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A DWI arrest can be overwhelming, especially when a blood test report shows a BAC just above the legal limit and you assume the case is already decided. You may be looking at a lab number, a court date, and a notice about your driver's license without knowing which deadline matters most or whether anyone can independently check the result.
You still have legal and scientific questions worth investigating. A blood test is evidence, not an automatic conviction. The retained portion of the sample, the collection process, the chain of custody, the laboratory method, and the timing of the draw may all affect how the State's evidence should be evaluated.
Why a Blood Test Result Is Not the Final Word
BAC, or blood alcohol concentration, is the measured concentration of alcohol in a person's blood at the time the specimen was collected and analyzed. It doesn't automatically answer the separate question of whether you were legally intoxicated while driving. Texas must prove the elements of the DWI offense based on the circumstances of driving, not place a laboratory number in front of a judge or jury.
That distinction matters when the blood draw happened after the traffic stop. Alcohol may still have been absorbing, or the concentration may have been changing, between the time you drove and the time medical personnel collected the sample. A defense toxicologist can examine that timing through scientific analysis, including retrograde extrapolation, which evaluates what the BAC may have been at an earlier point.
A report that reads just above the legal threshold can create understandable anxiety. But a number near the threshold may call for closer review of collection, storage, laboratory testing, and the assumptions used to connect the result to the time of driving. The legal significance of a result depends on the entire record, not the number in isolation.
Practical rule: Don't treat the lab report as the end of the investigation. Treat it as the beginning of the evidence review.
The retained split sample is central to that review. When a forensic blood specimen is divided and a portion remains available, the defense may ask the court to allow an independent laboratory to analyze it. That second analysis can test whether the original result is reproducible and whether the underlying records support the State's interpretation.
The timing of that request matters because Texas has separate criminal and administrative proceedings. In the administrative license process, ALR, or Administrative License Revocation, concerns whether the Texas Department of Public Safety should suspend or disqualify your driving privileges based on the applicable chemical-test evidence. DPS states that you have 15 days from the date notice is served to request an ALR hearing, and the hearing may take up to 60 days to set after the request is made. Those deadlines are explained in the Texas rules on alcohol absorption and DWI cases.
A field sobriety test is a roadside evaluation used by an officer to look for signs associated with impairment. It isn't the same as a blood test, and performance may be affected by balance problems, footwear, lighting, fatigue, medical conditions, or the instructions given. A careful defense examines every category of evidence instead of allowing one test result to define the case.
Texas Rules on Preserving and Destroying Blood Evidence
The strongest retesting request begins before the sample disappears. Texas DPS guidance describes a properly collected blood specimen as requiring a minimum of 10 milliliters of blood in a gray-top vial, refrigeration until submission, sealed packaging, and the appropriate toxicology submission forms. The people handling the evidence should also identify whether the testing concerns alcohol, drugs, or both. These requirements are described in the Texas DPS guidance for DWI blood evidence.
Those details aren't paperwork trivia. Refrigeration, sealed packaging, and prompt submission help reduce contamination risks and preserve the sample's evidentiary value. The forms also help establish what the laboratory was asked to test, which can matter when the defense needs alcohol testing, drug testing, or both.
Why the destruction date matters
Texas changed the authorization process for destroying toxicology evidence on September 1, 2021. For offenses occurring before that date, a judge's signature was still required before toxicology evidence could be destroyed. For current cases, destruction may be authorized immediately upon adjudication through a Toxicology Disposition Form or a court order. The change is discussed in Texas guidance about toxicology evidence and blood kits.
That rule makes preservation a time-sensitive defense task. A case can be resolved, dismissed, or otherwise adjudicated while the defense is still deciding whether an independent retest is necessary. If no preservation request is made, the retained sample may not remain available indefinitely.
Texas DPS also made Toxicology and Blood Alcohol Collection Kits a requirement effective September 15, 2024, according to the same customer-notification discussion. Standardized collection materials can improve consistency, but they don't eliminate the need to inspect the actual kit, vial, paperwork, seals, storage history, and chain-of-custody records in your case.
What preservation should address
A preservation request should identify the physical blood specimen and related materials, not just ask the State to “keep the evidence.” Counsel may seek preservation of:
- The retained split sample: The portion not consumed during the original analysis, including its vial, label, seal, and identifying information.
- Collection materials: The gray-top vial, kit information, packaging, and any documentation showing when and how the specimen was collected.
- Laboratory records: The toxicology report, chromatograms, calibration materials, analyst notes, quality-control records, and testing worksheets.
- Custody records: Every transfer, storage location, temperature record, submission form, and disposition document connected to the specimen.
- Related testing information: Records showing whether the laboratory was asked to test for alcohol, drugs, or both.
The right to seek a defense analysis is not the same as a guarantee that a usable specimen still exists. The defense must act early, identify the custodian, and ask the court for practical access under conditions that protect the sample and preserve the integrity of the retest. A discussion of that process appears in the right to an independent blood test in a Texas DWI case.
For a broader explanation of how chemical-test evidence can be questioned, Challenging Breath and Blood Tests in Texas DWI Cases is another useful resource. The important point is simple: preserve first, analyze second, and litigate the reliability of the evidence with the complete record in hand.
How to Locate the Sample and File the Right Motions
The defense usually begins with discovery, not with a laboratory appointment. A written request should seek the blood draw affidavit, warrant materials when applicable, toxicology report, submission forms, chain-of-custody records, storage information, analyst notes, chromatograms, calibration records, and any disposition paperwork.
Those documents often identify the sample's custodian. A specimen may be held by a Texas DPS crime laboratory, a hospital laboratory, a law enforcement evidence room, or another designated evidence custodian. The location matters because a request directed to the wrong office can delay access while the evidence remains subject to the applicable disposition process.
The documents that reveal the retest path
The blood draw affidavit may show who performed the draw and the person's stated qualifications. The toxicology report identifies the reported result and may identify the testing method. The chain-of-custody materials show whether the State can account for the specimen from collection through analysis.
A defense motion for independent testing generally asks the court to:
- Identify the specimen: Describe the defendant's retained blood sample by case number, vial information, laboratory identifier, or other available information.
- Order preservation: Direct the State and the custodian not to destroy, alter, consume, or transfer the sample outside approved conditions.
- Authorize access: Permit an identified defense toxicologist or accredited laboratory to receive and analyze the retained portion.
- Set handling terms: Establish who transports the specimen, how the transfer is documented, what quantity may be used, and how the remaining evidence is returned or preserved.
- Define the testing scope: State whether the analysis concerns alcohol, drugs, or both, consistent with the issues in the case.
The motion doesn't need to promise that the retest will help. Its purpose is to protect a meaningful opportunity to evaluate the State's evidence before the sample is gone.
Hospital blood requires a different review
Hospital blood isn't automatically equivalent to a forensic DWI specimen. Medical testing may be performed for diagnosis and treatment rather than litigation, and the collection materials, preservatives, documentation, storage practices, testing method, and custody records may differ.
That doesn't mean hospital records are irrelevant. It means the defense must determine what was collected, who handled it, where it was stored, what method was used, and whether enough specimen remains for a scientifically defensible analysis. A hospital result may support the State's theory, but its limitations may also create important admissibility or weight issues.
| Custodian of Sample | Documents to Request | Retest Consideration |
|---|---|---|
| Texas DPS crime laboratory | Toxicology report, chromatograms, calibration records, analyst notes, custody records | Confirm whether a retained split sample exists and whether the original forensic method can be independently evaluated |
| Hospital laboratory | Medical records, collection information, laboratory report, specimen history, storage records | Determine whether the specimen was collected and preserved for forensic analysis and whether enough material remains |
| Law enforcement evidence room | Submission forms, seals, transfer logs, storage records, disposition paperwork | Verify uninterrupted custody, proper storage, and whether destruction has been authorized |
| Other designated custodian | Custody policy, location records, transfer documents, testing request | Identify the person with legal control of the specimen and request court-approved access |
When you meet with counsel, bring the blood-test report, arrest paperwork, ALR notice, court papers, and any document showing the draw date or laboratory name. Ask whether preservation has been requested, whether the retained sample has been located, what testing method was used, and whether the defense has received the raw laboratory data.
Questions about how concentration affects the alleged charge and consequences can also be considered alongside Texas DWI BAC Levels and Enhanced Charges. The answer in your case will depend on the charged offense, the evidence, and the applicable law. Discovery and preservation should come before assumptions.
Choosing and Working With a Defense Toxicology Expert
A defense toxicologist does more than send a vial to a second laboratory. The expert compares the State's original analysis with an independent examination and determines whether the result is supported by reliable collection, custody, chemistry, and interpretation.
The first comparison concerns the method. A forensic laboratory may use headspace gas chromatography, a method that separates volatile substances in the air above a blood sample and identifies and measures compounds through the instrument's response. The expert should know the method used in the case and evaluate whether the laboratory applied it correctly.
What the expert should examine
The raw data often tells a more complete story than the final number. A qualified expert may review:
- Chromatograms: The visual records of the instrument's detected peaks, including retention time and peak shape.
- Calibration records: Evidence showing how the instrument was calibrated and whether the calibration supported the reported measurement.
- Analyst notes: The laboratory employee's observations, calculations, deviations, and explanations.
- Quality-control material: Records that may reveal an instrument problem, carryover, contamination concern, or inconsistent run.
- Custody and storage information: Documentation showing whether the sample remained sealed, refrigerated, and properly transferred.
- Independent results: A retest from an accredited laboratory that can be compared with the original work without consuming the evidence unnecessarily.
A retest is most useful when the independent laboratory has access to the original chromatograms, calibration records, and analyst notes. Without those materials, the expert may be limited to comparing two final numbers and unable to explain why they differ.
The State's analyst may defend the original result as valid because the instrument produced a report. The defense expert asks a broader question: Did the entire process support that result, and can another qualified laboratory reproduce it from the preserved specimen?
Selecting the right professional
Look for an expert or independent laboratory with recognized forensic credentials, relevant accreditation, experience with headspace gas chromatography, and experience explaining toxicology to Texas judges and juries. Trial experience matters because a technically correct opinion still has to be communicated clearly under cross-examination.
Independence also matters. The expert should be able to identify weaknesses in the defense position when the science requires it. A candid assessment may conclude that a retest won't materially change the case, or that the better challenge concerns collection, custody, storage, or the connection between the draw time and driving time.
The best expert isn't the one who promises a favorable number. It's the one who can explain what the evidence proves, what it doesn't prove, and why.
The working relationship typically begins with document review. If the records support further testing, counsel coordinates access to the retained sample and the laboratory performs the analysis under documented conditions. The expert then prepares an opinion that may address the original method, the independent result, any discrepancy, and the limits of the State's interpretation.
The attorney presents the legal significance. The expert translates the chemistry. You don't need to become a scientist to make informed decisions about your defense. You need a legal team that understands which scientific questions are worth asking and how to place reliable answers before the court.
For cases where expert testimony is central, review the discussion of DWI toxicologist testimony in Texas.
Reading the Retest Results and Building the Defense
A retest doesn't win a case because the number is different. The meaningful question is whether the difference, viewed with the collection and timing evidence, weakens the State's ability to prove intoxication when you were driving.
That issue appeared in a recent Texas case summary involving a court-ordered retest. The retest reportedly came back about 0.015 to 0.02 lower than the original result, and the defense used expert testimony to argue that the reported number didn't reliably establish intoxication at the time of driving. The case summary is available in this Texas DWI retesting case result.
The lesson isn't that every lower retest produces the same result. The lesson is that a lower result can become more significant when it supports a larger problem with the State's timeline, assumptions, or proof.
Why the time of driving controls the analysis
The blood draw gives a concentration at the draw time. The prosecution still has to connect that concentration to the time you operated a vehicle. Retrograde extrapolation attempts to work backward from the later result, but the reliability of that estimate depends on case-specific facts, including when drinking occurred, whether absorption was complete, and what other evidence supports the calculation.
A defense expert may therefore use the retest as one part of a broader opinion. The expert can explain that a lower independent result changes the starting point, while the drinking timeline and uncertainty about absorption make it difficult to state confidently what the BAC was during driving.
The retest is often most valuable when the original result is near 0.08, the alcohol-concentration threshold used in Texas administrative license suspension cases. At that level, relatively small collection, storage, transfer, or analytical problems may affect whether the result crosses the threshold. The threshold's role in administrative action is described in Texas ALR hearing guidance.
When the number stays close
A retest that is close to the original result doesn't necessarily end the defense. It may confirm that the result is consistent, but counsel must still examine whether the State can authenticate the specimen and show that the collection and analysis followed required procedures.
Other challenges may include:
- Chain of custody gaps: The records don't clearly identify every person who handled the specimen or every transfer between locations.
- Storage concerns: The documentation doesn't establish appropriate temperature control or shows a mismatch between the required conditions and the actual conditions.
- Collection problems: The vial, preservative, packaging, sanitation, qualifications of the collector, or submission forms don't match the expected forensic process.
- Laboratory interference: The chromatogram or analyst notes raise questions about carryover, peak shape, retention time, calibration, or another substance affecting the analysis.
- Timing uncertainty: The State's retrograde calculation depends on assumptions the evidence doesn't support.
The defense may challenge admissibility, reliability, or weight. Those are different arguments. An attorney may ask the judge to exclude evidence because a legal or evidentiary requirement wasn't met, or may present the weaknesses to the jury to show why the reported result shouldn't be accepted as proof beyond a reasonable doubt.
The same approach applies when the result is substantially higher. A retest may still reveal a laboratory or custody issue, but the defense lawyer and toxicologist must assess whether the discrepancy is meaningful, reproducible, and capable of changing the State's proof. Retesting is a tool, not a guarantee, and the strongest strategy uses the result with candor.
Your Next Move and Common Questions Answered
You don't need to solve the science before calling a lawyer. You do need to protect the evidence and the deadlines while the sample and records are still available.
A practical checklist is:
- Request preservation promptly: Ask counsel to identify and preserve the retained split sample and related records.
- Calendar the ALR deadline: DPS says the request for an ALR hearing must be made within 15 days after notice is served, and the setting may take up to 60 days, as described in its Administrative License Revocation program information.
- Obtain the laboratory file: Request the report, chromatograms, calibration records, analyst notes, custody documents, and collection materials.
- Evaluate the draw timeline: Compare the driving time, stop, arrest, and blood draw rather than focusing only on the result.
- Consult a toxicologist early: Determine whether the specimen is available, whether independent testing is technically sound, and whether the original analysis has identifiable weaknesses.
Common questions
How long does an independent retest take
There isn't one dependable answer. Timing depends on whether the sample has been located, whether a court order is needed, the custodian's response, the independent laboratory's schedule, and the complexity of the requested analysis. Delaying the request can create problems that no laboratory can fix later.
Who pays for the independent laboratory
The cost arrangement depends on the defense agreement, the laboratory selected, and the financial circumstances of the case. Ask your attorney for a written explanation of anticipated expert and laboratory expenses before authorizing the work.
Can a retest come back higher
Yes, an independent analysis can produce a higher result. That doesn't automatically establish guilt, but it may affect how counsel evaluates the retest and whether another issue, such as chain of custody, method reliability, or timing, provides a stronger defense path. A responsible expert should discuss that possibility before testing begins.
Does retesting help when the original result was far above the limit
It can, but the practical value may be different. A modest change may not alter the legal posture when the original result is substantially above the applicable threshold, while a clear methodological or custody problem could remain important regardless of the number. The expert must compare the result with the full evidence instead of promising that retesting will produce a dismissal.
The criminal case and the license case move on separate tracks. A lawyer may evaluate the stop, warrant, blood draw, field sobriety test, implied consent issues, chemical evidence, negotiations, and trial strategy together. Implied consent refers to the legal framework under which a person who drives in Texas is treated as having agreed to certain alcohol or drug testing consequences, subject to the facts and procedures of the case. It doesn't eliminate the right to challenge an unlawful draw, unreliable result, or improper license action.
The Law Office of Bryan Fagan, PLLC offers DWI defense, ALR hearing representation, chemical-test review, and coordination with defense toxicology experts for clients in Houston, Dallas–Fort Worth, Austin, San Antonio, and surrounding Texas communities. Request a free consultation or case evaluation, with 24/7 support, so your blood-test evidence, preservation deadline, and license concerns can be addressed before one test result is allowed to decide your future.
