Can a San Antonio DWI Lawyer Challenge the Blood Test in Your Case?

If you got arrested for DWI in Bexar County and the officer took a blood draw, you may have been told the number doesn't lie. That framing is wrong. Blood evidence in Texas DWI cases passes through multiple hands, storage facilities, and laboratory processes — and each of those steps is a point of vulnerability. A San Antonio DWI lawyer who knows the science and the law can find those vulnerabilities. Here is what the defense actually looks like.

What we look for in a blood DWI case:

• Under Art. 39.14(h) of the Texas Code of Criminal Procedure, the State must disclose all exculpatory, impeachment, and mitigating evidence related to your blood analysis — including lab audits and analyst credentials.

• Chain of custody gaps, improper storage, and equipment maintenance failures can all compromise a blood result.

• The preservative used in blood draw kits does not prevent ethanol contamination in every case.

• Retrograde extrapolation — using a later blood draw to estimate your BAC at the time of driving — is scientifically questionable and legally constrained under Texas case law.

• DWI blood cases in Bexar County can move fast once you get your blood results. Early involvement by a defense attorney matters.

  
  

What the State Must Turn Over Under Art. 39.14

Under Article 39.14(h) of the Texas Code of Criminal Procedure — known as the Michael Morton Act — the State must disclose any exculpatory, impeachment, or mitigating document, item, or information in its possession that tends to negate your guilt or would tend to reduce your punishment. In a blood DWI case, that obligation extends to:

• Lab accreditation certificates and all audit reports, including any documented compliance failures

• The forensic analyst's qualifications: testimonial evaluation forms, training history, and active certifications

• All instrument maintenance logs for the equipment used to test your blood

• Quality-control and quality-assurance documentation for the testing batch that included your sample

• The complete chain of custody report from the blood draw through the lab report

  
  

If any of these documents are missing, incomplete, or produced late, that is a discovery issue worth pressing — and it can shape the entire defense posture.

What to Attack in a Bexar County Blood DWI Case

Chain of Custody

A DPS blood kit in a typical Bexar County case can pass through a law enforcement officer, a local evidence coordinator, a FedEx shipment to a regional hub, a bio-refrigerator, a coordination unit, and finally the analyst who signs the report. The chain of custody log documents every one of those transfers. Any unrecorded transfer, container discrepancy, or unexplained break in the timeline is a legitimate question at trial: was the blood that was tested the same blood that was drawn?

Lab Maintenance and Accreditation Records

Labs used in Texas DWI cases are required to maintain accreditation and keep their instruments calibrated. Maintenance logs for the gas chromatograph headspace analyzers used to measure blood alcohol concentration are available through subpoena. Accreditation audit reports for Texas crime labs have documented real problems such as refrigerators recording temperature excursions outside the required range on dozens of occasions with no record of corrective action. Those findings are discoverable and relevant to the reliability of the testing process.

Sodium Fluoride and Specimen Contamination

Blood draw kits used in Texas DWI cases are supposed to contain sodium fluoride as a preservative. According to Garriott's Medicolegal Aspects of Alcohol (6th Ed.) — a widely recognized scientific reference in blood alcohol litigation — an increase in the concentration of ethanol in a biological specimen is associated with contamination, and sodium fluoride has been shown to inhibit most organisms except Candida albicans. If a sample was contaminated or improperly preserved, the BAC reading the State is relying on may be artificially higher than what was in the driver's blood at the actual time of the draw.

Analyst Credentials and Bias

Not every forensic scientist who testifies in a Bexar County DWI case has the credentials the jury assumes. Testimonial evaluation forms list the disciplines each DPS analyst is certified to testify in, when those certifications were issued, and when they expired. Blood alcohol testing and toxicology interpretive opinions are separate certifications. An analyst who testifies about the significance of a BAC result needs an active interpretive certification to offer that opinion.

Texas Rule of Evidence 803(18) allows statements from recognized scientific publications to be read into evidence when established as a reliable authority — either by the analyst's own acknowledgment on cross-examination, by another witness, or by judicial notice. Garriott's Medicolegal Aspects of Alcohol and Disposition of Toxic Drugs and Chemicals in Man are widely accepted in this area.

Retrograde Extrapolation: Why the BAC Number at the Time of Driving Is Contested

In many blood DWI cases, there is a gap between the traffic stop and the blood draw — sometimes an hour or more. The State may ask its analyst to testify that the driver's BAC was even higher at the time of driving, using retrograde extrapolation.

Retrograde extrapolation has significant scientific limitations. As Dubowski (1976) documented, peak blood alcohol concentration and elimination rates vary widely between individuals. The Jones (1993) study acknowledged: requests to back extrapolate a suspect's BAC from the time of sampling to the time of driving are often made, but this remains a dubious practice owing to the many variables and unknowns involved.

Texas courts have put legal constraints on this practice. The Court of Criminal Appeals addressed foundational requirements for retrograde extrapolation testimony in Mata v. State, 46 S.W.3d 902 (Tex. Crim. App. 2001), and the issue was developed further in Veliz v. State, 474 S.W.3d 354 (Tex. App. 2015). Without individual-specific data about absorption and elimination rate, a retrograde opinion may not have the foundation Texas law requires.

When to Call a DWI Attorney in San Antonio

The sooner a defense attorney gets into a blood DWI case, the more options exist. ALR hearings have a short window after arrest. Discovery requests for lab records take time, and pretrial motions challenging admissibility have to be briefed before the trial date.

If you have been arrested for DWI in Bexar County and blood was drawn, contact The Law Office of Christian Vega at (210) 424-4264. We handle DWI blood draw cases throughout San Antonio and the surrounding counties.

By Christian O. Vega, Attorney at Law — The Law Office of Christian Vega · 103 E. Travis St #425, San Antonio TX 78205

Past results do not guarantee similar outcomes in any future case. The information here is general legal information about Texas law, not legal advice for your specific situation. Reading this post does not create an attorney-client relationship.