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BAC & Breath Test Challenges in San Marcos & Hays County

A BAC reading is not a conviction. Breath test results depend on properly calibrated equipment, a certified operator, and an exact protocol — and blood draw results depend on proper collection, an unbroken chain of custody, and documented lab procedures. Any failure in that chain creates a challenge. We have used BAC evidence problems to get DWI cases dismissed in Hays County courts. Former prosecutor Allison Tisdale on staff. Trial-tested defense. Over 930 dismissals or rejected cases since 2021. Call 737-937-5786.

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Litigator of the Year 2023 — BAC breath test challenge attorney Mark Hull San MarcosMark Hull
Expertise.com Best DWI Defense Lawyers Hays County — BAC evidence challengesMark Hull*
National Trial Lawyers Top 100 — DWI BAC defense Hays County TexasMark Hull — 2022
Top 40 Under 40 — Allison Tisdale former prosecutor DWI breath test defenseAllison Tisdale — 2022
Lawyers of Distinction — DWI BAC challenge attorney San Marcos Hays CountyMark Hull
Criminal Defense Top 10 Attorney — breath test blood draw defense Hays CountyMark Hull

*Based on the quality and quantity of reviews and average minimum rating for a law firm practicing criminal defense in Austin, TX researched by expertise.com

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

A breath or blood BAC result is not self-proving evidence. In Texas, a DWI conviction under Penal Code §49.04 requires proof beyond a reasonable doubt that you were intoxicated — either because your BAC was 0.08 or above, or because you lacked normal use of your mental or physical faculties. The BAC number is evidence of the first standard. But that evidence is only as reliable as the machine that produced it, the person who operated it, and the procedures that governed every step from roadside stop to test result. When any of those elements fail, the number is challengeable.

I’ve reviewed breath test and blood draw evidence in Hays County DWI cases for over 20 years. Allison Tisdale prosecuted DWI cases as a Texas state prosecutor — she understands exactly how DA Kelly Higgins’s office uses BAC results to build its case, and where that evidence is vulnerable. We pull every relevant record on every DWI case: Intoxilyzer 9000 calibration logs, operator certification records, blood draw documentation, and lab quality assurance files. Those records produce suppression and exclusion arguments more often than most attorneys expect.

A high BAC number does not close your case. We have gotten DWI charges dismissed in Hays County courts despite BAC results that most attorneys would have accepted as insurmountable. The question is never just what the number was — it’s whether the number was obtained correctly, and whether it accurately reflects your actual BAC at the time you were driving. Call 737-937-5786 the day of the arrest.

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DWI BAC defense attorneys San Marcos — Mark Hull and Allison Tisdale challenging breath test and blood draw evidence

Two Ways Texas Can Prove DWI — And Two Ways We Fight Back

Texas Penal Code §49.01 defines intoxication two ways. The per se standard: a BAC of 0.08 or above. The impairment standard: not having the normal use of mental or physical faculties due to alcohol, drugs, or a combination. Either standard alone is sufficient for a DWI conviction. The prosecution does not need both. And critically, Texas does not require a BAC reading to convict you of DWI — officer observations alone, if believed, can satisfy the impairment standard.

This dual standard shapes the entire defense strategy. On the BAC track, we challenge the reliability and accuracy of the test result itself. On the impairment track, we challenge the officer’s observations — the dashcam and bodycam footage, the field sobriety test administration, and whether the officer’s conclusions are actually supported by what the camera shows.

When both tracks are contested simultaneously, the prosecution has to prove its case on two fronts. That creates real pressure. DA Kelly Higgins’s office in Hays County, like any DA’s office, makes cost-benefit calculations about which cases to push to trial and which to negotiate. A case where both the BAC evidence and the officer’s impairment testimony are subject to documented challenges is a fundamentally weaker prosecution file. That’s where the path to dismissal opens.

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

The two primary BAC testing methods have different vulnerabilities. Here is what we look for on each.

Intoxilyzer 9000

Breath Test Challenges

Texas uses the Intoxilyzer 9000 for evidentiary breath testing. Every challenge starts with the same records: the calibration log, the maintenance log, and the operator’s certification. The device must be calibrated according to TxDPS schedule, operated by a certified technician, and administered after a minimum 15-minute deprivation period during which the subject has not belched, vomited, or introduced anything into their mouth. Two samples must be taken and must agree within 0.02 of each other. Any deviation from these requirements is a ground for challenge.

Beyond the procedural requirements, the Intoxilyzer 9000 can produce elevated readings from mouth alcohol (residual alcohol from recent belching, dental work, or GERD), radio frequency interference, and certain medical conditions including diabetes and ketogenic diets — both of which produce acetone that the device can misread as ethanol.

Calibration records • Operator certification • 15-min deprivation • Mouth alcohol • Medical conditions
Blood Draw

Blood Draw Challenges

Blood draws in Texas DWI cases must be performed by a qualified person under Texas Transportation Code §724.017 — a licensed physician, qualified technician, chemist, registered nurse, or licensed vocational nurse. The blood must be drawn using an approved non-alcoholic swab, stored in proper preservative tubes, transported under documented chain of custody, and analyzed by a certified lab using documented quality assurance procedures. The result is only as reliable as every step of that process.

We pull the full chain of custody documentation, the lab’s quality assurance records, the analyst’s certification, and the collection records. Improperly stored samples can ferment, artificially elevating BAC. Contaminated collection tubes, breaks in chain of custody, and inadequate lab QA procedures are all grounds to challenge the admissibility or weight of the result.

TTC §724.017 qualified draw • Non-alcoholic swab • Preservative tubes • Chain of custody • Lab QA records
Biological and medical factors affecting BAC readings — DWI defense San Marcos Hays County

Why the BAC Number May Not Reflect What You Actually Had When You Were Driving

The most important concept in BAC defense is the difference between the result the machine produced and what your actual BAC was at the time you were behind the wheel. These are not always the same number, and the gap between them is where the defense lives.

↑ Rising Blood Alcohol

Alcohol continues to absorb into the bloodstream for 30–90 minutes after the last drink. If you had drinks shortly before driving, your BAC at the time of the stop may have been lower than your BAC at the time of the test. A breath test administered an hour after the stop can show a higher number than what existed when you were actually driving.

⚐ Mouth Alcohol Contamination

The Intoxilyzer 9000 tests the air from deep in your lungs. Residual alcohol in your mouth — from a recent belch, acid reflux episode, dental work, or mouthwash used hours earlier — can falsely elevate the reading. The 15-minute deprivation requirement exists precisely because of this problem. If the officer failed to observe the full deprivation period, the result is unreliable.

⚠️ GERD and Acid Reflux

Gastroesophageal reflux disease causes stomach contents — including stomach alcohol — to rise into the esophagus and mouth. During a breath test, this stomach alcohol registers as elevated breath alcohol. The Intoxilyzer 9000 cannot distinguish between deep lung air and mouth or esophageal air contaminated by reflux. A driver with GERD who has had a small amount to drink can test significantly above their actual BAC.

🍍 Diabetes and Ketogenic Diets

Individuals with diabetes or those following a low-carbohydrate or ketogenic diet produce acetone as a metabolic byproduct. The Intoxilyzer 9000, like earlier generations of breath testing equipment, can misread isopropanol (derived from acetone) as ethanol. A driver with elevated ketone levels can register a false positive or an artificially elevated result even with no alcohol consumption at all.

⚖ Body Weight and Physiology

Alcohol distributes through body water, not body mass. A smaller person with less body water will generally reach a higher BAC from the same amount of alcohol than a larger person. Women typically reach higher BAC from the same consumption as men of comparable weight due to differences in body water percentage and alcohol metabolism. The breath-to-blood ratio assumed by the Intoxilyzer (2100:1) does not match every individual equally.

💋 Partition Ratio Variability

The Intoxilyzer 9000 converts breath alcohol to blood alcohol using a fixed partition ratio of 2100:1 (meaning 2100 ml of alveolar air contains the same amount of alcohol as 1 ml of blood). The actual ratio in living humans ranges from roughly 1700:1 to 2400:1. A person with a lower-than-average partition ratio will produce a breath result that overstates their actual BAC. This is a documented, scientifically accepted source of inaccuracy in breath testing.

DWI BAC evidence challenge strategy San Marcos — Intoxilyzer 9000 calibration and blood draw defense Hays County

How We Challenge BAC Evidence in Hays County DWI Cases

Every DWI case involving BAC evidence gets the same systematic review. Here is how that works in practice.

01
Request and review all calibration and maintenance records

For breath tests: we request the Intoxilyzer 9000 calibration log, maintenance log, and the instrument’s complete service history. Texas DPS regulations require calibration at set intervals. An out-of-calibration device produces unreliable results. We have found out-of-specification calibration records in Hays County DWI cases that formed the basis of successful suppression motions.

02
Verify operator certification

The Intoxilyzer 9000 must be operated by a certified breath test operator under Texas DPS certification. We verify the arresting officer’s certification was current at the time of the test, that the officer followed the required observation procedure, and that the 15-minute deprivation period was actually observed — not just documented. Dashcam and bodycam footage often tells a different story than the paperwork.

03
Identify medical and physiological factors

We ask about every relevant medical condition: GERD, acid reflux, diabetes, low-carb diet, dental work, medications. Any condition that affects the reliability of the breath test result becomes part of the defense. Where a condition is identified, we work with expert witnesses to present the scientific basis for challenging the result in Hays County court.

04
Pull the full blood draw chain of custody

For blood cases: we request the blood draw documentation, the collection kit batch records, the transport and storage documentation, the lab intake records, the analyst’s certification, and the lab’s quality assurance records for the analysis period. A gap anywhere in that chain is a challenge point. Improperly stored samples that show signs of fermentation are a grounds for exclusion.

05
Evaluate the rising BAC defense

If you had drinks in the hour or two before the stop, your BAC may have been lower while driving and higher by the time the test was administered. We evaluate the timeline — what was consumed, when, and what the absorption rate would produce — to determine whether the test-time result accurately reflects the drive-time BAC. Retrograde extrapolation by the prosecution is subject to challenge on the same scientific grounds.

06
File suppression motions and challenge admissibility

If any of the above produces a legitimate ground to challenge the BAC result, we file a motion to suppress under Texas Code of Criminal Procedure Chapter 38 or a motion in limine to exclude the evidence as unreliable. In Hays County courts, a successful suppression motion on BAC evidence typically results in a dismissed case — because DA Kelly Higgins’s office knows what its remaining evidence looks like without it.

Texas BAC legal limits and DWI thresholds — Hays County DWI defense attorney San Marcos

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

Texas law sets different BAC standards for different categories of driver. Understanding which standard applies to your case — and what the difference means for the charge level and the defense strategy — is the foundation of any DWI defense.

The 0.08 standard for standard adult drivers is the per se threshold: at or above that level, intoxication is presumed by the BAC result alone. But the prosecution can also prove intoxication through the impairment standard, independent of any BAC reading. A result below 0.08 does not automatically mean a dismissal if the officer observed impairment indicators — and it does not automatically mean a conviction either, because impairment observations are challengeable on their own terms.

The 0.15 threshold matters because it triggers a charge enhancement from Class B to Class A misdemeanor, with dramatically different penalties: up to one year in jail versus up to 180 days, and a fine up to $4,000 versus $2,000. It also triggers a mandatory ignition interlock device requirement on any probation. On cases near the 0.15 threshold, we scrutinize the testing methodology especially carefully, because the difference between a Class B and Class A result can come down to a margin of error that the device is not required to hit.

Driver Category BAC Threshold Charge / Consequence
Standard Adult Driver (21+)Penal Code §49.04 0.08% or above DWI per se — Class B Misd. • Intoxication presumed by BAC result alone
Any Driver — Enhanced BACPenal Code §49.04(d) 0.15% or above Class A Misd. • Up to 1 year jail • Up to $4,000 fine • Mandatory IID on probation
Commercial Driver (CDL)Texas Transportation Code §522.081 0.04% or above CDL disqualification • DWI charge still possible • 1-year CDL suspension on first conviction
Driver Under 21Texas Alcoholic Beverage Code §106.041 Any detectable DUI (zero tolerance) • Class C Misd. • Escalates to DWI if BAC ≥ 0.08
Any Driver — Impairment OnlyPenal Code §49.01(2)(B) No BAC required DWI through impairment standard — officer observations alone can be sufficient • Challengeable on evidence

Why Technical BAC Defense Requires More Than Knowing the Law

Most DWI attorneys in Texas know the penalty ranges. Fewer know how to read a calibration log, cross-examine an Intoxilyzer operator on the TxDPS technical supervisor manual, or challenge a lab analyst on their QA documentation. BAC evidence defense requires understanding the science behind the test — the partition ratio assumptions, the interferent screening limitations of the Intoxilyzer 9000, the fermentation risk in improperly stored blood samples.

Mark Hull has practiced Texas criminal defense for over 20 years. He has tried DWI cases to verdict in Hays County, Travis County, and Williamson County, and has cross-examined arresting officers and breath test operators in those courts. Allison Tisdale prosecuted DWI cases as a Texas state prosecutor before joining our firm. She knows how DA Kelly Higgins’s office uses BAC results in its case files, and where those results are most susceptible to challenge. Together, we have achieved Over 930 dismissals or rejected cases since 2021 across Central Texas — including cases with BAC readings well above 0.08.

◆ Trial-Tested Criminal Defense

20+ years of Texas criminal defense experience. Trial experience in Hays County DWI cases.

◆ Former State Prosecutor on Staff

Allison Tisdale prosecuted DWI cases before joining the defense. She knows how BAC evidence is used to build the prosecution’s file.

◆ Calibration Records & Lab Files

We pull every relevant document on every BAC case: calibration logs, operator certs, chain of custody, and lab QA records. These produce challenges more often than most attorneys expect.

◆ Over 930 dismissals or rejected cases since 2021

Across Hays, Travis, and Williamson Counties — including cases with breath test results and blood draw evidence.

Over 930dismissal or rejected cases since 2021
5.0Google Rating (363 Reviews)
20+Years in Hays County Courts

BAC & Breath Test Questions — San Marcos & Hays County

What clients ask us most about BAC evidence, breath tests, and blood draws in Texas DWI cases — answered with specific legal and scientific detail.

Yes — and it happens more often than most people expect when the records are actually reviewed. Breath test results are challengeable on calibration failures, operator error, improper deprivation period observation, and medical conditions including GERD, diabetes, and ketogenic metabolism. Blood draw results are challengeable on chain of custody breaks, improper collection procedure, inadequate lab quality assurance, and sample fermentation from improper storage. We pull every relevant record on every DWI case involving BAC evidence. In Hays County courts, a successful challenge to BAC evidence typically results in a dismissed case because DA Kelly Higgins’s office knows what the remaining evidence looks like without it. Call 737-937-5786 the day of the arrest.

Texas uses the Intoxilyzer 9000, manufactured by CMI, Inc., as its evidentiary breath testing instrument. The device uses infrared spectroscopy to measure the ethanol content in a deep lung breath sample and converts that result to a blood alcohol equivalent using a fixed partition ratio of 2100:1. Texas DPS maintains a list of approved breath test instruments, and the Intoxilyzer 9000 must be calibrated according to TxDPS schedules and operated by a certified breath test operator. The device is not foolproof — it can produce elevated readings from mouth alcohol, certain medical conditions, and interferents that produce signals similar to ethanol in the infrared spectrum.

Texas DPS regulations require a 15-minute continuous observation period before administering an evidentiary breath test. During this period, the subject must not eat, drink, smoke, belch, vomit, or place anything in their mouth. The requirement exists because mouth alcohol — residual alcohol in the oral cavity from any of those activities — can produce artificially elevated readings on the Intoxilyzer 9000. If the officer failed to properly observe the full 15-minute deprivation period, the result is subject to challenge as unreliable. Dashcam and bodycam footage often contradicts the officer’s notation that the period was properly observed. This is one of the first things we look for when reviewing a breath test case.

Yes — and this is a scientifically documented source of error in breath testing. Gastroesophageal reflux disease causes stomach contents, including stomach alcohol, to move into the esophagus and oral cavity. When this occurs during a breath test, the Intoxilyzer 9000 reads the stomach alcohol in the breath sample as though it originated from the deep lung. The result is an artificially elevated BAC reading. A driver with GERD who consumed a moderate amount of alcohol can test significantly above their actual blood alcohol level. If you have GERD, acid reflux, or any condition affecting your digestive system, tell us immediately — it may be directly relevant to your defense.

Alcohol continues to absorb from the stomach and small intestine into the bloodstream for 30 to 90 minutes after consumption. During the absorption phase, BAC is still rising. If you had drinks in the hour or two before driving and were stopped relatively soon after, your BAC at the time of the stop may have been below 0.08, even though your BAC at the time of the breath or blood test — administered later — was at or above the threshold. The prosecution sometimes attempts to rebut this with retrograde extrapolation — calculating backward from the test result to estimate BAC at the time of driving. We challenge retrograde extrapolation on the scientific assumptions it relies on, which are highly individualized and frequently overstated in police reports.

Yes. Texas Penal Code §49.01(2)(B) defines intoxication as not having the normal use of mental or physical faculties by reason of introduction of alcohol into the body. The prosecution does not need a BAC reading to prove DWI under this standard — officer observations of impairment, dashcam footage, field sobriety test performance, and witness testimony can each contribute to proving the impairment standard. If you refused a breath or blood test, or if your BAC result was below 0.08, the prosecution may rely on the impairment standard instead. We challenge the impairment evidence on the same grounds we challenge everything else: the legality of the stop, the field sobriety test administration, and what the camera footage actually shows versus what the officer wrote down.

Refusing a breath or blood test in Texas has consequences on both tracks. On the administrative license revocation track, refusal triggers a 180-day license suspension for a first offense if you do not request an ALR hearing within 15 days of arrest — compared to 90 days for a failed first-offense test. On the criminal track, the prosecution can introduce your refusal before the jury as evidence of consciousness of guilt. However, refusal also means there is no BAC number in the prosecution’s file. The case then rests entirely on the officer’s impairment observations, the field sobriety test results, and available video. Whether refusal helps or hurts depends heavily on the specific facts of the stop. Call 737-937-5786 and we will evaluate your specific situation directly.

Blood draw challenges in Texas DWI cases focus on four areas. First, who drew the blood: Texas Transportation Code §724.017 requires the draw to be performed by a licensed physician, qualified technician, chemist, registered nurse, or licensed vocational nurse. Any draw performed by a person not meeting those qualifications is subject to challenge. Second, how the blood was collected: the collection kit must use a non-alcoholic swab and appropriate preservative and anticoagulant tubes. Third, chain of custody: every transfer of the sample must be documented from draw to lab intake. Any gap in documentation creates a challenge point. Fourth, the lab analysis itself: we review the lab’s quality assurance records for the analysis period, the analyst’s certification, and the specific methodology used.

A BAC of 0.15 or above triggers an enhancement under Penal Code §49.04(d) from a Class B misdemeanor to a Class A misdemeanor on a first offense. The practical difference is significant: the fine ceiling doubles from $2,000 to $4,000, jail time increases from 180 days maximum to one year maximum, and a mandatory ignition interlock device is required as a condition of any probation. On cases testing at or near the 0.15 threshold, we scrutinize the testing methodology especially carefully. The Intoxilyzer 9000 has a documented margin of uncertainty, and the difference between a Class B and a Class A result can fall within that margin. A result of 0.153 is not the same as a provably accurate reading of 0.153, and that distinction matters in court.

Call 737-937-5786 the day of the arrest. The 15-day ALR deadline starts the moment of arrest — not when you are released, not when charges are filed. Beyond the license, the earlier we get involved, the more complete the record we can build: we need to preserve dashcam and bodycam footage before it is overwritten, request calibration and maintenance records while they are current, and evaluate whether the deprivation period was properly observed before the officer’s memory of the event fades. A breath or blood test result is not the end of a DWI case — it is one piece of evidence, and it is subject to challenge on multiple independent grounds. We have gotten cases dismissed in Hays County with BAC results well above 0.08. The key is getting an attorney involved immediately so the review starts before the evidence window closes.

DWI Defense Resources — San Marcos & Hays County

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DWI Charge in San Marcos? Challenge the BAC Evidence Now.

A breath or blood test result is one piece of evidence — and it is subject to challenge on calibration, operator error, chain of custody, and biology. The 15-day ALR deadline starts the moment of arrest. Call our San Marcos DWI defense team at 737-937-5786 — 24 hours a day, 7 days a week.

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For educational purposes only. Not legal advice. Contact us for a free, confidential consultation specific to your case.

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