By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist
A blood test result in a Texas DWI case carries significant weight. Juries tend to trust numbers, prosecutors lead with them, and judges sentence based on them. But a BAC figure on a laboratory report is not a fact. It is the end product of a chain of human decisions, biological processes, chemical reactions, and analytical procedures, any one of which can introduce error.
Understanding where that number comes from, and where it can go wrong, is the foundation of an effective blood test defense.
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How Blood Testing Works in Texas DWI Cases
When a Texas driver is arrested for DWI and either refuses a breath test or is suspected of drug impairment, law enforcement will typically seek a blood draw. Post-Missouri v. McNeely (2013), officers generally need a warrant for a non-consensual blood draw. In most Texas jurisdictions telephonic warrants are now routinely obtained within an hour or less of a refusal.
The blood is supposed to be drawn by a qualified medical professional, typically at a hospital, jail clinic, or mobile phlebotomy unit. The sample is collected into a specific type of tube containing preservative and anticoagulant chemicals, sealed, labeled, and entered into a chain of custody. It is then transported to a laboratory which may be a Texas Department of Public Safety facility, a county lab, or an out-of-state private contractor such as NMS Labs in Pennsylvania. It is then analyzed for alcohol content or the presence of drugs.
The result that comes back is what the prosecution will call the evidence. What it actually is, is a measurement produced under a specific set of conditions that deserve scrutiny.
Where Blood Test Evidence Can Break Down
The Collection Tube: Preservative and Anticoagulant Requirements
This is one of the most technically significant and least-understood attack vectors in blood test defense and it begins before the sample ever reaches a lab.
Blood drawn for forensic alcohol analysis must be collected into a tube containing two specific chemicals: sodium fluoride (NaF), which acts as a preservative and inhibits microbial fermentation, and potassium oxalate, which acts as an anticoagulant to prevent clotting. Both are essential. Both must be present in the correct amounts.
If the wrong tube is used (a plain red-top tube, a serum separator tube, or any collection vessel without the correct preservative) the sample is not properly preserved for forensic alcohol analysis. Microbial activity in an improperly preserved sample can produce ethanol endogenously, meaning alcohol can be generated in the tube itself after the draw, resulting in a BAC reading that is higher than what was actually in the defendant’s blood at the time of driving.
This is not a theoretical concern. It is a documented phenomenon with a well-established scientific basis, and the type of tube used in your blood draw is one of the first things we examine in any blood test case.
Post-Draw Fermentation: When BAC Rises After the Draw
Even in a properly preserved sample, conditions can arise that allow microbial fermentation to occur, particularly if the tube was not properly sealed, if the sample was stored at improper temperatures, or if the sodium fluoride concentration was insufficient to fully inhibit bacterial activity.
Fermentation produces ethanol. A sample that sits in a warm environment, or that was improperly handled during transport or storage, may register a higher BAC at the time of testing than was present in the defendant’s blood at the time of the draw. That difference can be the margin between a result above and below the legal limit.
Storage temperature logs, transport documentation, and the elapsed time between draw and analysis are all relevant to whether fermentation could have affected the result in a given case.
The Draw Itself: Site Sterilization and Technique
The manner in which blood is drawn matters. A phlebotomist who sterilizes the venipuncture site with an isopropyl alcohol swab and then draws before the site fully dries can introduce alcohol contamination directly into the sample. Forensic blood draw protocols specify the use of non-alcoholic antiseptics (typically a betadine or chlorhexidine solution) and require that the site be fully dry before the needle is inserted.
Draw technique also matters with respect to the tube itself. Proper inversion of the collection tube (eight to ten proper inversions immediately after the draw) is required to ensure the preservative and anticoagulant are thoroughly mixed with the sample. Failure to properly mix the contents can result in clotting or uneven distribution of the preservative, either of which affects the integrity of the sample.
Chain of Custody: The Paper Trail That Has to Be Perfect
Chain of custody documentation tracks every person who handles a blood sample from the moment it is drawn to the moment the analysis is complete. Every transfer, every storage entry, every laboratory receipt must be documented. Any gap such as an uninitialed seal, an undocumented transfer, a missing storage log entry or a discrepancy in sample identification numbers creates a question the prosecution must answer.
Chain of custody gaps do not automatically render a result inadmissible, but they open the door to a foundational challenge: how do we know the sample tested is the same sample drawn from this defendant, handled without tampering or substitution, and maintained under conditions that preserved its integrity?
In cases where the sample traveled from a Texas jurisdiction to an out-of-state laboratory like NMS Labs, the chain of custody documentation spans multiple handlers, shipping protocols, and jurisdictional handoffs. Each transition is an opportunity for documentation failure.
The Analytical Process: A Brief Note on GC-FID
The standard method for analyzing blood alcohol content in a forensic laboratory is gas chromatography with flame ionization detection (GC-FID). The method involves extracting alcohol from the headspace above the blood sample and injecting it into a gas chromatograph, which separates and quantifies the compounds present.
When properly performed with appropriate internal standards, calibration, and duplicate analysis, GC-FID is a reliable methodology. The questions a defense attorney asks are not whether the method is valid in principle (it is) but whether it was executed correctly in this case, with this instrument, by this analyst, on this sample. Calibration records, internal standard performance, duplicate analysis results, and the analyst’s qualifications are all part of that review. A standalone piece on GC-FID methodology is forthcoming but in any blood test case, those records are always part of what we request in discovery.
Retrograde Extrapolation: The Timing Problem
Blood is drawn after arrest and sometimes significantly after. The BAC measured at the time of the draw is not necessarily the BAC at the time of driving, which is the legally relevant moment.
Alcohol metabolizes at a relatively predictable average rate, but that rate varies by individual based on body weight, sex, metabolic factors, food consumption, and the type and timing of alcohol consumed. Retrograde extrapolation (calculating backward from the measured BAC to estimate what the BAC was at an earlier time) requires assumptions about the individual’s absorption and elimination rate that are rarely verified in a given case.
If the measured BAC is 0.09 at the time of the draw, and the draw occurred 90 minutes after the traffic stop, the defendant’s BAC while driving could have been higher, lower, or approximately the same depending on where they were in the absorption and elimination curve. A prosecution expert who testifies to a specific BAC at the time of driving based on a later blood test is presenting an estimate with significant margins of error and those margins deserve to be examined.
Case Results
Your Right to Independent Testing
Under Texas law, a defendant has the right to have an independent analysis performed on a portion of their blood sample. This right is established in Texas Transportation Code §724.019, and it is one of the first things we pursue in any blood test case.
An independent analysis performed by a qualified forensic toxicologist can confirm, dispute, or contextualize the state’s result. It can identify whether the sample shows signs of fermentation, whether the internal standards in the state’s analysis were appropriate, and whether the reported BAC is consistent with the circumstances of the case. If the state’s result cannot be replicated on the same sample, that is significant.
What a Blood Test Defense Actually Looks Like
A defense built around blood test evidence is not built on the hope that the jury will ignore the number. It is built on a methodical examination of how that number was generated.
That means requesting complete laboratory discovery and not just the one-page report or a litigation packet. Full lab discovery will include instrument run logs, calibration records, internal standard data, chain of custody documentation, analyst certification records, and the lab’s accreditation scope. It means examining the collection tube type, the draw protocol, the storage and transport records, and the elapsed time between draw and analysis. And it means retaining an independent expert when the evidence warrants it.
My background in pharmaceutical science and forensic toxicology is directly relevant to this work. Understanding what GC-FID chromatograms actually show, what fermentation artifacts look like in a sample, and what proper collection protocol requires is not something most defense attorneys bring to a blood test case. It is something I bring to every one.
Blood Test Defense at Deandra Grant Law
Managing Partner Deandra Grant brings more than 30 years of DWI defense experience, a Master’s Degree in Pharmaceutical Science, and an ACS-CHAL Forensic Lawyer-Scientist designation to every blood test case. The number on the report is the beginning of the analysis and not the end of it. Call (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation.
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