits in Texas — What Each Threshold Means for Your Case

BAC Defense in Bexar County — Technical Knowledge, Local Courts

BAC & Breath Test Questions — San Antonio & Bexar County

DWI Defense Resources — San Antonio & Bexar County

San Antonio DWI Lawyer

BAC & Breath Test Challenges

First-Time DWI Defense

DWI Conviction Consequences

DUI Defense (Under 21)

Drug Charges Defense

Expungement San Antonio

San Antonio Criminal Defense

What Our Clients Say

DWI Breath Test or Blood Draw in San Antonio? Challenge the Evidence.