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Understanding Blood Tests in Texas DWI Cases

Typically, in Texas DWI cases, breath tests are used to determine whether a person is unlawfully intoxicated per se while operating a vehicle in a public place. In other words, a breath test will show if the driver's blood alcohol concentration is 0.08% or higher. Sometimes, though, an officer will choose to use a blood test. This happens often when:

  • a driver refuses a breath test;
  • a driver is unconscious; or
  • a driver is suspected of being intoxicated by controlled substances or other drugs – including prescription or over-the-counter drugs.

Blood tests tend to be more accurate than breath tests. That said, there are inherent problems with the tests just as there are human-made problems with the tests. Because of these problems, the results can be successfully challenged so long as the attorney challenging the tests understands the technical and scientific aspects of them. 

Carl Ceder has a deep understanding of blood tests: how they work, how to identify problems, why the tests fail, including the human error, technology, and science that factor into these failures. Below is an overview of blood tests and what you need to know if you had a blood test taken during a DWI investigation in the Dallas-Fort Worth metro area. If you have been arrested for a DWI, Contact Carl Ceder to schedule a free initial consultation. 

Understanding Blood Tests in a Texas DWI Case

When an officer (as opposed to a medical professional) requests a blood sample for a blood test, Texas Transportation Code, Chapter 724, controls how it is done. Certain rules and procedures must be followed. First, according to § 724.012, the officer may request a blood sample upon arrest when there is "reasonable grounds to believe the person" was "intoxicated [while] operating a motor vehicle in a public place."

The police officer must request that a qualified person take the sample in a sanitary place. According to § 742.017, qualifying persons include 

  • physicians
  • technicians
  • chemists
  • registered professional nurses, or
  • licensed vocation nurses.

The Transportation Code also states that the subject consents to the blood draw (or breath sample), by virtue of being arrested for driving while intoxicated. So, you can refuse a breath or blood sample prior to an arrest. If a person refuses to provide a breath or blood sample upon arrest, the police officer must obtain a warrant to have blood drawn for testing. A search warrant that authorizes a police officer to obtain a person's blood is an evidentiary search warrant according to the Texas Code of Criminal Procedure Art. 18.02(10). Because the latter is true, the police officer must:

  1. get a DWI blood search warrant signed by a magistrate and not by a justice of the peace or a municipal judge;
  2. provide an affidavit to accompany the warrant that establishes sufficient probable cause showing the alleged DWI offender committed the DWI offense (e.g., failed field sobriety tests); and 
  3. show that the evidence to be seized (e.g., the blood sample) will be evidence necessary to prove a DWI offense.

It is important to note that if you refuse to give a breath sample and the officer obtains a warrant to take a specimen for analysis – you do not get to choose if the specimen is a breath or blood sample. Blood tests are more accurate than breath tests, so this is important to keep in mind. After you refuse and after a warrant is obtained, the officer can compel you to give a blood sample.

There are times when it is mandatory (meaning you cannot refuse) an officer's request for a blood (or breath) specimen. Mandatory taking of a specimen occurs when the subject  

Challenging Blood Test Results in Dallas-Fort Worth DWI Cases

Blood test results are challenged through different means. Some are highly technical while others don't require a scientific mind. Some of the strongest arguments against the validity, accuracy, or reliability of blood tests are listed here.

  • Blood Vial Challenges. An attorney may be able to challenge a blood sample's admissibility based on how the blood sample was kept. Typically, blood samples must be stored in blood vials with an anticoagulant, a preservative, and a tamper-proof seal. If not, then the means to keep the blood may have impacted the integrity of the sample.
  • Skin Cleaner Challenges. If the person withdrawing the blood used certain disinfectant toilettes on the alleged DWI offender, then enough ethanol may have made its way into the blood sample, increasing the blood alcohol content.
  • Equipment Issues. The equipment used to draw the blood may not have been sterile. This is especially true if, for example, the alleged DWI offender was taken to the ER where a nurse had already prepped the room and removed needles and other items from their packages.
  • Distorted Readings. Depending on whether serum or whole blood was used, the results could be inaccurate. Serum (or plasma) often produces a higher BAC than whole blood samples. 
  • Problems with the Handling of the Blood Sample. The blood specimen could have been neglected at the lab or if left for some time, the sample wills tart to decompose, and this process produces more alcohol in the blood.
  • Chain of Custody Gaps. A record must be provided of who and where the blood sample was/is at all times. When there is a gap in the custody of the blood sample, the accuracy of the test results is questioned.

Challenging blood tests requires a thorough understanding of the process, including the technology and science behind it. Only a skilled, experienced DWI defense attorney will have the kind of knowledge required to put forth a viable challenge.

Contact a Knowledgeable DWI Defense Attorney in Dallas-Fort Worth Today

If you have been charged with a DWI and a blood draw was taken to analyze your BAC, contact Carl David Cedertoday. He will thoroughly review the process and your DWI case and provide honest, candid answers to your questions.