Challenging Blood Alcohol Testing in Texas DWI Cases: Calibration Errors, Chain of Custody, and Suppression Strategies

Challenging blood alcohol testing in Texas DWI cases requires examining how the sample was obtained, whether it remained identifiable and uncontaminated, how the laboratory tested it, and whether the prosecution can establish reliability under Texas law.

Texas defines intoxication as either losing the normal use of mental or physical faculties because of alcohol or another substance, or having an alcohol concentration of 0.08 or more. Even with a result above the legal limit, the State must prove that the evidence was lawfully obtained and scientifically dependable.

Have a Texas DWI lawyer investigate those weaknesses before the BAC reaches the jury.

The State Must Prove More Than A Laboratory Number

Blood testing has two stages: obtaining the sample and analyzing it.

A blood draw is a search under the Fourth Amendment. Police generally need a valid warrant, voluntary consent, or a recognized exception. Alcohol’s natural dissipation does not create an automatic emergency in every DWI investigation. Texas implied-consent statutes permit officers to request a specimen after certain arrests, but implied consent alone does not authorize every warrantless blood draw.

When officers obtain a warrant, a Texas DWI lawyer may challenge whether the affidavit established probable cause, correctly identified the driver, and supported the requested draw. Counsel may also examine whether officers exceeded the warrant or conducted the procedure unreasonably.

Texas Rule of Evidence 702 requires scientific testimony to be relevant and reliable. Kelly v. State requires proof that the scientific theory is valid, the technique is valid, and the technique was properly applied.

Calibration Errors Can Distort The Reported BAC

Texas laboratories commonly use headspace gas chromatography to identify and measure ethanol in blood. The method can be reliable only when the instrument is properly maintained, calibrated, and operated.

Calibration compares the instrument’s response with samples containing known alcohol concentrations. Controls, blanks, internal standards, and duplicate testing help determine whether a testing run performed within accepted limits. A DWI defense attorney may request the complete laboratory packet, including:

• Calibration curves and acceptance limits
• Quality-control and blank results
• Chromatograms and peak data
• Duplicate results and rerun explanations
• Maintenance, repair, and error logs
• Reagent records and expiration dates
• Analyst training and proficiency records

A failed control, unexplained recalibration, irregular peak, or maintenance issue may support exclusion or force the analyst to defend the result under cross-examination. The defense should also examine manual adjustments, repeated failed runs, improper rounding, and results outside laboratory tolerance limits.

Measurement uncertainty matters when the reported BAC is close to 0.08. Every scientific measurement has a range of uncertainty. A result of 0.081 may be less conclusive if the laboratory’s uncertainty range reaches below the legal limit.

Timing Can Disconnect The Result From Driving

A blood test measures alcohol concentration when the sample is taken, not necessarily when the person was driving. If substantial time passes, the State may use retrograde extrapolation to estimate an earlier BAC.

That calculation depends on drinking times, alcohol quantity, food intake, and whether alcohol was still being absorbed. Without reliable facts, the estimate may rest on assumptions. Mata v. State requires careful consideration of the available facts and underlying assumptions.

Chain Of Custody Must Identify And Protect The Sample

Chain of custody documents who collected, sealed, transported, stored, opened, and tested the blood. The State must connect the laboratory sample to the accused and show that it remained suitable for analysis.

The defense should compare the collection kit, labels, seals, transfer forms, property-room records, courier logs, accession records, and laboratory timestamps. Common problems include:

• Expired collection kits
• Missing initials, dates, or seal information
• Conflicting specimen numbers
• Unexplained refrigeration or delivery delays
• Improper storage temperatures
• Broken or undocumented seals
• Leaking or partially filled tubes
• Unrecorded transfers
• Sample consumption preventing independent testing

Texas courts often treat minor custody gaps as matters affecting evidentiary weight rather than admissibility. A strong challenge must connect the defect to a realistic possibility of contamination, fermentation, substitution, degradation, or mistaken identification.

Collection Errors May Affect Sample Integrity

Texas law limits who may draw blood in certain implied-consent cases and requires collection in a sanitary place. The defense may investigate the blood drawer’s qualifications, location, equipment, and compliance with accepted medical practices.

Incorrect tubes, inadequate preservatives, poor mixing, and improper storage may affect sample integrity. Not every deviation requires exclusion, but several connected departures may undermine confidence in the result.

Medical records, collection forms, testimony from the blood drawer, and body-camera footage may reveal whether established procedures were followed.

Suppression And Exclusion Serve Different Purposes

A motion to suppress challenges the legality of obtaining the blood. It may apply when officers relied on an invalid warrant, exceeded its authority, obtained involuntary consent, or conducted an unreasonable draw. Texas Code of Criminal Procedure article 38.23 may require exclusion of evidence obtained in violation of federal or Texas law, subject to applicable exceptions.

A motion to exclude scientific evidence attacks reliability. Under Rule 702 and Kelly, calibration failures, invalid controls, unexplained reruns, analyst mistakes, or departures from written procedures may show that the method was not properly applied.

Authentication objections question whether the State connected the tested tube to the accused. Other objections may require the prosecution to produce a witness who can explain and defend the testing rather than rely solely on a laboratory report.

Not every defect produces complete suppression. Some weaknesses are more effective before the jury. A defense toxicologist may explain why the number is uncertain or why it does not prove intoxication while driving.

Early Discovery Can Shape The Defense

Blood evidence should be investigated immediately. Counsel may seek the warrant, affidavit, body-camera footage, collection records, kit information, laboratory procedures, raw data, accreditation materials, maintenance logs, audit findings, and communications about testing problems.

The remaining sample should also be preserved. Independent testing may confirm the reported BAC, produce a different result, or reveal a condition affecting reliability. Delay may lead to destruction of the sample or loss of records.

Challenge The Evidence Before It Controls The Case

A BAC report should be tested against the warrant, collection records, custody history, raw laboratory data, and Texas evidentiary standards. Paul F. Tu Attorney at Law can identify weaknesses in blood evidence and pursue suppression, exclusion, negotiation, or trial; contact us today at 713-774-2800 to discuss a Texas DWI charge.