Harris County runs "no refusal" year-round in practice: refuse a breath test and an officer will get a warrant for your blood within the hour. Clients walk in convinced a 0.12 or 0.15 blood result ends the discussion. As a former Vehicular Crimes prosecutor who was dispatched to blood-draw scenes, Brian Foley knows it does not — and explains why in this video.
How blood evidence is made
A blood result is not a single fact; it is the end product of a chain of human steps, each of which can fail:
- The warrant. A blood draw requires consent or a search warrant supported by a sworn affidavit. Affidavits written at 3 a.m. by an officer using boilerplate language frequently contain errors — wrong dates, wrong names, missing facts — that can invalidate the warrant.
- The draw. Texas law requires a qualified person (a physician, nurse, EMT-P, or qualified technician) to draw blood in a sanitary place. Draws done by an unqualified person or in the back of a patrol car are challengeable.
- The tube. Gray-top tubes contain a preservative and an anticoagulant. An expired tube, an under-filled tube, or a tube that sat in a hot car for days can ferment — producing alcohol that was never in your body.
- Chain of custody. Every hand the tube passes through must be documented. Gaps are common.
- The lab. Blood is analyzed by gas chromatography. The instrument must be calibrated, the batch must include controls, and the analyst must be qualified to testify. Discovery of the lab's raw data, not just the one-page result, is where cases are won.
"But the test was two hours later"
Blood is drawn long after driving. To prove you were at 0.08 or more while driving, the State either relies on the result alone or calls an expert to "retrograde extrapolate" — estimate backward using assumptions about absorption and elimination rates that vary widely between individuals and depend on facts (when you last drank, what you ate) the State often does not have. Cross-examining that extrapolation is one of the most effective tools in a blood test case.
The video still matters
Juries watch the in-car and body-camera footage. A driver who is polite, coherent, steady on their feet, and follows instructions does not look like a 0.15 — and jurors are entitled to weigh what they see against a number from a lab. High-blood cases are won when the science is questioned and the video does not match.
Watch the companion video on beating a blood test DWI, read our DWI blood tests page, or call or text (713) 703-1718 for a free consultation with our Houston DWI attorneys.