Can a Blood Test Be Challenged in Bexar County?

Yes — Blood Test Evidence Is Challengeable in Bexar County

Blood draws in San Antonio DWI cases are typically analyzed at the Bexar County Crime Laboratory. The complete chain of custody — from the venipuncture through the lab analyst’s report — must be documented at every link. We request the complete lab file, not just the summary result, on every blood draw DWI case in Bexar County. The complete file includes collection documentation, phlebotomist qualifications, tube type, preservation records, storage logs, analyst certification, and gas chromatograph calibration records.

The Blood Draw Warrant

SAPD routinely obtains blood draw warrants under Transportation Code §724.017 when defendants refuse the breath test. The warrant must be supported by probable cause. If the warrant lacked a sufficient factual basis — if the affidavit supporting it overstated the officer’s observations or omitted contrary facts — the blood draw is an illegal search and the results are suppressible under the Fourth Amendment regardless of the BAC number found.

Phlebotomist Qualifications

Under Texas Transportation Code, a blood draw must be performed by a physician, qualified technician, chemist, registered nurse, or licensed vocational nurse. If the person who drew the blood did not hold a qualifying certification at the time of the draw, the evidence may be inadmissible in Bexar County courts.

In Vitro Fermentation — Post-Collection Error

Improperly preserved blood samples can ferment after collection, producing additional ethanol not present at the time of the draw. If the sodium fluoride content in the collection tube is insufficient, or if the sample was stored at elevated temperature before delivery to the Bexar County Crime Lab, the BAC measured can be higher than the actual BAC at the time you were driving. We examine preservation documentation on every Bexar County blood draw case.

Chain of Custody Documentation

Every transfer of the blood sample between the collection site and the Bexar County Crime Lab analyst must be documented. A gap in the chain of custody documentation — an undocumented transfer, missing refrigeration log, or absent lab receipt record — creates a challenge to the integrity of the evidence. See our San Antonio BAC and blood test defense page.