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DWI Breath Tests in Texas — How They Work and How to Challenge Them

When the prosecution presents a blood test result in a DWI case, jurors tend to treat it as objective, scientific proof. The number appears precise. The laboratory report looks authoritative. And most defense attorneys lack the scientific training to question it.

At Deandra Grant Law, our attorneys hold ACS-CHAL Forensic Lawyer-Scientist designations, which include rigorous training in the chemistry and methodology of blood alcohol testing. We don’t just argue that the number might be wrong. We understand why it can be wrong, how errors occur, and where the prosecution’s evidence is scientifically vulnerable. In addition, the largest chapter in Deandra’s book, The Texas DWI Manual, is devoted to blood alcohol analysis.

How DWI Blood Tests Work: GC-FID Analysis

The standard method for analyzing blood alcohol content in Texas DWI cases is gas chromatography with flame ionization detection (GC-FID). This is a laboratory technique that separates and identifies the chemical components of a blood sample.

The process works as follows:

• A small portion of the defendant’s blood sample is vaporized and injected into the gas chromatograph
• The vaporized sample travels through a separation column, where different chemical compounds separate based on their physical properties
• As each compound exits the column, it passes through a flame ionization detector, which generates an electrical signal proportional to the amount of the compound present
• The instrument produces a chromatogram which is a graph showing peaks that correspond to each compound detected
• The peak identified as ethanol (drinking alcohol) is measured and used to calculate the blood alcohol concentration

This process sounds precise, and under ideal laboratory conditions, GC-FID is a reliable analytical method. But DWI blood testing is not conducted under ideal conditions. It involves human blood samples sometimes collected in a jail, transported to laboratories, and analyzed by technicians who must follow exacting protocols at every step. Errors can and do occur at every stage of this process.

Deandra is an Assistant Chromatography Instructor at Axion Labs in Chicago and teaches there 4 weeks each year.

Where Blood Test Evidence Can Be Challenged

Blood Draw Procedures

Texas law requires that blood samples in DWI cases be drawn by qualified personnel using approved methods. Challenges to the blood draw include:

• Use of alcohol-based skin antiseptic. Non-alcohol antiseptics (such as betadine or povidone-iodine) should be used for forensic blood draws. If an alcohol-based swab is used it violates standard operating procedures and may be subject to challenge.
• Improper collection tubes. Blood samples must be collected in tubes containing the correct preservative (sodium fluoride) and anticoagulant (potassium oxalate) in the proper amounts. If the wrong tubes are used, or if the preservative is insufficient, the sample may undergo chemical changes that lead to inaccurate results.
• Failure to mix properly. After blood is drawn into the collection tube, it must be gently inverted multiple times to distribute the preservative and anticoagulant throughout the sample. Failure to mix properly can result in uneven preservation and lead to the potential for invalid results.
• Unqualified personnel. Texas law specifies who may draw blood for DWI testing. If the blood was drawn by someone not authorized under the statute, the results may be subject to suppression.

In Vitro Fermentation

One of the most significant challenges to blood test accuracy is in vitro fermentation which is the production of alcohol inside the blood collection tube after the blood was drawn. This occurs when microorganisms, particularly Candida albicans (a common yeast), metabolize glucose in the blood sample and produce ethanol as a byproduct.

In vitro fermentation can occur when:

• The blood sample is not properly preserved with sufficient sodium fluoride
• The sample is stored at elevated temperatures
• There is a delay between collection and analysis
• The defendant has an elevated blood glucose level (diabetic or pre-diabetic individuals are particularly susceptible)

When in vitro fermentation occurs, the BAC reported by the laboratory is higher than the defendant’s actual blood alcohol level at the time of the draw. The laboratory is measuring alcohol that was produced in the tube, not alcohol that was in the defendant’s bloodstream.

Coelution

Coelution occurs when two or more volatiles exit the GC column at the same time and produce overlapping peaks on the chromatogram. If a substance other than ethanol coelutes with ethanol, the instrument may overcount the ethanol, producing an artificially elevated BAC result. Substances that can coelute with ethanol include acetone (elevated in diabetics), isopropanol, and other volatile organic compounds.

Chain of Custody

The blood sample must be documented, stored, and transported under controlled conditions from the moment it is drawn to the moment it is analyzed. Any gap in the chain of custody raises questions about whether the sample was properly preserved, whether it was contaminated, or whether it is even the correct sample.

Laboratory Procedures and Quality Control

The laboratory that analyzed the sample must follow validated protocols, maintain calibrated equipment, run appropriate quality control samples, and document its work. Our forensic training allows us to review laboratory procedures, quality control records, and analyst qualifications to identify deviations from accepted practice.

Challenging the Warrant

Under Missouri v. McNeely (2013), the police generally need a warrant to draw blood in a DWI case. If the warrant was not supported by probable cause, if it was obtained through false or misleading information, or if the blood draw exceeded the scope of the warrant, the blood test results may be subject to suppression.

Why ACS-CHAL Training Matters for Blood Test Challenges

Most defense attorneys know that blood tests can be challenged. But knowing that a test can be challenged and knowing how to challenge it are two different things. Our ACS-CHAL Forensic Lawyer-Scientist training gives us the scientific knowledge to:

• Read and interpret gas chromatography data, including chromatograms and calibration records
• Identify coelution, contamination, and other analytical errors in laboratory results
• Depose and cross-examine forensic analysts on their methodology, quality control, and conclusions
• Retain and work with independent forensic toxicology experts who can present alternative analyses
• Explain complex scientific concepts to juries in terms they can understand and apply

Defend Your DWI Case with Science — Contact Deandra Grant Law

If you are facing DWI charges in Texas, contact Deandra Grant Law for a free, confidential consultation. Our attorneys hold ACS-CHAL Forensic Lawyer-Scientist designations, giving us the scientific training to challenge the prosecution’s evidence at a level most defense attorneys cannot match. Lead attorney Deandra Grant has defended thousands of DWI cases over 30+ year.