Blood Tests (+ lab errors)
A blood test feels like the end of the road, surely a number from a crime lab is beyond dispute. It isn’t. A DWI blood result is only as trustworthy as the warrant that authorized it, the person who drew it, the way it was stored, and the lab that tested it. Any weak link can make the result unreliable, or inadmissible.
Did the police have a warrant?
In most cases, police need a warrant to take your blood. The Texas Court of Criminal Appeals held in State v. Villarreal (2014) that Texas’s mandatory-blood-draw statute cannot override the Fourth Amendment’s warrant requirement, and the U.S. Supreme Court held in Birchfield v. North Dakota (2016) that a warrantless blood draw cannot be justified simply as a search incident to arrest. The Supreme Court had already ruled in Missouri v. McNeely (2013) that the natural dissipation of alcohol in the bloodstream is not, by itself, an emergency that excuses getting a warrant.
What this means in practice: if an officer forced a blood draw without a warrant and without a genuine emergency, the result may be suppressed: thrown out entirely. This is one of the most powerful defenses in a blood-test case, and it’s the first thing we investigate.
Watch: Blood Draws, Warrants and No Refusal Weekends
When police can take your blood, when they need a warrant, and what a Texas “no refusal” weekend really means.
Can police take your blood without consent?
What to do during a “no refusal” weekend
How blood evidence gets contaminated or corrupted
Even with a valid warrant, a blood result can be wrong:
- Who drew it, and how. Texas law requires blood to be drawn by qualified personnel using proper technique. Using an alcohol-based swab, a bad draw, or an unqualified person creates problems.
- Fermentation and clotting. If a vial lacks the correct preservative and anticoagulant, or is stored at the wrong temperature, blood can ferment and actually produce alcohol after the draw, inflating the result, or clot and skew the measurement.
- Chain of custody. Every hand the sample passes through must be documented. Gaps raise real doubt about whether the blood tested was even yours, or whether it was compromised.
- Whole blood vs. serum. Serum readings run higher than whole-blood readings; mixing them up produces a falsely high BAC.
- Retrograde extrapolation. Because blood is often drawn an hour or more after driving, the State may try to “extrapolate” your BAC backward to the time you were driving, a guess that depends on assumptions a skilled lawyer can dismantle.
Texas crime labs are not infallible
Texas has a documented history of crime-lab problems: from the shuttered Houston Police Department crime lab to statewide DPS lab issues, including mishandled samples, calibration failures, and analyst misconduct. A blood result is a human product of a fallible system, and it deserves scrutiny, not blind trust.
Facing a blood-test DWI in Houston? Call Blass Law at (713) 664-4000. We examine the warrant, the draw, the storage, the chain of custody, and the lab’s records, and we bring in independent experts when the science demands it.
(Related: breath test defense · field sobriety tests · ALR license hearing)
Related: How we challenge DWI tests · Breath test defense (Intoxilyzer 9000) · Field sobriety test defense
Reviewed by Jay Blass Cohen, founder of Blass Law PLLC — ACS-CHAL Forensic Lawyer-Scientist, certified Drug Recognition Expert, and SFST instructor, serving Houston since 2009. This article is general information, not legal advice for your situation. Prior results do not guarantee a similar outcome.