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Blood Alcohol Concentration (BAC)

Blood Alcohol Concentration in Texas DWI Cases: What the Number Means, How It’s Measured, and Why It Can Be Wrong

In most Texas DWI cases, the prosecution’s central exhibit is a number: a BAC result from a breath or blood test. Prosecutors present that number as objective scientific fact. In reality, it is a measurement produced by a process and every step of that process, from the moment of collection through laboratory analysis, involves variables that can introduce error.

Understanding what BAC means, how it is measured, what can go wrong, and how those errors are identified requires scientific training. At Deandra Grant Law, Managing Partner Deandra Grant holds a Master’s Degree in Pharmaceutical Science, a Graduate Certificate in Forensic Toxicology and holds the ACS-CHAL Forensic Lawyer-Scientist designation. She is trained to operate and maintain the Intoxilyzer. That training is not background. It is the foundation of how chemical test evidence is evaluated in every case this firm handles.

The Two Ways Texas Law Defines Intoxication

Most people assume that a BAC of 0.08 or above is required for a DWI conviction. It is not. Texas Penal Code §49.01 defines intoxication in two distinct ways, either of which is sufficient for a DWI charge:

Per se intoxication.  A blood or breath alcohol concentration of 0.08 or above at the time of testing creates a legal presumption of intoxication.

Loss of normal use.  A person is also intoxicated under Texas law if they have lost the normal use of their mental or physical faculties due to the introduction of alcohol, a controlled substance, a drug, or any combination. This prong does not require any specific BAC reading. An officer’s observations, field sobriety test performance, and in-car behavior can be used to support a loss-of-normal-use charge even when the chemical test shows a BAC below 0.08.

The practical consequence: a defendant whose blood result comes back at 0.06 can still be prosecuted for DWI if the officer’s observations and the field sobriety test evidence are used to support the loss-of-normal-use prong. The defense must address both prongs, not just the number.

Breath Testing: The Intoxilyzer 9000

Texas uses the Intoxilyzer 9000 as its approved breath testing instrument. The device measures alcohol concentration in deep lung air (alveolar air) and uses an infrared spectroscopy method to estimate the blood alcohol concentration. The result is reported as a breath alcohol concentration (BrAC) that is then correlated to an estimated BAC using a fixed partition ratio of 2,100:1, meaning the instrument assumes that 2,100 milliliters of alveolar breath contains the same amount of alcohol as one milliliter of blood. That ratio is a population average. It does not apply uniformly to every individual, and deviation from that ratio in either direction affects the reported result. Learn more about how breath tests work and where they can fail.  

Breath Test Challenge Points

Calibration and maintenance records.  The Intoxilyzer 9000 must be calibrated and inspected according to Texas Department of Public Safety protocols. Records of those inspections are discoverable. Gaps in maintenance, out-of-range calibration results, or use of an instrument with an expired certification are all grounds for challenging the reliability of the result.

The observation period.  Texas protocol requires a continuous 15-minute observation period before a breath test is administered, during which the subject must not eat, drink, smoke, belch, or regurgitate. The purpose is to ensure that no residual mouth alcohol contaminates the sample. An officer who failed to maintain the required observation period, or who allowed the subject to be out of their direct line of sight during that period, compromises the validity of the test.

Mouth alcohol.  Alcohol trapped in the mouth (from belching, acid reflux, recent alcohol consumption, or alcohol-containing products like mouthwash or breath spray) can cause the instrument to produce a falsely elevated BrAC result. The Intoxilyzer 9000 has a slope detector designed to identify mouth alcohol, but the detector is not infallible and can be defeated by conditions that present similarly to alveolar air.

Medical conditions.  Individuals with gastroesophageal reflux disease (GERD) or acid reflux are at higher risk for mouth alcohol contamination because stomach contents (including any alcohol present) can migrate into the esophagus and mouth even without an obvious belch. Diabetics in a state of ketoacidosis produce isopropanol and acetone, compounds that the Intoxilyzer can misidentify as ethanol, producing a falsely elevated reading. These are documented, peer-reviewed phenomena that courts have recognized as valid grounds for challenging breath test results.

Blood Testing: The Laboratory Analysis

Blood testing in Texas DWI cases is performed by forensic laboratories — the Southwestern Institute of Forensic Sciences (SWIFS) in Dallas County and the Tarrant County Medical Examiner’s Forensic Laboratory (TCME) in Tarrant County, among others. The standard methodology is headspace gas chromatography with flame ionization detection (GC-FID). The analysis involves multiple steps and each of them can introduce error. Learn more about blood test methodology and challenge points.  

Blood Test Challenge Points

Collection tube and preservative.  Blood for BAC analysis must be collected in the correct tube: a gray-top tube containing sodium fluoride as a preservative and potassium oxalate as an anticoagulant. Sodium fluoride inhibits microbial fermentation of glucose in the blood, which would otherwise produce ethanol and artificially elevate the reported BAC. If the tube contains insufficient sodium fluoride, or if the sample is stored at an elevated temperature, in vitro fermentation can occur after collection which could produce a BAC result that is higher than the defendant’s actual BAC at the time of driving.

Chain of custody.  The blood sample must be documented continuously from collection through laboratory analysis. Every transfer of the sample must be logged, and the identity of every person who handled the sample must be recorded. Gaps in the chain of custody raise questions about whether the sample analyzed in the laboratory is the same sample collected from the defendant, and whether the sample was stored properly during any period of transfer.

Laboratory methodology and analyst qualifications.  The GC-FID analysis must be performed by a qualified analyst using a properly calibrated instrument. The analyst’s training, the instrument’s calibration records, the reference standards used, and the laboratory’s quality assurance protocols are all discoverable and subject to challenge. A laboratory that does not follow its own standard operating procedures, or that uses reference standards outside their certified range, produces results that cannot be relied upon.

The retrograde extrapolation problem.  The blood draw typically occurs 30 minutes to several hours after the stop. The BAC at the time of the draw is not necessarily the BAC at the time of driving. If the defendant was still absorbing alcohol at the time of the stop, the BAC was rising which means the BAC at the time of the test may actually be higher than it was at the time of driving. Conversely, if the defendant had stopped absorbing and entered the elimination phase, the BAC at the time of the test would be lower than at the time of driving. Evaluating which scenario applies requires pharmacokinetic analysis.

The Rising BAC Defense

Alcohol absorption into the bloodstream is not instantaneous. After consumption, alcohol moves from the stomach and small intestine into the bloodstream over a period of 30 minutes to 2 hours, depending on the individual, what was consumed, whether food was present, and other variables. During the absorption phase, BAC rises. The legal question is what the BAC was at the time of driving (not at the time of the test).

The rising BAC defense applies when the defendant consumed alcohol close to the time of driving and was still in the absorption phase at the time of the stop. In that scenario, the BAC at the time of the test (after a traffic stop, field sobriety testing, and transport to a testing facility) may be meaningfully higher than the BAC at the moment the defendant was behind the wheel. Establishing this defense requires pharmacokinetic analysis: consumption timeline, body weight, absorption rate, and the time between driving and the test. That is precisely what a Master’s Degree in Pharmaceutical Science is designed to evaluate.

What Deandra Grant Law Brings to Chemical Test Evidence

Most DWI defense attorneys challenge chemical test results through legal argument: the observation period was not maintained, the calibration records are missing, the chain of custody has a gap. Those arguments are valid and important. But they are incomplete without the ability to evaluate the underlying science independently.

Deandra Grant’s Master’s Degree in Pharmaceutical Science and ACS-CHAL Forensic Lawyer-Scientist designation means that chemical test evidence in the firm’s cases is evaluated at a scientific level and not just based on the paperwork surrounding it. The difference between a breath result of 0.07 and 0.09, the significance of an in vitro fermentation finding or the validity of a retrograde extrapolation opinion are all questions that require scientific training to answet. This firm has that training.

Call (214) 225-7117 to speak with Deandra Grant Law about your DWI charge and the chemical test evidence in your case.