By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist
The question: Can a DWI blood test be wrong?
The short answer: Yes. Blood alcohol and blood drug testing is presented to juries as the gold standard of forensic evidence, but every step between the needle and the laboratory report is a potential source of error. Blood can be drawn into the wrong kind of tube, stored at the wrong temperature, contaminated by microorganisms that produce alcohol after collection, misidentified in the laboratory, tested on a chromatograph that was not properly calibrated, and reported with a number that does not reflect the true measurement uncertainty. A blood test result is evidence. Like all forensic evidence, it is defensible if, and only if, the chain from collection to report holds up under scrutiny.
Here is the longer answer: what can go wrong with a DWI blood test, where the errors come from, and how a forensic defense takes a blood result apart.
"Deandra Grant Law – Criminal & DWI Defense handled my case with diligence and professionalism. Deandra Grant's reputation is stellar and now I know why. She has a team of individuals who provide quality service."
- N. Coulter
"Deandra Grant Law – Criminal & DWI Defense fights hard for their clients and is always willing to go above and beyond. They are the best firm for DWI cases in DFW and beyond. Definitely hire them to represent you in any pending cases."
- P. Williams
"Deandra Grant made a tough situation so much better. She listened to my concerns and helped me so much with my case. I would recommend her to anyone needing legal services."
- M. Haley
How Blood Testing Actually Works in a Texas DWI Case
When blood is drawn in a Texas DWI case (whether by consent, under a warrant, or under a mandatory-testing statute) the blood is collected into specialized tubes, sealed, and transported to a laboratory for analysis. For alcohol testing, the laboratory typically uses gas chromatography with a flame ionization detector (GC-FID). For drug testing, the laboratory uses liquid chromatography-tandem mass spectrometry (LC-MS/MS) for most drugs, though older instruments and methods still appear in some reports.
The testing technology itself, when properly applied, is reliable. The problems arise in how it is applied from the moment the needle enters the vein to the moment the number appears on the report. A defense that takes the problems seriously requires walking through every step of that process, and that requires counsel trained in the underlying science.
Collection Tube Problems
Blood collection tubes are not all the same. For forensic blood alcohol testing, the correct tube is a 10 mL glass gray-top tube containing sodium fluoride as a preservative and potassium oxalate as an anticoagulant. The sodium fluoride stops microbial activity that could produce or destroy alcohol in the sample after collection. The potassium oxalate prevents the blood from clotting so it can be tested properly.
When the wrong tube is used (ex. a clinical tube without sodium fluoride, a partially-filled tube with inadequate preservative concentration, or an expired tube with degraded preservative) the sample is vulnerable to post-collection changes that can increase or decrease the reported alcohol concentration:
- Microbial fermentation. If sodium fluoride is absent or inadequate, yeast and bacteria in the blood sample can ferment glucose into ethanol. Candida albicans, a yeast commonly present on human skin and in the bloodstream of some patients, is particularly efficient at producing alcohol under the right conditions.
- Loss of alcohol during storage. Improperly stored samples can also lose alcohol through evaporation, oxidation, or partitioning into the tube’s headspace.
- Clotting and hemolysis. Inadequate anticoagulant allows clotting, which changes the physical and chemical properties of the sample and can affect the reliability of downstream analysis.
The solution is simple on paper: use the correct forensic tubes, fill them properly, and document the lot numbers and expiration dates. In practice, tube-related problems show up more often than the State would like to admit and every properly-resourced defense reviews the tube documentation as a routine part of case analysis.
Who Drew the Blood, and Where
Texas Transportation Code §724.017 specifies who is qualified to draw blood for DWI testing: a licensed physician, a qualified technician, a registered professional nurse, a licensed vocational nurse, or a licensed or certified emergency medical technician-intermediate or EMT-paramedic. Blood drawn by anyone else (or by a qualified person acting outside the scope of their qualification) raises chain-of-custody and reliability questions that can be developed in pre-trial motions.
The setting matters too. Blood drawn in a hospital, by trained clinical staff, under sterile conditions with documented supplies is a different evidentiary profile than blood drawn in a jail, by a contract phlebotomist with a rolling cart, with an audience of officers in the room. Neither is automatically invalid. Both can be examined on their merits. The difference often surfaces in how cleanly the collection was done, how clearly the chain of custody was documented, and how well the collection conditions can withstand cross-examination.
Chain of Custody
A blood sample is only as good as its chain of custody which is the documented record of everyone who had possession of the sample from draw to test. Chain-of-custody problems are not hypothetical. They are a recurring source of successful suppression and exclusion motions in Texas DWI cases. The problems tend to be:
- Gaps in the documentation. Periods during which the documented custody does not account for the sample’s location.
- Improper storage. Failure to refrigerate samples during transport or storage, allowing temperature-related changes in the sample.
- Mismatched identifiers. Samples labeled with one name or case number and received by the laboratory under another.
- Unsealed or compromised packaging. Evidence that the sample was handled, opened, or substituted outside documented procedures.
Every kit and every sample generates paperwork. Requesting and reviewing that paperwork (collection logs, seal records, transport logs, receipt logs, storage logs) is routine defense work in blood cases, and it pays for itself when a gap is found.
Laboratory Testing: Gas Chromatography and Measurement Uncertainty
Once the blood reaches the laboratory, most alcohol testing is done by headspace gas chromatography with a flame ionization detector (GC-FID). A small portion of the blood is placed in a sealed vial and heated. Alcohol volatilizes into the headspace above the blood. A portion of the headspace is injected into a gas chromatograph, which separates the volatile components of the sample. A flame ionization detector measures the amount of alcohol present, and the instrument calculates a blood alcohol concentration.
GC-FID is the appropriate method for ethanol, and when properly run it is a defensible technique. The challenges in a defense case are:
- The instrument must be calibrated using certified reference standards on a defined schedule. Calibration failures, expired standards, and out-of-tolerance results should be documented and sometimes are not.
- Quality control samples. Each analytical batch should include known-concentration control samples to verify that the instrument is producing accurate results. Out-of-range controls call the batch into question.
- Other volatile compounds in the blood can co-elute with ethanol, producing peaks that inflate the reported reading if not properly resolved.
- Duplicate testing. Forensic best practice is to run each sample in duplicate and report the average. Variation between duplicates is itself information about the reliability of the result.
- Residue from a prior high-concentration sample can contaminate the next injection if the instrument is not properly flushed between samples.
- Measurement uncertainty. Every forensic measurement has uncertainty. A result of 0.12 percent reported without its uncertainty is an incomplete measurement. The true value could be above or below the reported number by an amount that depends on the specific laboratory’s method validation.
These are not minor quibbles. They are foundational forensic science issues that qualified defense experts and forensic-trained attorneys develop in every serious blood case.
Drug Testing Is a Different Analysis
Blood tests in DWI cases are not always alcohol tests. In DUID (driving under the influence of drugs) cases and combined DWI cases, the laboratory tests for controlled substances, their metabolites, or both. The preferred confirmatory methodology for most drugs in biological specimens is LC-MS/MS (liquid chromatography with tandem mass spectrometry) which provides high specificity and sensitivity when properly run.
Drug testing carries its own set of defensible issues:
- Metabolite problem. A positive result for a drug metabolite (such as carboxy-THC, the non-psychoactive metabolite of cannabis) establishes past use, not impairment at the time of driving. Under Texas Penal Code §49.01, intoxication requires loss of normal mental or physical faculties due to introduction of a substance, or a per-se alcohol level. There is no per-se drug level in Texas DWI law, which means metabolite positivity alone does not establish intoxication.
- Cross-reactivity in screening. Immunoassay screens used for initial drug detection have well-known cross-reactivity problems. Confirmed results from LC-MS/MS should always be the basis for any courtroom conclusion.
- Chirality issues. Some drugs exist as multiple stereoisomers with different pharmacological properties and different legal implications. Methamphetamine, for example, has both a pharmacologically active d-isomer and a largely inactive l-isomer found in certain over-the-counter nasal inhalers. Laboratories that do not distinguish the isomers can produce positive results that do not reflect actual drug use. For more information see our blog on L-Meth vs D-Meth.
- Quantitation limits and method validation. Drug concentrations near the lower limit of quantitation carry higher uncertainty than concentrations in the middle of the calibration range.
Drug testing is not the gold-standard evidence it is often presented as. In some respects it raises more complicated forensic questions than alcohol testing, not fewer.
Laboratory Accreditation and Analyst Qualifications
Texas laboratories performing forensic toxicology are subject to accreditation requirements under the Texas Forensic Science Commission framework. Accreditation tells the court and the parties that the laboratory has met defined quality standards, but accreditation does not mean every result is automatically reliable. Forensic laboratories are audited, and audits sometimes find problems. Analysts are trained, but training records can be incomplete. Method validations are documented, but documentation can be out of date.
The questions defense counsel should routinely ask about the laboratory:
- Is the laboratory currently accredited under Texas Forensic Science Commission standards? Accreditation status is publicly available.
- Has the laboratory had any accreditation or proficiency testing problems? Proficiency testing failures and audit findings are discoverable.
- Is the analyst who tested this sample qualified under the laboratory’s SOPs? Training records, competency assessments, and authorization to perform the specific analysis should all be documented.
- What is the laboratory’s method validation for this specific analyte? Method validations document the laboratory’s assessment of accuracy, precision, linearity, selectivity, and matrix effects.
Rising BAC and Retrograde Extrapolation
One of the most important questions in any blood case is when the blood was drawn relative to when the driving occurred. Texas Penal Code §49.04 requires proof that the defendant was intoxicated at the time of driving, not at the time of the blood draw. Because alcohol follows an absorption, peak, and elimination curve, the blood alcohol concentration at the time of driving is not necessarily the same as the blood alcohol concentration at the time of the draw.
- If the defendant was still in the absorption phase at the time of driving (the alcohol had not yet been fully absorbed from the stomach and small intestine into the bloodstream), the blood alcohol concentration at the time of the draw (often one, two, or three hours later) can be meaningfully higher than the concentration at the time of driving.
- If the defendant was past peak and in the elimination phase, the reverse is true, and the State may attempt retrograde extrapolation to estimate a higher historical BAC.
- Retrograde extrapolation is not a simple calculation. It requires specific inputs (time of last drink, nature and quantity of food consumed, individual physiological factors) that are often unavailable or in dispute. A defense expert can address the analysis and identify the scenarios where the extrapolation is unreliable.
The timing of the draw, the timing of the last drink, and the pharmacokinetics of alcohol in the specific defendant are forensic questions that require graduate-level training in pharmaceutical science and toxicology to address properly.
The Warrant Affidavit and the Draw Itself
A defense review of a blood case is not complete without examining the warrant that authorized the draw. In Texas, warrantless non-consensual blood draws are constitutionally problematic outside specific exceptions, as the Court of Criminal Appeals held in State v. Villarreal, 475 S.W.3d 784 (Tex. Crim. App. 2014), and as the United States Supreme Court held in Missouri v. McNeely, 569 U.S. 141 (2013). Most blood in contested DWI cases is drawn pursuant to a search warrant, and every warrant rests on a supporting affidavit.
Affidavits are defendable. Common defense targets:
- Insufficient probable cause on the face of the affidavit. Conclusory language, boilerplate recitations, and missing factual detail can all support a motion to suppress.
- Material omissions or misrepresentations. Under Franks v. Delaware, 438 U.S. 154 (1978), a defendant may challenge the veracity of the affidavit through a Franks hearing.
- Staleness, jurisdictional problems, and scope issues. Defects in the warrant itself, separate from the affidavit, can also invalidate the draw.
A warrant that does not survive scrutiny is a blood result that does not come into evidence. That is one of the most powerful outcomes available in a blood DWI case and it starts with careful review of the warrant and the affidavit.
Case Results
What This Means for Your Case
Every blood DWI case involves a series of specific questions:
- What tubes were used, and were they properly preserved?
- Who drew the blood, where, and under what conditions?
- Does the chain of custody account for the sample from draw to report?
- What does the chromatogram actually show?
- What is the laboratory’s accreditation status, and what is the analyst’s qualification record?
- What is the true measurement uncertainty of the reported result?
- What was the timing of the draw relative to the driving, and what does that mean pharmacokinetically?
- Is the warrant and its supporting affidavit defensible?
A defense that answers all of these questions is a defense that takes the blood case seriously. A defense that does not is leaving forensic evidence on the table.
The Bottom Line
A DWI blood test can be wrong. It can be wrong because of the tube, the draw, the storage, the chain of custody, the instrument, the calibration, the analyst, the method, the interpretation, the timing, or the warrant that authorized the draw. It can also be right but that determination requires work, not an assumption. The State presents blood evidence as the gold standard. A forensic defense examines the gold for what it actually is. If you have been charged with DWI based on a blood test, you are entitled to a defense that understands both the science and the law.
Blood Test Defense at Deandra Grant Law
Deandra Grant Law defends DWI and intoxication-offense cases across North and Central Texas including Dallas, Fort Worth, Plano, McKinney, Frisco, Allen, Lewisville, Denton, Rockwall, and Waco. We handle blood alcohol cases and drug cases with the same forensic rigor, examining tube documentation, chain of custody, chromatography, method validation, warrant affidavits, and the pharmacokinetic timing analysis that every serious blood case requires. Our team includes an ACS-CHAL Forensic Lawyer-Scientist with a Master of Science in Pharmaceutical Science and a Graduate Certificate in Forensic Toxicology.
If a blood test is being used against you in a DWI case, call Deandra Grant Law at (214) 225-7117 or visit texasdwisite.com to schedule a confidential consultation. If you have not yet addressed the license side of the case, the 15-day ALR deadline runs from the date of service of the notice of suspension.
Have a DWI question you want answered in this series? Submit it at texasdwisite.com — you might see it featured in a future Ask Deandra post.
Firm Accolades
Better Business Bureaus
D Magazine
