Breathalyzer vs. Blood Test in Houston DWI Cases: How We Challenge Both

When a Houston police officer suspects DWI, the arrest typically produces one of two types of chemical test evidence: a breath test using the Intoxilyzer 9000, or a blood sample analyzed by gas chromatography. Both are presented to juries as scientific proof of intoxication. Both have documented vulnerabilities that an experienced Houston DWI attorney can attack — and the specific vulnerabilities differ completely depending on which test was used.

This article explains how each test works, what the science actually says about its accuracy, and how Herman Martinez challenges each type of evidence in Harris County courts.

Part One: The Breathalyzer — How It Works and Where It Fails

What the Intoxilyzer 9000 Actually Measures

Texas has used the Intoxilyzer 9000 (manufactured by CMI Inc.) as its approved breath testing device statewide. It is important to understand what this machine does not do: it does not directly measure the alcohol concentration in your blood. It measures alcohol in your deep lung air and uses a mathematical conversion — a fixed 2100:1 ratio — to estimate what the blood alcohol concentration must be.

This ratio assumes that 2100 mL of alveolar (deep lung) air contains the same amount of alcohol as 1 mL of blood. It is a population average — not your personal ratio. Research consistently shows that the blood-to-breath ratio varies from person to person in a range from approximately 1700:1 to 2400:1. A person whose actual ratio is 1700:1 will produce a breath test reading 24% higher than their actual BAC when the machine assumes the standard 2100:1 ratio. In practical terms: a person with an actual BAC of .065 (below the legal limit) can produce a breath test reading of .08 due solely to their individual physiology.

The 15-Minute Observation Period

Texas requires officers to observe a DWI suspect for a continuous, uninterrupted 15-minute period before administering a breath test. The purpose is to ensure the suspect does not belch, burp, vomit, or otherwise introduce mouth alcohol into the oral cavity before the test. Mouth alcohol inflates breath test readings dramatically — even residual alcohol from mouthwash can skew results.

In practice, Harris County officers frequently violate this requirement. An officer who looks away, uses the radio, or allows the suspect to speak to another officer during the observation window has not completed a valid observation. Herman requests all dashcam and bodycam footage specifically to verify whether the observation period was conducted correctly — and whether it was truly uninterrupted for all 15 minutes.

Medical Conditions That Affect Breath Test Results

Gastroesophageal Reflux Disease (GERD)

GERD causes gastric contents — including alcohol vapors absorbed from food and beverages — to travel up the esophagus and contaminate the oral cavity. A driver with active GERD who consumed a single glass of wine with dinner can produce a breath test reading that significantly overstates actual BAC, because the machine cannot distinguish between alcohol from the lungs and alcohol from the stomach. GERD affects approximately 20% of the American adult population, making it one of the most common and most significant sources of breath test error.

Diabetes and Acetone in the Breath

Diabetic ketoacidosis and controlled diabetes on ketogenic diets produce acetone as a metabolic byproduct of fat metabolism. The Intoxilyzer 9000 uses infrared spectroscopy to identify alcohol in the breath — and acetone absorbs infrared light at wavelengths similar to ethanol. Some studies have documented acetone-caused false positive readings approaching .08 in diabetic individuals who have consumed no alcohol. The machine’s acetone filter is not foolproof, and DKA-level acetone production can overwhelm it.

Elevated Body Temperature

For each 1°C increase above normal body temperature (37°C), breath test readings increase by approximately 7%. A person with a mild fever of 38°C (100.4°F) — not an unusual condition — will produce a breath test reading approximately 7% above their actual BAC. For someone at .074 (below the legal limit), a fever-induced inflation of 7% produces a reading of .079 — above .08 and legally over the limit. Flu, infections, and even vigorous exercise that raises body temperature can have this effect.

Breathing Patterns

How you breathe before and during the test measurably affects the result. Research shows that hyperventilating for 30 seconds before the test reduces breath test readings by an average of 22%. Holding your breath for 30 seconds before blowing increases readings by approximately 16–20%. These effects are caused by changes in the composition of deep lung air due to altered breathing patterns — not by any change in actual BAC. Neither effect is something the machine accounts for or can detect.

Calibration and Maintenance: The Paper Trail Herman Subpoenas

Every Intoxilyzer 9000 in Texas must be calibrated by a certified technical supervisor at least every 90 days. The machine must pass calibration checks before and after each test. Herman subpoenas the following records in every breath-test DWI case:

• Calibration logs for the specific machine used, going back at least 2 years
• Maintenance records documenting any repairs, replacements, or service
• Technical supervisor’s certification file (training, certification dates, renewal history)
• Operator’s certification for the officer who administered the test
• The specific test record showing all reference readings taken during the test session

Gaps in calibration records, failed calibration checks, or lapsed certifications can undermine the admissibility or weight of breath test results.

Radio Frequency Interference (RFI)

The Intoxilyzer 9000 includes an RFI detector designed to shut down the test if radio frequency interference from police radios, cell phones, or other sources is detected. However, the detector is not perfectly sensitive to all frequencies. If an officer’s portable radio transmitted during the test in a frequency range the detector doesn’t monitor, the resulting interference may not have been caught — yet it could affect the reading. Herman requests information about all radio transmissions during the test period as part of the standard breath-test discovery request.

Part Two: Blood Tests — The Gold Standard With Its Own Vulnerabilities

After the U.S. Supreme Court’s decision in Missouri v. McNeely (2013), Texas law enforcement increasingly relies on blood draws obtained either by consent or by warrant. Blood tests are marketed to juries as more accurate than breath tests — and in theory, gas chromatography provides a direct measurement of blood alcohol rather than an estimate. In practice, blood test results have their own significant vulnerabilities.

The Blood Draw: Who, Where, and How Matters

Texas law requires blood draws in DWI cases to be performed by a licensed person — a physician, qualified technician, chemist, nurse, or phlebotomist. The draw must be conducted in a sanitary place. Issues that arise at the collection stage include:

• Alcohol-based skin swabs: Using an alcohol (isopropyl or ethanol) swab to clean the draw site before inserting the needle can introduce external alcohol into the sample, artificially elevating the result. The correct procedure is to use a non-alcohol cleanser. Herman reviews collection documentation for the specific swab type used.
• Unlicensed drawer: If the person who drew the blood was not properly licensed or trained, the draw itself may not have been conducted properly — and chain of custody issues begin at the draw site.
• Collection kit issues: Texas DPS blood draw kits contain specific anticoagulant and preservative chemicals (sodium fluoride and potassium oxalate) in specified ratios. These chemicals prevent clotting and fermentation of the blood sample. If the tube was expired, improperly stored, or damaged, the preservative/anticoagulant balance may be off.

Fermentation: How BAC Can Rise After the Blood Is Drawn

Blood contains living bacteria that metabolize sugars — including the trace sugars in blood — and produce ethanol as a byproduct. This process is called in vitro fermentation. A blood sample that is not properly preserved and refrigerated will ferment, producing alcohol that was not present in the living blood at the time of the draw.

The sodium fluoride in the collection tube inhibits fermentation, but it must be present in the correct concentration. If the tube contains insufficient NaF, or if the tube was not properly inverted to mix the chemicals with the blood after collection, fermentation can occur — and the resulting BAC measurement will be higher than the actual BAC at the time of driving.

Herman requests the lab’s storage temperature logs and the specific collection tube lot numbers to evaluate whether fermentation was a realistic possibility in your case.

Chain of Custody: Every Hand That Touched the Sample

From the moment blood is drawn to the moment it is analyzed in the lab, every person who handles the sample must be documented. Gaps in chain of custody documentation — a missing signature, an undocumented transfer, an unaccounted period of time — create legitimate questions about whether the sample analyzed by the lab is the same sample drawn from you, and whether it was stored properly throughout.

Herman traces the complete chain of custody for every blood sample: from the draw, through transport, to intake at the lab, through refrigerated storage, to the analyst who performed the test, through the final report. Any break in the chain is documented and presented to the jury.

Gas Chromatography: The Lab Analysis and Its Vulnerabilities

The Texas DPS labs that analyze DWI blood samples use gas chromatography with headspace analysis — the gold standard method for blood alcohol testing. But even gold standard methods can produce erroneous results under the right conditions:

• Instrument calibration: Gas chromatographs require regular calibration with reference standards. Herman subpoenas calibration records and quality control logs to verify the instrument was functioning correctly on the day your sample was analyzed.
• Internal standard verification: Proper headspace GC analysis uses an internal standard (n-propanol) to verify that the ethanol peak identified is actually ethanol and not a co-eluting compound. If the internal standard wasn’t used correctly, the result may not be reliable.
• Analyst qualifications: The analyst who tested your blood must be certified and properly trained. Herman requests the analyst’s curriculum vitae, certifications, and proficiency test results.
• QA/QC records: Lab quality assurance documentation shows whether the lab’s own controls passed on the day of your analysis and whether the run your sample was part of was within acceptable parameters.

Your Right to Independent Blood Testing

Under Texas Code of Criminal Procedure Art. 38.35, you have the right to request that a portion of your blood sample be preserved and provided for independent testing by an expert of your choosing. This “blood split” must be requested before the state destroys the sample. Herman makes this request in every blood draw DWI case, because an independent analysis by a defense expert can compare results, evaluate fermentation markers, and sometimes produce a reading that differs materially from the state’s test.

Retrograde Extrapolation: The BAC at Driving, Not at Testing

Here is a detail that the prosecution often glosses over: your blood was not drawn at the moment of driving. It was drawn hours later — sometimes two, three, or more hours after the traffic stop. The question the jury must answer is not what your BAC was when the blood was drawn; it’s what your BAC was at the time you were operating the vehicle.

Forensic toxicologists use a process called retrograde extrapolation to estimate the BAC at an earlier time from a known measurement at a later time. This extrapolation requires assumptions about absorption rate, distribution, and elimination rate — all of which vary significantly from person to person and from situation to situation depending on food consumption, body composition, drinking pattern, and individual metabolism.

When the state’s expert testifies that “based on retrograde extrapolation, the defendant’s BAC at the time of driving was .XX,” that number is an estimate built on multiple assumptions, not a measurement. Herman challenges those assumptions in cross-examination and, where appropriate, presents a defense toxicology expert who can explain the range of uncertainty in the extrapolation.

Breath Test vs. Blood Test: Which Is More Defensible?

There is no universal answer to which test is more defensible. It depends on the specific facts: how far above .08 the reading was, what medical conditions are present, what the chain of custody looks like, whether calibration records are complete, and what defenses are available from the traffic stop and field sobriety test administration. Herman evaluates every specific case on its merits.

If you took a breath or blood test in a Houston DWI case and the result was at or near the legal limit, or if you have any of the medical conditions described above, call Herman Martinez at (713) 242-1779 for a free case evaluation. The science behind these tests is more vulnerable than the prosecution wants you to believe.