Charged with DWI based on a blood test?Houston defense by a former police officer trained in gas chromatography.
Texas blood-test DWI cases are won and lost on technical details that most defense lawyers cannot effectively challenge. The blood draw, the chain of custody, the calibration of the gas chromatography machine, the qualifications of the lab analyst — every step has procedural and scientific requirements. Every step is a potential defense angle.
How a Texas DWI blood test is actually performed.
The number that ends up on a Texas DWI lab report did not come from a single moment of testing — it came from a multi-step process, and every step has documented procedures that the State must follow. Understanding the science is the first step in challenging it.
Draw
Blood is drawn by qualified personnel under Tex. Transp. Code § 724.017
Preservation
Vacuum tubes with anticoagulant and sodium fluoride preservative
Chain of Custody
Sealed, labeled, refrigerated, transported to lab
Gas Chromatography
Headspace GC separates and identifies ethanol
Reporting
Analyst reviews chromatogram, certifies BAC result
The State must establish a proper foundation for every link in this chain. If any link breaks, the result may be suppressed or excluded under Tex. R. Evid. 702 or 901. The defense’s job is to find the breaks.
When can the State get your blood?
Texas operates under an implied consent framework under Tex. Transp. Code Ch. 724: a driver is deemed to consent to a blood or breath test if lawfully arrested for DWI. Refusal triggers a license suspension. But implied consent is not the same as actual consent for Fourth Amendment purposes.
The current law on warrantless blood draws:
- Missouri v. McNeely (2013) — the natural metabolism of alcohol does not, by itself, create per se exigency to justify a warrantless blood draw
- Birchfield v. North Dakota (2016) — warrantless blood tests are not authorized as searches incident to a lawful arrest
- State v. Villarreal (Tex. Crim. App. 2014) — Texas implied consent statutes do not, by themselves, authorize a warrantless blood draw absent another recognized exception
That means: in most Texas DWI cases involving blood, the State should have a search warrant. If it did not, the blood draw may be subject to suppression. If it did, the warrant itself is the next target — for adequacy of probable cause, particularity, and execution.
Mandatory blood draw situations under Tex. Transp. Code § 724.012:
- DWI involving an accident with serious bodily injury
- DWI involving a fatality (intoxication manslaughter)
- DWI with a child passenger under 15
- Prior DWI convictions on the record
- Suspended driver’s license at the time of arrest
Even in “mandatory draw” cases under § 724.012, the post-McNeely warrant requirement still applies. The statute does not override the Fourth Amendment.
How a blood-test DWI is defended.
Blood-test DWI cases create more defense angles, not fewer. The science is detailed. The procedural requirements are exact. The State must establish every link in the chain. Each link is a potential motion to suppress.
Was there a valid search warrant?
If the blood draw was warrantless, was a recognized exception present? If a warrant issued, was probable cause adequate? Was the warrant particular? Was it executed properly?
Who drew the blood?
Under Tex. Transp. Code § 724.017, only specifically qualified personnel may draw blood for evidentiary purposes. An unqualified draw can suppress the result.
What was the chain of custody?
Every transfer of the sample must be documented. Gaps in the chain — even short ones — create reasonable doubt about whether the tested sample is the sample drawn.
Was the GC machine properly calibrated?
Gas chromatography machines require regular calibration with known standards. Calibration logs must be available. Out-of-spec or missing calibrations can disqualify results.
Was the lab analyst qualified?
The analyst who certified the BAC must be qualified under lab protocols, trained in the methodology, and available for cross-examination under Bullcoming v. New Mexico.
What is the analytical uncertainty?
Every BAC result has a measurement uncertainty — typically ±0.005 to ±0.01 BAC. For results near the 0.08 statutory threshold, uncertainty can put the actual BAC below the legal limit.
Blood-test cases follow a different license-suspension clock.
Blood-test cases work differently from breath cases on the license side. Because the blood is sent to a lab, there is often no 15-day deadline at the scene. The Administrative License Revocation (ALR) process under Tex. Transp. Code Ch. 524 is triggered only if and when the lab result comes back at 0.08 or higher. If that happens, DPS mails a Notice of Suspension and a 20-day deadline to request an ALR hearing applies from the date of that notice. That delay can work in your favor — it allows time to investigate how the blood was drawn, stored, and tested.
Gas chromatography training. Police officer experience.
Attorney Kenneth V. Mitchell is trained on the gas chromatography methodology used in Texas DWI blood testing. Combined with 13 years as a Texas police officer — including service in the Hit and Run and Vehicular Crimes Division of the Houston Police Department — that training produces a defense built from both the law enforcement side and the forensic science side.
For a blood-test DWI, that combination matters. Cross-examining the lab analyst about chromatogram peaks, baseline drift, and calibration standards requires actual familiarity with the methodology. Cross-examining the arresting officer about the draw, the warrant, and the chain of custody requires actual familiarity with law enforcement protocols. Few Houston DWI lawyers can do both.
If your case involved an Intoxilyzer 9000 breath sample rather than a blood draw, the defense issues are different. See DWI breath test defense.
The science is detailed. So is the defense.
A Texas DWI blood test result is not a single number — it is the product of a multi-step procedural and scientific chain. Every step can be challenged. The State must prove every link. The defense’s job is to find the breaks.
Free, confidential case review. Flat-fee blood-test DWI defense across Harris, Montgomery, and Galveston Counties. Kenneth reviews every blood-test DWI inquiry personally.