September 11, 2023
Breath and blood tests do not measure alcohol in the same specimen or use the same scientific process. A breath instrument estimates alcohol concentration from deep-lung breath. A forensic blood test measures ethanol in a blood specimen, usually through laboratory analysis. Neither result should be accepted or rejected based only on the label attached to the test.
In a Texas DWI case, the useful question is whether the State can connect the reported number to the person, the relevant time, and a reliable testing process. That requires reviewing the surrounding records—not simply assuming that blood is always better than breath or that every machine is unreliable.
How evidentiary breath testing works
An evidentiary breath test is different from a portable device an officer may use during a roadside investigation. The roadside device may help the officer decide what to do next, while an evidentiary breath instrument produces a result the State may seek to use in court.
The person provides breath specimens into an approved instrument. The device analyzes the breath and reports an alcohol concentration under the governing Texas program. The test record may include the operator’s documentation, test sequence, reference checks, instrument-status information, maintenance records, and other quality-assurance materials.
A defense review may examine:
- whether the operator and instrument satisfied the requirements applicable to the test;
- whether the observation and testing procedure was followed;
- whether the specimens met the program’s acceptance criteria;
- whether reference checks and status records support the test sequence;
- whether maintenance, repair, or error records require follow-up;
- whether the reported result fits the video, driving, timing, and other evidence; and
- whether a physical condition or interfering substance is actually supported by the facts.
An unusual record does not automatically invalidate a test. Its meaning depends on the instrument, procedure, timing, and complete test record.
How forensic blood-alcohol testing works
A blood test involves a longer chain of events. Police must obtain the specimen through a legally authorized draw, identify and seal the tubes, maintain custody, transport and store the kit, and submit it to a laboratory. The laboratory then prepares a portion of the specimen and analyzes it.
Many forensic laboratories use headspace gas chromatography to measure ethanol. The laboratory runs the specimen with calibrators, controls, blanks, and other quality materials. The instrument creates chromatograms and data that analysts review before issuing a report.
The final blood report may be short, but the underlying record can include:
- the warrant or consent documentation;
- blood-draw and collection records;
- tube labels, seals, submission records, and chain of custody;
- transportation and storage information;
- sample-preparation records;
- the analytical sequence;
- calibration data, controls, standards, and blanks;
- chromatograms, duplicate results, and processing history;
- instrument and maintenance records; and
- technical and administrative review documentation.
Blood testing can provide valuable evidence, but the laboratory instrument cannot independently prove that every collection, identity, custody, and reporting step was correct.
For a detailed explanation, see our guide to Texas DWI blood-test errors and laboratory review.
Is a blood test more accurate than a breath test?
There is no responsible one-word answer. The methods measure different specimens and depend on different procedures.
A properly collected and analyzed blood specimen can provide a direct measurement of ethanol in that specimen. But the reliability of the reported result still depends on specimen identity, collection, custody, storage, preparation, calibration, quality controls, data review, and reporting.
An evidentiary breath instrument does not draw blood or require laboratory transport, but its result depends on proper operation, accepted specimens, instrument checks, maintenance, and the relationship between breath alcohol and the allegation in the case.
The strongest comparison is case-specific:
- What test was actually used?
- When was it administered relative to driving?
- What procedure applied?
- Were the required records produced?
- Do those records support the result?
- Does the number fit the rest of the evidence?
A categorical claim that one method is always accurate and the other is always flawed is not sound analysis.
Why breath and blood results can differ
Breath and blood results can differ without proving that either number was fabricated. Possible reasons include:
- the tests were taken at different times;
- alcohol absorption or elimination was still occurring;
- breath and blood measure different specimens;
- the methods use different analytical and reporting conventions;
- biological and measurement variation;
- a collection, instrument, or laboratory issue; or
- the two results came from different evidentiary circumstances.
Timing matters because a later measurement is not automatically the exact alcohol concentration at the time of driving. Any attempt to estimate an earlier concentration depends on assumptions and case facts. The report should not be treated as a time machine.
Warrants, consent, and refusal in Texas
Texas DWI investigations may involve requests for breath or blood specimens, consent questions, refusals, and search warrants. The legal consequences and available challenges depend on what occurred, what officers requested, whether a warrant was obtained, how it was executed, and the procedural posture of the case.
This article is not a recommendation to provide or refuse a specimen. That decision can carry legal and driver-license consequences, and broad internet advice cannot account for a person’s circumstances. If an investigation or arrest has already occurred, the useful next step is to preserve the facts and promptly review the officer’s request, any warrant, the draw or breath procedure, and applicable deadlines with counsel.
The reported number is only part of the DWI case
Chemical evidence does not replace the rest of the case. The State still must prove the legal allegation with admissible evidence.
A complete review may include:
- the reason for the stop;
- driving behavior;
- body-camera and dash-camera video;
- field sobriety testing;
- statements and observations;
- the time between driving and chemical testing;
- the warrant or consent process;
- the collection or breath-test procedure; and
- the scientific records supporting the reported result.
A high number may be important evidence. It is not a substitute for verifying that the number came from the correct person through a reliable process and relates to the relevant time.
Which detailed testing issue applies?
Different issues belong in different reviews:
- For blood collection, chain of custody, headspace gas chromatography, calibration, controls, and laboratory data, read Texas DWI Blood Test Errors.
- For the broader differences among blood, breath, and drug testing, read Forensic Toxicology in Texas DWI Cases.
- For analyst-quality incidents and disclosure issues in State v. Momin, read Can Problems With a DPS Blood Analyst Affect a Texas DWI Case?.
How The Napier Law Firm can help
The Napier Law Firm reviews the test that actually exists and the records behind it. In a breath case, that may include the operator, sequence, reference checks, maintenance, and video. In a blood case, it may include the draw, custody, laboratory method, analytical batch, review history, and disclosures.
If you are facing a DWI charge in Houston, Harris County, Fort Bend County, Montgomery County, Conroe, The Woodlands, or the surrounding area, call (713) 470-4097 or request a free consultation.
This article provides general educational information and is not legal advice. Reading it does not create an attorney-client relationship. Testing procedures and legal issues vary by instrument, laboratory, date, jurisdiction, and case facts.