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Understanding Blood Alcohol Concentration (BAC) in Texas DWI Cases

Calculating My BAC After a DWI Arrest in Houston, TX

If you're facing a DWI charge in Texas, you undoubtedly have many questions about what happens next and what you should do. But first, you should understand the DWI prosecution process and how the police and state will determine the evidence against you. A big part of this will be calculating your blood alcohol concentration after a DWI arrest. Under Texas law, you may be "intoxicated" if your BAC measures .08% or higher, or .04% or higher if you have a commercial driver's license.

Calculating Your BAC

In the 1930s, a Swedish scientist named Dr. Erik Widmark came up with a way to estimate blood alcohol concentration using a formula that considers:

  • A person's weight,
  • The number of drinks they've ingested,
  • The size of the drink,
  • The proof or concentration of the alcohol,
  • Gender, and
  • The time of drinking.

Using these factors, it became possible to estimate a person's BAC, although there are individual differences. For example, alcohol is more concentrated in women's cells, so their BACs will typically be higher than men's, even at the same body weight.

Then, in 1938, Dr. Rolla Harger developed the first portable breath BAC test, which was jokingly called the "Drunkometer." Other roadside BAC tests followed, and in 1954, Professor Robert Borkenstein developed the first breathalyzer test.

Breath Test BAC

Today, Texas police departments use a machine called the Intoxilyzer 9000. The Intoxilyzer uses infrared spectrometry, or the absorption of infrared light, to look for the presence of alcohol in a breath sample. Because ethyl alcohol absorbs infrared light in a unique way, the machine can identify and calculate the amount of alcohol in a breath sample. The Intoxilyzer measures grams of alcohol per 210 liters of breath, which is the standard required under Texas law. See Tex. Penal Code. § 49.01(1)(a).

In some cases, the police will ask you to voluntarily take a BAC test during a stop for suspected DWI. If you agree, they will probably conduct a breath test with the Intoxilyzer 9000. However, the Intoxilyzer can still give inaccurate BAC results in many situations because:

  • Breath tests don't measure the amount of alcohol in the blood or the brain.
  • The temperature, breathing technique, and breath and blood partition ratio can all affect the accuracy of breath tests.
  • Tests are typically calibrated and standardized for men, and the results for women may be inaccurate because of metabolic differences.
  • Interference from radio frequencies or cell phones can affect the results of the Intoxilyzer 9000.
  • The police officer performing the test may be improperly or inadequately trained.
  • The Intoxilyzer 9000 equipment may not be properly maintained, cleaned, or calibrated.

Blood Test BAC

Today, blood BAC testing typically gives more accurate results than breath tests. Blood tests use gas chromatography to determine your BAC. "Headspace gas chromatography with flame-ionization detection" is now the gold standard for analysis because it can be easily automated and is generally fairly accurate. The gas chromatography process analyzes chemicals in the blood without decomposing them. A technician mixes a solvent into a blood sample during the process and injects it into the gas chromatograph. The machine heats the liquid sample and vaporizes it into the gas phase, and the gas carrier separates the components of the sample in a long tube. As the sample hits the detector at the end of the tube, it records the sample reaching the detector on the y-axis and the amount of time it took to reach the detector on the x-axis. The result is a series of peaks proportional to the amount of each component found in the sample. The tech can then correlate these peaks to the number of grams of alcohol per 100ml of blood, as Texas law requires.

While modern blood tests are generally more accurate than breath tests, blood tests can also produce inaccurate results for a variety of reasons. Some of the reasons occur when:

  • The tech fails to follow laboratory protocols,
  • The tech fails to store samples properly,
  • The medical professional collecting samples fails to use the proper amount of anticoagulant,
  • The tech mishandles paperwork or fails to label samples properly,
  • The lab mixes up samples,
  • The tech fails to collect enough samples, and the lab destroys all the blood in testing, failing to retain samples for retesting,
  • Blood samples ferment or coagulate, or
  • Someone breaks the chain of evidence.

Problems can also arise when the equipment used for gas chromatography isn't properly maintained, cleaned, or calibrated.

Hire an Experienced Houston DWI Defense Attorney

If you're facing a DWI in Texas, it's important to remember that, although the consequences of a DWI conviction are serious, you are innocent until proven guilty. Just because you have a BAC over .08 or .04% does not mean that a court will convict you of a DWI. Rather, if you have a lawyer who is experienced in DWI defense, you'll have the best possible chance of fighting the charges. A skilled Texas DWI lawyer can analyze your case and determine your best path forward.

Attorney Doug Murphy is one of only two attorneys in Texas who are Board Certified in both Criminal Defense Law and DWI Defense. Doug's Board Certification means that his peers regard him as an attorney with extensive knowledge and experience in DWI defense. His certifications also mean that he has broad experience handling complex DWI cases and understands the technical aspects of BAC testing and the legal nuances of DWI defense.

Doug is also a past President of the Harris County Criminal Lawyers Association and serves as Dean Emeritus on the Board of Regents for the National College for DUI Defense. Doug has more than twenty years of DWI and criminal defense litigation experience in Texas, working passionately to defend his clients in and out of the courtroom. The Harris County Criminal Lawyers Association awarded him the Sharon Levine Unsung Heroes Award when he helped expose problems with the Houston Police Department's Blood Alcohol Testing vans. Doug's work with others eventually led to the police department decommissioning the vans, making DWI stops fairer for everyone.

If you're facing a DWI in Texas, Doug can help you too. Call the Doug Murphy Law Firm, P.C. at 713-229-8333 or contact Doug Murphy online to schedule your consultation.

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