The state builds most DWI cases on a number — a breath printout or a lab report. But that number is only as strong as the machine, the procedure, and the people behind it. Challenging chemical test evidence is the heart of modern DWI defense.
Breath tests: the machine is not infallible
Texas uses approved breath-testing instruments that require strict protocols: a 15-minute observation period, certified operators, scheduled maintenance, and reference-sample checks. Defense attorneys subpoena:
- Maintenance and calibration records — missed checks and error logs undermine every result the machine produced.
- Operator certification records — an expired or lapsed certification invalidates the test.
- The observation period. Burping, reflux, or mouth alcohol within 15 minutes of the test can inflate readings dramatically.
Blood tests: a long chain with weak links
Blood evidence must survive scrutiny at every step:
- The draw itself. Was there a valid warrant or valid consent? Was the draw performed by qualified personnel using proper (non-alcohol) antiseptic?
- Chain of custody. Every hand that touched the vial must be documented. Gaps create doubt.
- Storage and preservation. Improperly preserved samples can ferment — producing alcohol that wasn't in your bloodstream.
- The lab. Independent retesting of the retained sample sometimes tells a different story than the state's report.
Rising BAC: timing matters
Alcohol takes 30–90 minutes to absorb. If you drank shortly before driving, your BAC at the station an hour later may be significantly higher than your BAC behind the wheel — and Texas law criminalizes intoxication while driving, not while sitting in a chair at the jail.
Refusal cases are defensible too
If you refused testing, the state loses its number — but gains a suspension and an argument. Defense then focuses on the stop, the officer's observations, and the refusal warnings themselves. See our refusal guide.
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