Learn how DWI blood alcohol testing and gas chromatography work, and the many ways a Texas DWI defense attorney can challenge a blood test result in court.
Blood alcohol testing is not that complex. A skilled DWI defense lawyer can always fight a blood sample lab test. Not all criminal defense attorneys know how to fight one, including some less experienced DWI attorneys. Your DWI lawyer must understand how the labs develop a blood alcohol concentration number. Knowing what matters and why tells the lawyer what to look for and what questions to ask. That knowledge is essential when evaluating evidence and communicating about it in court.
Let’s demystify crime lab blood testing for alcohol content. I can already hear something along the lines of “Oh, no! Science! Math!” No worries. You don’t need much science or math to understand the main concepts. I’ll explain it mainly using non-technical terminology, but I’ll tell you a few proper terms. I’ll leave out many details and science jargon to make it more easily understood.
Please don’t be intimidated. It’s not magic. Labs have no mysterious power. Lab workers are not wizards. They don’t have any superpowers. You can probably understand everything they do. You can do much of what they do, even though you don’t meet the job qualifications.
Lab workers need a formal scientific education to qualify for the job. A degree in science is proof to the lab bosses that the lab worker has enough understanding to do the job right. Lab workers need to understand why their actions are essential as they do it. You don’t need their formal education to get a good idea of how it works.
The name might sound intimidating but relax. It’s much easier to understand than you think. Gas Chromatography is the testing method they use. The abbreviation to remember is GC.
A GC Autosampler machine handles your sample. It’s called an autosampler because it does all the work after the lab worker puts the samples into the machine. Lab workers can leave work at the end of the day, and the GC machine crunches through the analysis overnight. When finished, the GC spits out a report of the blood components. The lab workers review this data and create reports. That kind of work requires knowing what the data means and what to do with it.
Blood labs work routinely with tubes that contain blood from someone’s arm, which are then put into smaller glass vials that fit into the machine. They pour blood from the sample collected into another vessel. They use a unique device like a magic wand to suck up a measured portion of blood to put in the smaller tube. The device is called a pipette. You could do this part with some practice. The tubes are sealed by a cap with a rubbery center called a septum.
Some space is left between the sample and the top of the liquid level, and the lab worker secures a cap onto the vial to seal it. The lab worker places the vial with other blood samples onto a tray with many other vials and then sticks them into the GC Autosampler machine.
The machine heats the sample. Heating causes tiny amounts of the blood’s contents to rise into the space between the blood and the cap. The gas in that space is called headspace gas. After sufficient heating, the machine sticks a needle through the rubbery top of each tube and sucks out a small amount of the gas above the liquid. The GC does not test the blood sample; it tests the gas above the liquid sample instead.
The headspace gas sample enters a specially treated tube called a column. This tube is extremely long, roughly 100 feet, and coiled to fit into the machine.
Pressurized gas pushes the headspace gas sample through the tube. The type of gas used should not cause any chemical reaction.
The time it takes for a substance to come out of the tube tells the machine what each substance is. During the gases’ journey through the tube, substances in the gases separate. Different substances exit the tube at different times because they separate as they travel. As sophisticated as it is, that’s the simple central concept.
GC machines leave the factory unconfigured. Lab decision-makers program them to recognize certain substances based on their time signatures.
Lab workers run a known substance through the GC machine. When that known substance emerges from the tube at a specific time, lab personnel program the machine to identify it as the known substance. The GC machine will later identify a substance coming out of it at that time as the known substance programmed into it. Alcohol is one of the substances it identifies, of course.
Labs get their training samples from NIST (National Institute of Standards and Technology), an entity of the federal government, or from a NIST-approved vendor. NIST produces the highest quality training samples known, called standard reference materials. These materials are used in many labs because they are widely accepted and set the bar for purity.
Measuring the amount of substance is different than identifying what it is, but it isn’t magic, either. At the tail end of the GC machine’s tube is a flame ionization detector (FID). I know, I know, that sounds like science. Relax. It’s just a flame burning at the end of the tube, and it burns whatever comes out. The FID measures the ions electronically. The measurement corresponds to a quantity of the substance. (This YouTube video helps visualize the FID process starting at the 3:20 mark.)
Imagine tossing a half-inch square piece of paper into a fire. It will vanish quickly. If you throw a one-inch square piece of paper into a fire, it will flash and then burn longer. Similarly, the FID electronically measures the size of the flash and burn and assigns a value.
If you’ve gotten this far, you know more about blood testing than most. If you can understand this, so can a jury.
Here’s a fact everyone should know: no lab has the power to render a guilty verdict.
I’ve heard many variations on this theme: “You can’t challenge blood test results.” That’s absolute nonsense. The only way anyone could honestly say such a thing is to not know enough about the topic. Trials are all about contesting evidence, even if it looks overwhelming on its face.
A jury decides what weight to give a lab result if any is given at all. Any member of a jury may disregard every bit of the expert’s testimony. A juror is not required to give a reason, either.
As you read further, don’t worry about whether you have enough scientific knowledge because you don’t need much at all. It may help to understand jurors in DWI cases are rarely scientific experts, just like most of us. The things I’ll point out in this blog are meant to appeal to them.
This blog post will demonstrate you CAN challenge blood results in a DWI case. Many things in a DWI blood case can give jurors good cause to doubt the result of a lab report.
I am not dismissing the powerful effect blood evidence can have on the attitudes of everyone involved. It can be an imposing hurdle to the defense. It’s typically an important piece of evidence, but certainly not the only evidence which matters.
Prosecutors are more likely to believe they have a stronger case with it, which may affect what options they are willing to consider. A high result is also something prosecutors may be willing to punish more severely.
Jurors may want to give credence to a number produced by a lab. A multitude of CSI (Crime Scene Investigation) television programs over two decades has indoctrinated people to think labs perform miraculous work without flaws. It’s fiction, but it affects attitudes.
Everyone knows that labs are an example of where science applies to the real world. The number provided by the lab can have powerful effect.
It’s the DWI defense attorney’s job to pull back the curtain, so to speak. Sound scientific theories help make the modern world. However, no matter how sound the theory, methods must be applied properly to be reliable.
The main point is to understand a lab report of a blood test does not automatically determine the result in a trial.
You should be aware of the legal context before we dive into some ways to challenge a blood test.
The default position is innocence. A person charged with DWI is presumed innocent. Potential jurors can be excluded from serving, if they don’t agree with this concept.
The state’s attorney is obligated to overcome the presumption of innocence to prove DWI was committed. Failure to do so means a “not guilty” verdict. The defense, in contrast, has no burden at all and may rely upon the presumption of innocence.
The prosecutor must prove guilt in the DWI case “beyond a reasonable doubt.” That’s the highest burden of proof needed to win in a trial court.
You may not have considered what “beyond a reasonable doubt” means. Take a quick look at the actual words. ONE reasonable doubt is enough to lead to “not guilty” because the prosecutor’s case must be “beyond” that single doubt. If the prosecutor doesn’t negate that doubt, the proper verdict should be “not guilty.”
Defendants have absolutely no obligation to prove anything. For example, the defense does not have to demonstrate the lab’s result is wrong.
A critical goal for a DWI defense attorney is to expose every reason possible to cause jurors to have a reasonable doubt. We want to show the jury where and why they should doubt the evidence. Each reason to doubt we can point out may be the difference between a “guilty” verdict and a “not guilty” verdict.
The source of a reasonable doubt could be the production of no evidence, insufficient evidence to meet the burden, or even evidence the juror simply doesn’t trust. If the prosecutor does not answer or dispel the questions raised by a defense lawyer or in the juror’s own mind, a juror may consider that a source for a reasonable doubt.
Each juror is the sole judge of what is a reasonable doubt. Jurors do not need to agree on the specific reasonable doubt to reach a verdict of “not guilty”. If every juror has a reasonable doubt, and each doubt is different, that supports a “not guilty” verdict.
You may be surprised to learn that a verdict of “guilty” or “not guilty” must be unanimous. Failure to reach unanimity leads to a mistrial.
DWI defense attorneys obviously want to get “not guilty” verdicts for their clients. However, a mistrial can be considered a successful defense. The result of a mistrial is to force the prosecutor to decide what to do next. Prosecutors sometimes decide to abandon prosecution altogether. A mistrial may also prompt the prosecutor to make a more lenient offer or reduce the charge. Retrial is another choice for the prosecutor, meaning the entire trial process would be repeated. The choice made can depend on many factors, including how many jurors voted each way.
Getting the lab’s blood level number thrown out or undermined doesn’t necessarily mean a win. Other evidence in the case may be enough to convince a jury to vote “guilty.”
I don’t claim to have an exhaustive list of every possible way to challenge a blood test. Each situation has the potential to create a new and unique set of questions. By the time you finish reading this post, you’ll know lab results from a blood sample in a DWI case can be challenged.
The burden is on the prosecutor to show intoxication “while” operating a motor vehicle. Even if literally everything went perfectly from start to finish, the result reported by the lab doesn’t equal intoxication at the time of driving.
Police may have no idea when driving occurred, such as when they happen upon an unreported crash. Usually, a traffic stop by police leads to arrest, but hours may elapse from arrest until a blood sample is drawn from the arm of a person arrested for DWI.
A DWI defense lawyer must understand how alcohol is absorbed into and eliminated from the blood stream. Alcohol levels in a blood sample taken after arrest may be much different than what the levels were while the person was driving.
The ability to raise that question effectively depends upon the knowledge of the attorney. Factors which could affect the answer include the time of the last drink, how much alcohol was consumed overall over what period of time, sex and weight of the drinker, what, when, and how much was eaten relative to drinking, and more. A reasonable doubt may be raised by “when” the driver was intoxicated, even if he was intoxicated at some time.
Is the person who drew the blood available to testify? This question matters for every potential witness in the chain of custody through testing and reporting the result. If one or more witnesses is not available to testify, it could lead to exclusion of the result or casting doubt upon the reliability of the result.
Was the blood draw consensual? If not, was a warrant obtained? Questions could be asked about whether the application for a warrant was faulty. Whether the warrant was properly issued or executed is another valid line of questioning.
Where the blood was drawn? If it was not a “sanitary” place, it could be excluded from evidence. Was the blood drawn by a qualified person?
What materials were used to complete the draw? Was the blood kit purchased through DPS or an authorized vendor? Was anything in the blood kit expired? Was there a recall from the manufacturer of the blood tubes? Did the tubes contain the proper substances?
Whether proper technique was used to draw the blood. Was the site of the arm the proper place to draw? Was the area properly disinfected? Was the substance something other than alcohol? Did the disinfectant have time to disinfect, i.e. dry?
How many tries did it take to get a sample? Each stick increases the likelihood it will have skin in the blood, among other things aside from blood.
Were the samples properly marked to identify them? Was this verified at every step?
Was the blood properly handled? Were the tubes properly inverted the correct number of times? Were the tubes shaken instead?
Were there any issues affecting chain of custody from the point of the blood draw to the testing of the sample and beyond?
How did the arresting officer handle the samples?
Where did the arresting officer keep the samples and for how long? For example, were the samples kept in a trunk or in an unairconditioned car? What was the reason for holding onto the samples for any length of time? Were the samples handled by anyone else? Were the samples kept in a place anyone could tamper with them?
How were they transported to the lab?
All the actions taken at the lab are bound by protocols. Protocols are procedures lab personnel are required to follow. They apply from the moment the lab gets the blood until they are authorized to dispose of the blood. They are written after much consideration. The people who run the lab may adopt protocols already approved by another entity, such as an accreditation agency. These protocols easily reach into the hundreds of pages. Protocols are in place to ensure the proper procedures are followed. Failure to follow them can give rise to a reasonable doubt and much more reason to consider the result unreliable. Each failure to follow protocol is an opportunity to create doubt about the reliability of the result.
Two examples of procedure manuals applicable to Texas crime labs are the “Toxicology (Alcohol/Volatiles) Procedures and “Crime Laboratory Division Manual.”
Were the samples stored in a freezer? When did that begin? What temperature were the samples kept at in the freezer? What thermometer was used to verify the temperature? How do you know the temperature was at a certain degree and not interrupted? Who had access to the samples in the freezer?
When were the samples removed from the freezer and by whom? What was the condition of the samples when they were removed from the freezer?
How long were the samples out of the freezer before being opened? Who opened them? Were they identified as coming from a certain person? How?
Were the tubes clearly marked? Was the information accurate? Were the seals intact? Were the tubes intact or broken? Were the stoppers secure? Was there any indication of an improperly sealed tube? Were any leaks evident?
What color was the blood in the tubes? Were the samples in liquid form? Did the samples contain chunks? Was the blood coagulated?
Was the person or persons involved qualified for the job? Is there any reason to believe he/she/they have a record of mistakes or dishonesty at work or in testimony before a court?
How and when were samples taken from the tubes for testing? What instrument was used? (Pipette) Have there been any recall or advisories from the pipette manufacturer issue? Was the pipette in good working order? What documentation demonstrates that it had been recently calibrated? How long ago was calibration completed? How often is it required to be calibrated? Was that requirement met? Where are the records of such calibration?
Were there any discrepancies in the batches immediately before or after my client’s testing batch? If so, how were the discrepancies addressed by lab management? What remedial steps were taken to identify the cause for the discrepancy? What steps were taken to rectify the discrepancy, so that it would not occur again? Is there a history of discrepancies with the machine testing my client’s sample?
How many subject samples were tested in the batch of my client’s? Was the blood sample of the accused placed in the right spot in the sample tray?
What process was used to ensure no carryover occurred from one pipetted sample to another? Were disposable pipette tips used on each sample?
Was the pressure in the carrier gas within acceptable range? Was the gauge which measured the pressure properly calibrated and certified? Was the gas an acceptable type of gas? Was the source of the gas traceable, so as to be sure it was the claimed gas?
Has proper and scheduled maintenance on the GC been performed and documented? Has the maintenance schedule been followed? Were any problems discovered?
Were the columns trimmed and reattached? If so, was the GC recalibrated for the shorter length of time it would take for substances to pass through the shorter column? If so, where are the records of recalibration? Was any testing at all done to confirm the accuracy of the calibration after modifying the columns?
Were the columns replaced? If so, what is the source of the columns? Is the supplier approved? Has there been any recalls? If the columns were replaced, has the GC been recalibrated for all substances the lab tests for?
I could belabor this discussion indefinitely. The list of questions about what could go wrong at a lab is practically inexhaustible. Trying to cover every possible question would fill a book. Anything about the entire process which was supposed to happen is fair game. How what was supposed to happen is fair game, too. Any discrepancy, no matter how seemingly insignificant, may provide a juror with a reasonable doubt.
So, if someone ever tells you a blood test in a DWI case cannot be challenged, you know better.
© 2019-2026 Law Office of Phillip W. Goff. All Rights Reserved. Privacy Policy
