Can Blood or Urine Test Results Be Challenged in a Texas DWI Case?

 Posted on May 15, 2026 in Criminal Defense

If you have been charged with a DWI in Texas in 2026, the state's case against you may rest heavily on a blood or urine test. Many people think that once a test result exists, the outcome is set. That is not true. Even when prosecutors rely heavily on chemical test evidence, a DWI case is not always guaranteed to move forward. According to the Texas Department of Public Safety’s 2024 DWI Summary Report, 2,538 DWI arrests in Texas resulted in release with no charges in 2023.

These tests are only as good as the people and steps behind them, and errors happen more often than most people know. A Marble Falls, TX DWI defense attorney can look at the testing process in your case and find any problems that could help your defense.

What Does Texas Law Say About Chemical Testing in DWI Cases?

Under Texas Transportation Code § 724.011, drivers arrested for DWI are treated as having agreed to provide a breath or blood specimen. Urine testing may still come up in some drug-related DWI cases, but it is not specifically listed in the implied consent statute. Officers choose which type of test to request based on the situation. Blood tests are especially common when drug use is suspected or when a breath test is not available.

Refusing to take a requested test can lead to consequences such as a driver's license suspension. Even so, agreeing to a test does not mean the results will automatically hold up in court. The law sets rules for how samples must be collected, handled, and tested. When those rules are not followed, the results may be challenged.

How Can Errors in a Blood Test Affect a Texas DWI Case?

Blood tests are often seen as the strongest evidence in a DWI case because they measure alcohol or drugs directly from a person's blood. But a blood test is only reliable when every step is done right. Common problems include:

• Improper collection: Texas law requires that blood be drawn by a qualified person in a sanitary place. If those rules were not followed, the result may not be valid.
• Chain of custody failures: Every person who handles a blood sample must be logged. If there are gaps in that record, the defense can argue the sample may have been mixed up or tampered with.
• Storage and preservation errors: Blood samples must be preserved and stored correctly. Poor storage conditions or improper handling can affect the accuracy of the results.
• Lab errors: Testing labs can make mistakes in how they handle or record results, and their equipment must be properly maintained.

Any one of these problems can give your attorney grounds to challenge the results or ask the court to throw them out.

How Can Urine Test Results Be Challenged in a Texas Drug DWI Case?

Urine tests are most often used when drug use is suspected, and they come with their own set of problems. Unlike blood tests, urine tests do not show how much of a substance was in your system while you were driving. They only show that a substance was present in your body at some point. Many drugs stay in the body for days or even weeks after their effects have worn off. A positive urine test does not prove you were impaired at the wheel.

Other issues include false positives from certain medicines or foods, and contamination during collection. A defense attorney can sometimes use scientific analysis to show that the test result does not prove impairment at the time of the stop. When a urine test is the main piece of evidence, that difference can matter a great deal in your case.

Speak With a Fredericksburg, TX DWI Defense Lawyer

When chemical test results are the main evidence against you, your defense depends on someone who understands the science behind those tests. Attorney Russ Baker holds a Bachelor of Science in Biology from Texas Christian University. His background in science equips him to examine how blood and urine evidence is collected, stored, and analyzed in DWI charges. With over 16 years of legal experience, he knows what to look for and how to use it. Contact the Marble Falls, TX criminal defense attorney at the Law Office of Russ Alan Baker, PLLC by calling 325-216-2006 today.