Challenging a DWI Blood Test

December 11th, 2020 by Tad Nelson in DWI

Those who are arrested for and charged with driving while under the influence of drugs or alcohol in Texas can only be convicted if a prosecutor has enough evidence to prove guilt. In some cases, this may take the form of a police officer’s testimony about a driver’s behavior, the results of a roadside breath test, or even the results of a chemical or blood test administered at the police station. Fortunately, even when those results indicate that a driver was intoxicated, he or she isn’t out of legal options. In fact, an experienced Houston DWI lawyer could even help a driver avoid conviction or get his or her charges reduced if able to successfully challenge a blood test in court. 

Errors in Blood Drawing Procedures

While blood tests are generally considered to be more accurate than roadside breath tests, they are by no means foolproof. There are, for instance, a number of simple mistakes that, if made during a blood drawing procedure, could significantly alter a person’s results, including:

  • Blood sample contamination, which often occurs when an officer fails to take sanitary precautions before drawing blood, such as using needles that have been sanitized with non-alcoholic solutions, 
  • Drawing blood from an artery instead of a vein, which is more likely to result in a higher, albeit less accurate, concentration of alcohol in a person’s bloodstream;
  • Drawing an inadequate amount of blood to perform the test;
  • Using alcohol swabs to clean the skin prior to drawing blood, which can adversely impact test results; and
  • Drawing blood without the proper qualifications

All of these mistakes could have significant repercussions on the accuracy of a driver’s blood test. Fortunately, an experienced DWI lawyer will be well-versed in these arguments and able to use them to challenge a tainted sample. 

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Improper Tests

Drawing blood is not the only difficult part of taking a BAC blood test, as the processes required to test the blood itself are also extremely complex. This means that even a minor error during testing can lead to a skewed result. Failing to recalibrate plasma blood test results to whole blood value, for instance, can adversely affect a blood sample, as can failing to properly mix a sample with the correct preservatives or anticoagulant agents. Failing to properly clean and calibrate the equipment used to perform a blood test is also a relatively common occurrence that can result in the improper testing of hundreds of samples. 

Storing Samples

Once they have been drawn, blood samples are stored by law enforcement officers who are required to comply with strict chain of custody procedures, which ensures that samples are not lost or tainted. If the chain of custody for a sample is broken, that sample will often be barred from entry as evidence in a DWI case. Similarly, blood samples must be refrigerated before being taken to a lab for testing. Failing to store blood samples in safe containers of a certain temperature can affect the integrity of the sample itself. If either of these errors is discovered, a court could throw out the test results and if officers failed to secure additional vials for retesting, could even dismiss the charges entirely. 

Hiring a Houston DWI Lawyer to Dispute Your Blood Test Results

If your blood was not properly drawn or tested after a DWI arrest, you may be able to avoid conviction. Please call The Law Offices of Tad Nelson & Associates at 281-962-7817 to learn more about defending yourself against DWI charges from a dedicated DWI lawyer in Galveston, Houston or League City. 


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