Ask Deandra: Can a DWI Blood Test Be Wrong?

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.

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.

Blood is not taken because an officer wants it. There has to be a legal basis, and in Texas it comes from one of three places: you agreed to give a specimen, a statute required one in specific circumstances, or a judge signed a warrant. Warrants are now the common route, and in most Texas jurisdictions a telephonic warrant is obtained within an hour or less of a refusal.

What follows the draw is a route, not a reading. Somewhere between your arm and the report sits a tube, a courier, a refrigerator, a batch queue, an instrument, and a person who signed off. Every one of those steps has a written specification and generates a record. The sections below follow that route in order.

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.

The quantities matter as much as the labels. The forensic tube carries about 100 mg of sodium fluoride, enough to stop both microbial activity and glycolysis from altering the sample, and about 20 mg of potassium oxalate. Clinical gray-top tubes look identical and often carry significantly less preservative, because a hospital is testing your blood now, not defending a number in court months later. The Texas blood kit that officers carry is built around the forensic tube and holds two of them, so that a second specimen exists.

Three collection details decide whether the tube actually does its job:

• Mixing. The additives arrive as a dry powder at the bottom of the tube. Eight to ten proper inversions immediately after the draw are required to distribute the preservative and anticoagulant through the sample. A tube that was never inverted can clot or ferment in the portion the preservative never reached.
• Antiseptic. The site is supposed to be cleaned with a non-alcohol antiseptic. That is not a technicality, it is why forensic kits carry povidone-iodine rather than the isopropyl swab used for ordinary venipuncture. Hospitals and emergency rooms frequently stock only alcohol swabs, and staff drawing a forensic sample out of habit will reach for one.
• Fill volume. Overfilling the tube dilutes the sodium fluoride below the concentration it was measured out to provide, which undercuts the protection the tube was chosen for in the first place.

Too little preservative is what makes post-collection alcohol production possible, a documented phenomenon covered in What Is In Vitro Fermentation? And the tube is only the first link in a chain that runs from the phlebotomist’s antiseptic to the laboratory refrigerator, walked through in the chain-of-custody section below. When we review a blood case, the tube type, the additive content, the fill, and the mixing procedure are on the checklist before the number is ever taken at face value.

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 statute names a chemist as well, and it carries a second requirement that is easy to miss: the specimen must be taken in a sanitary place. Two details in that list matter more than they look.

• Phlebotomist is not one of the named categories. A draw performed by someone whose job title is phlebotomist depends on that individual being shown to qualify as a qualified technician. That is a question of proof, not an assumption.
• The list governs officer-request draws, not warrant draws. Texas courts have held that section 724.017 applies to a specimen taken at the request or order of a peace officer. A draw taken under a search warrant rests on a different authority and gets a different analysis, so a 724.017 argument does not automatically transfer to a warrant case.

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. Texas courts have held that a medical environment is not required at all. What matters is whether the place was safe and whether the draw followed accepted medical practice, which is a question of evidence rather than a question of address.

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.

What that paperwork tracks is a physical object moving through a defined process. Once the tube is filled it is sealed with a tamper-evident seal across the stopper, labeled with your name and the collection information, packed into an absorbent pouch and a screw-cap container inside the kit box, and the box is sealed and initialed. The submission form travels on the outside of the box rather than inside it. From there the sample goes into evidence and then to a laboratory, which may be a Texas Department of Public Safety facility, a county laboratory, or an out-of-state private contractor such as NMS Labs in Pennsylvania. The guidance directs that the specimen be protected from extreme temperatures and refrigerated if submission is delayed. Every one of those movements is supposed to be written down, and every one of them is a place where the writing can stop.

Inside the Laboratory: Accessioning, Aliquots, and the Batch

At the laboratory the sample is accessioned, meaning it is logged in, given a laboratory identifier, and checked against the paperwork that arrived with it. It goes into storage until it is scheduled.

When its turn comes, the analyst does not test the tube. A small measured portion called an aliquot is drawn out with a pipette and mixed with a known quantity of an internal standard, then sealed into a smaller glass vial. Your sample is prepared alongside calibrators, quality control samples, and blanks, and all of them are loaded into an autosampler as a batch. Your blood sits in a rack with other people’s cases and with the laboratory’s own check samples, and the whole batch runs together.

That last point is worth sitting with, because it is where a great deal of the available defense work lives. Your result is not produced in isolation. It is produced by a run, and that run’s calibrators, controls, and blanks are the evidence that the run was working when your sample went through it. A batch that failed its own controls does not announce itself on the one-page report.

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:

• Calibration. 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.
• Coelution. 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.
• Internal standard. A known quantity of another alcohol, usually n-propanol, is added to every sample and every standard. The reported concentration comes from the ratio between your ethanol peak and that one, which is what corrects for small variations in injection volume and instrument response. It also means an internal standard that was degraded, contaminated, or added at the wrong volume moves your number in a predictable direction: too much of it reads low, too little reads high.
• Carryover. 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 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.

What the Laboratory Report Does Not Say

What arrives in the case file is usually one page. It carries your name, a case identifier, a laboratory number, a date, a concentration, and a signature.

What it does not carry is nearly everything the analysis produced: the chromatogram, the retention times, the peak areas, the internal standard data, the calibration curve, the quality control results, the batch sequence, the instrument maintenance logs, the storage records, and the measurement uncertainty. None of that is hidden exactly. It simply is not on the report, and it does not arrive unless someone asks for it. How that request works is covered on What You Are Entitled to See in a Texas DWI Case.

That is the practical difference between a blood case that gets examined and one that does not. A defense working from the one-page report is working from a summary of the evidence rather than from the evidence. Everything on the list above is obtainable, and obtaining it is where a blood defense actually starts.

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 accredited under the Texas Forensic Science Commission framework. Accreditation means the laboratory has met defined quality standards. It does not mean the result in your case is reliable, and accredited laboratories have produced scientifically indefensible results for years at a stretch before anyone caught it.

Four things are worth asking in any case that turns on a laboratory number, and all four are discoverable: whether the accreditation is current, whether the laboratory has proficiency testing failures or audit findings, whether this analyst was authorized under the laboratory’s SOPs to run this analysis, and whether a method validation exists for this specific analyte. What accreditation actually evaluates, what it does not, and what happened inside three accredited laboratories that failed anyway, is set out on Why Lab Accreditation Does Not Guarantee Your Test Results Are Correct.

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 calculation itself, the assumptions it rests on, and the foundation Texas courts require before an expert may present it are on Retrograde Extrapolation. The absorption scenario is on The Rising BAC Defense.

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.

Your Two-Hour Right to Your Own Specimen

Two different rights sit under this heading and they are easy to confuse.

The first is statutory, and it expires the night of the arrest. Under section 724.019 of the Transportation Code, a person who gives a specimen at the request or order of a peace officer may, on request and within a reasonable time not to exceed two hours after the arrest, have a physician, qualified technician, chemist, or registered professional nurse of their own choosing take an additional specimen of blood for analysis. You are entitled to a reasonable opportunity to contact that person.

Three limits come attached, and all three are in the statute. Police are not required to transport you anywhere to make it happen. Failing to obtain the second specimen does not keep the State’s result out of evidence. And no officer or agency is liable for damages arising from the request. So this is a right you have to exercise, quickly, while in custody, with no help from anyone. Very few people know it exists at the moment it would matter, which is exactly why it is worth knowing now.

What it buys is a specimen drawn at a known time by someone with no stake in the outcome. Where the State’s number and an independently drawn number disagree, that disagreement is difficult for the State to explain away.

The second right is not that one, and the difference matters. Testing the State’s own retained sample later is a separate thing, and it does not come from section 724.019. It is pursued by motion, asking the court to order that a portion of the sample be released for independent analysis. A qualified forensic toxicologist can then confirm, dispute, or contextualize the State’s result, and a State result that cannot be reproduced on the same blood is a serious problem for the prosecution. That route depends on two things the two-hour right does not: the sample still existing, and it still being in a condition worth testing.

How Long the Result Takes, and What the Wait Costs You

Blood results are usually not available at the time of arrest, and in many cases they are not available for weeks or months afterward. The laboratory queue, not the courthouse, sets the pace. What that wait looks like from your side is covered on Blood Results Timeline.

The gap has a consequence worth understanding. A criminal case can sit with very little happening until the result lands, because the result is often what the prosecution’s charging decision turns on. Meanwhile the license side of the case runs on its own clock and does not wait for anything. In a blood case the notice of suspension frequently arrives by mail well after the arrest, and the deadline to request a hearing runs from service of that notice rather than from the date the result comes back.

Blood Test or Breath Test

They are different measurements of different things. A breath instrument measures alcohol in a breath sample and converts it to a blood equivalent using a fixed assumption. A blood test measures a blood sample directly, which removes that conversion but adds a physical specimen that has to be collected, preserved, transported, stored, and analyzed. Blood also detects drugs, which breath does not, so a case where impairment by something other than alcohol is suspected generally goes to blood. Which one you face is mostly a function of what you were asked for and what you did, not a judgment about which method is more accurate.

How breath testing works is covered on The Breath Test. What happens when you decline to provide a specimen at all is covered on Refusing the Test.

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?
• Were you given a reasonable opportunity to obtain your own specimen within two hours of the arrest?
• What did the laboratory report leave out, and has any of it been requested?

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.

Key Terms

The words that come up most in a blood case, in plain English.

• Headspace gas chromatography. The method Texas forensic laboratories use to measure blood alcohol. The sealed vial is heated so volatile compounds evaporate into the space above the liquid, that gas is pushed through a column that separates the compounds, and each one is burned to produce a measurable signal. The instrument is taken apart in detail on The Instrument That Decides DWI Blood Cases.
• Internal standard. A known quantity of another alcohol, usually n-propanol, added to your sample before testing. Your result is calculated from the ratio between your ethanol peak and this one, so too much of it makes your result read artificially low and too little makes it read artificially high.
• Retention time. How long a compound takes to travel through the chromatography column. Because different molecules move at different speeds, retention time is what identifies a peak as ethanol rather than something else. Under proper conditions ethanol exits at a predictable, consistent point.
• Calibration curve. The mathematical relationship the instrument uses to convert what it detects into a concentration, built by running standards of known strength. It carries a correlation coefficient written as R squared, and a value below 0.99 may indicate a problem. If the curve is wrong, every result taken from it is wrong.
• Serum or plasma alcohol. A result measured on the liquid fraction of blood rather than whole blood, which is what hospitals test. Because that fraction holds proportionally more water, it reads higher than the whole blood figure Texas law defines the offense against, so it has to be converted. The conversion, the factors experts use, and what it does to a result near 0.08 are worked through on Legal Blood vs Hospital Blood.

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.

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