If you have been charged with  driving while intoxicated (DWI) in Texas, police may have obtained a sample of your blood, breath or urine as evidence to support the charge. Breath samples are not designed to detect blood alcohol content. Instead, they measure alcohol concentration in a person's breath, not their blood.

Blood draws are considered more invasive than a breathalyzer test, and police must obtain a warrant or valid consent to take a blood sample. It may be possible to challenge the results of a blood test which can lead to a dismissal of your case or not guilty verdict.

Overview Of DWI Blood Draws In Texas

Blood samples are generally considered more accurate than breath or urine samples. The accuracy of a breath test can be affected by can be affected by a number of factors such as the amount of air a person is able to expel. Breath tests can also be contaminated by recent ingestion of alcohol. Urine samples are more likely to show  positive results for drugs that were ingested previously even if a person is not under the influence of them.

Though blood tests are considered more accurate than other types of tests, law enforcement officers face greater procedural hurdles when obtaining a blood sample. Courts have held that there is a heightened expectation of privacy in a person's blood. To obtain a blood sample, the police must obtain valid consent or get a warrant. Unlike a breath or urine test, a blood draw involves piercing a person's skin.

In  Birchfield vs. North Dakota, the United States Supreme Court ruled that the Fourth Amendment prohibition of warrantless searches applies to blood draws because they cannot be easily taken at the side of the road like a breath test, and there is a greater privacy interest in blood than there is in a person's breath because the samples can be stored and tested later. While you cannot be criminally penalized under state implied consent laws for refusing to take a blood test, you can face other penalties like suspension of your driving privileges.

False Positives In Blood Testing

In some cases, blood test results are inaccurate. The possibility that the result is a false positive can raise a reasonable doubt about your guilt. If the sample was taken after alcohol swabs were applied to the skin or if the sample was mixed improperly with a solvent, this could cause a false positive on the test.

Other substance in the blood such as lactic acid can sometimes produce a positive result. In some cases, ingestion of certain foods or medications could cause a blood sample to yield a positive result for a drug that you have never ingested. If samples were not properly preserved, this can cause a blood test to yield a positive result.

Challenging Blood Draw Results

If police obtained a blood sample from you in violation of your rights or there was a problem with the way that the sample was taken or stored, it may be possible to get the results of the sample excluded from evidence or challenge its validity. If this is the most substantial evidence in the state's case against you, this can often result in a dismissal of the case.

Most challenges to a blood draw in a DWI case involve challenges to the way the sample was taken or challenges to the validity of the sample itself. Following are some of the most common reasons a blood draw is challenged:

  • False promises of leniency by law enforcement made to obtain the sample
  • Defective warrant
  • Coercive threats made by law enforcement during the process of obtaining a sample
  • The sample was taken by individuals without the proper qualifications – a blood sample requested by police must be taken by a physician, qualified technician, chemist, registered nurse, or licensed professional nurse.
  • The sample was improperly labeled or stored
  • Alcohol swabs to swab the skin prior to taking the sample
  • Police destroyed all blood samples and failed to preserve samples for retesting
  • Improper calibration of equipment used to test the sample
  • Failure to take a blood sample in a sanitary place

An attorney may be able to get the results of a blood test thrown out by filing a “Motion to Suppress” in cases that involved a faulty warrant or another violation of law by law enforcement. If evidence is excluded, the  prosecution is prohibited from using it. Even if the prosecution is allowed to present the results of a blood test to a judge or jury, there are numerous ways to attack the accuracy of the test.

Within two hours of blood testing, a person asked to submit to a blood draw may, within a reasonable time not to exceed two hours, request for another sample to be taken by a person qualified to take the sample in addition to the sample taken at the request of the investigating officer.

Blood Testing and Retrograde Extrapolation Testimony

To prove that a subject had a BAC level of 0.08 or above at the time that they were driving, the state may attempt to introduce expert testimony on retrograde extrapolation. This strategy is intended to demonstrate the subject's BAC level at the time of driving, which is what counts in a DWI charge rather than the BAC level at the time of testing. Like all scientific expert testimony, an expert must be able to explain the theory and reliability behind the science. If the expert is unable to do so, courts have the discretion to exclude the expert's testimony. A court may consider factors such as:

  • The length of time between the offense and administration of the test
  • The number of tests given
  • The interval between each test given
  • The individual characteristics are known to the expert at the time of testing such as the person's weight, gender, tolerance for alcohol, the amount of alcohol the person consumed, the time of the last drink, and what the person had to eat before and while drinking

Retrograde extrapolation testimony can sometimes be challenged if an expert is unable to explain all these factors that affect the reliability of their finding. These factors are often not taken into account by arresting officers during a DWI investigation.

A court will consider whether the proffered testimony is “more prejudicial than probative.” If there are irregularities in testing such as an interval between tests, a court may exclude this type of testimony because it would not assist the trier of fact in a determination of a defendant's guilt.

It is important to have an experienced  Texas DWI defense attorney review the evidence in your case and never plead guilty to a DWI without a lawyer. Sometimes errors by law enforcement and other weaknesses in the state's case can result in a dismissal or not guilty verdict.

Intoxication Manslaughter – Second Degree Felony Punishment

If you have been charged with a DWI, contact an  experienced defense lawyer right away. A DWI conviction can have serious consequences including fines, jail time, the loss of your driver's license, community service, and it can affect your career going forward.  Contact Jason S. English Law, PLLC online or call (512) 454-7548 to schedule a consultation.