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Criminal defense challenges to BAC blood test results in Drunk Driving prosecutions

If you have been arrested or charged with driving while intoxicated (DWI) in the State of Texas, and your blood test result comes back at 0.08 g/dL or higher blood alcohol content (”BAC”) , then what’s left for your defense? Once you’ve had a blood test that returns over the legal limit, is there any way to fight a Driving While Intoxicated conviction on your driving record?

Yes. There are solid legal defenses to Texas DWI charges even after the police have a blood sample with a BAC over the legal limit. Discrediting a “bad blood test” is one way to challenge that DWI charge and defend against having a drunk driving charge on your record, and there are several ways to do this.

These BAC tests, like other kinds of forensic evidence, are not immune to challenge on a variety of grounds. This article is intended to be a summary of possible defenses to BAC blood test results in Texas DWI cases, from the perspective of an experienced DWI criminal defense lawyer. Insofar as these defense strategies work as scientific challenges, they may also be successful in other state jurisdictions as well.


Typically, a sample of blood is drawn and tested for alcohol content (i.e., “BAC” or “blood alcohol content”) in only two DWI situations: (1) when requested by law enforcement officers in connection with a suspected drunk driving case, or (2) at the request of hospital medical personnel in connection with diagnosis and treatment of a patient who has been brought in after an accident.

Legal Blood vs. Medical Blood in Driving While Intoxicated (DWI) Charge

In the former situation, the blood is sometimes referred to as “legal blood” and whole blood is typically collected; in the latter, it is sometimes called “medical blood” and is typically serum. The methods used to draw, prepare, track, and test legal blood often differ from those used for medical blood.
There is a strong argument that serum blood samples, due to problems with accuracy, show lack of trustworthiness as forensic evidence of the actual blood alcohol reading of a patient.


First, every individual has a different serum or whole-blood ratio that varies from 1.90 to 1.35. Furthermore, an arbitrary or average conversion ratio does not give an accurate result for a particular individual.

Second, when medical staffers draw “legal” blood, they follow procedures promulgated specifically for blood alcohol testing. Often, state regulations or statutes will specify a particular type of “kit” that has alcohol-free swabs in the package, as well as a series of labels, seals, and “packing” forms to assure that all necessary steps are taken to obtain a sample that can be admitted in evidence.

Challenge the Admissibility of a “Legal” Blood Sample in DWI Case

In order to challenge the admissibility of a “legal” blood sample into evidence, the attorney must question the medical personnel involved to determine whether blood was draw and tested in compliance with state regulations. The defense attorney should also determine whether the regulations that were designed to ensure accuracy and reliability were followed.

Tamper-Proof Seals

These state regulations often specify that the vials shall contain anticoagulant and a preservative and be “stopped” or capped (or sealed) to prevent loss of contents through evaporation. These tamper-proof seals also will reveal any tampering with or compromise of the vials. Also, some jurisdictions mandate that all vials be marked with certain essential tracking information. Regulations or statutes may even specify the method of cleansing the subject’s skin before blood is drawn (i.e., with soap or Betadine, not alcohol).

Blood Vials

Blood vials that are to be used for legal samples generally contain an anticoagulant and a preservative, and generally have a gray-colored septum (indicating that vials have these two chemicals inside the vacuum tube). Other vials, used for various medical or laboratory purposes, may have red, yellow, purple or other colors signifying whether the vial contains certain chemicals.

Vials used in collecting medical blood samples may lack anticoagulant and preservatives. These vials generally have red septums, meaning that the blood collected is pure blood without anticoagulant or preservative added to the vial. A red-stoppered vial will become clotted (due to lack of anticoagulant) and will “deteriorate” much more quickly than those with gray stoppers, due to lack of a preservative.

DWI Defense Attorney Approach

The attorney should ascertain the condition of the tube used, because an improperly capped or filled tube may result in an incorrect BAC value. If the hospital (or police) sends blood to an outside private lab or the state crime laboratory for testing, counsel should check to see if the blood was continuously refrigerated, because the red-stoppered vials used by hospitals typically lack preservatives, and the alcohol content in the sample might actually increase if left at room temperature for an extended period of time. Jurors are shocked to learn that normal blood (with no alcohol in it at all) that decomposes actually creates ethyl alcohol as a by-product of the decomposition.

DWI Defense Expert Witness Testimony

Through use of expert witnesses, the party opposing the admission of blood test evidence should attempt to bring out evidence of the unreliability of the blood test results in the current case. First and foremost, emphasize that every individual has a different serum or whole-blood ratio and the use of an arbitrary or average conversion ratio does not give accurate results for a particular individual. If an incorrect color cap was used on a vial, or if an anticoagulant or preservative was omitted, this may offer “reasonable doubt” about the integrity of the blood test result, particularly if gaps exist in the chain of custody.

Almost all blood collection vials have an expiration date. Manufacturers claim that this is only relevant to the warranty on the vacuum within the vial. Jurors may see the issue differently, if defense counsel analogizes to milk or bread sold after the expiration date.

Important Defense Questions Challenging Blood Samples – DWI Cross Examination:

1. Who requested this sample and for what purpose?
2. Did the suspect actually “consent” to the test?
3. What type of vial was the specimen collected in?
4. Was the vial properly capped, and with what color cap?
5. Did the vial have a preservative in it?
6. Did the vial have an anticoagulant in it?


Defense attorneys should be aware and investigate the possibility that there may be a problem with contamination of the blood specimen. For example, the antiseptic used to cleanse the area may contain alcohol in amounts sufficient to produce measurable amounts of alcohol on a gas chromatograph. In emergency room settings, it is routine to use an alcohol-based antiseptic to clean the skin. It may also be routine to use Vacutainers and vials that are not in compliance with state regulations and statutes.

Alcohol Contamination through Antiseptics and Skin Cleansers

Any antiseptic that was provided in blood alcohol collection (BAC) kits (or otherwise used to draw blood specimens) should be analyzed by an independent laboratory to determine whether the antiseptic contains alcohol in sufficient amounts to produce measurable amounts of alcohol. While many jurisdictions have regulations and procedures governing blood testing, and despite the fact that most specifically prohibit using any alcohol solution as a skin cleanser, some of the swabs that are used in blood collection kits contain a substance known as benzalkonium chloride as an active ingredient. This compound contains approximately 2 percent (2%) or more ethanol concentration.

Alcohol Contamination through Clinipad7™, PDI7™, and Triad 7 ™ towelettes

Ethanol in measurable quantities also has been found in three types of disinfectants: Clinipad7™ brand antiseptic towelette, PDI7™ brand towelette and Triad 7 ™ brand towelettes. Swabbing the skin with ethanol before taking a blood sample for measuring blood ethanol concentration may increase the apparent blood ethanol level by up to 0.018 percent (g/dL), even if the skin is allowed to dry before the sample is taken. When a “wet” site is used, and a “wet” swab is held over the site to withdraw the needle (which has a vacuum to draw in the blood from the suspect’s vein), the potential additive effect is much greater.

Defense attorneys frequently succeed in convincing a jury that a percent of alcohol that was found in the blood sample was added to the test result because the person who withdrew the blood improperly used alcohol on the skin. As sample cross-examination could go as follows:

Important Defense Questions Challenging Blood Specimens – Sample DWI Cross Examination

Q. Doctor, suppose just one drop of alcohol was added to 100 drops of normal blood, what percentage of alcohol would you find?
A. If the drop of alcohol were the same size as a drop of blood, I would expect to find 1.0 percent by volume, of 0.80 percent by weight.
Q. What state of intoxication would you expect to find with 0.80 percent alcohol in the blood?
A. That person would be dead.
Q. Now, if only half a drop of alcohol was added to the 100-drop sample, what would you expect to find in the blood test?
A. Approximately 0.40 percent by weight.
Q. And what stage of intoxication would that percent correspond?
A. Marked intoxication, probably unconsciousness.
Q. Now suppose that one one-tenth drop of alcohol got into the 100 drops of blood, how would that affect the blood test?
A. The reading would be approximately 0.08 percent.

The attorney can now successfully argue to the jury (as I did) that a mere fraction of a drop of extraneous alcohol that had been applied to the skin had been responsible for a possible miscarriage of justice. (My client was acquitted.)

Contamination of Blood Taken from DWI Suspect involved in an Accident

The blood taken from a driving while intoxicated (DWI) suspect involved in an accident or car crash may also be contaminated if there was an intravenous fluid being administered at the time the blood was drawn.


The specimen may be inaccurate either (1) because of the increased volume of fluid in the circulatory system changes the blood alcohol level (lowers it) or (2) the blood may have been drawn from the same extremity that the IV is in, and therefore the sample is contaminated with the intravenous fluids. If there was injury to the defendant, and the paramedics responded, it is very likely that an IV was started en route to the emergency room.

Contamination of Blood Taken from DWI Suspect When Equipment is not Sterile

Contamination can also occur if the equipment used to draw the blood is not sterile. This may not pose as big a problem today as it did years ago before pre-wrapped disposable needles became the standard. Nevertheless, it still happens on occasion.

In some emergency room settings, nurses “set up” their trays and carts anticipating an emergency. That is, needles have been removed from the package and placed on the table along with other routinely used items so that these items will be readily available when an emergency comes in. If this is the case, a needle of syringe may have been left exposed for an undetermined amount of time and is no longer sterile. Check your state regulations concerning the propriety of using such items, in a non-sterile situation.


To challenge a serum blood alcohol result, attorneys must know the differences between whole and serum (or plasma) blood readings. The admissibility of a blood alcohol test as evidence at trial will depend on the specific language of the relevant state statute and on the ability of the attorney opposing the test result to identify all the ways in which it is unreliable. In some states, no published regulation controls how serum or plasma blood readings will be “adjusted” to reflect approximate whole blood alcohol levels.

Serum Levels Can be 16-21% Higher than Whole Blood BAC Levels

A potential source of error involves reporting plasma or serum values that are assumed to be whole blood values. The defense attorney should try to discover whether whole blood, serum or plasma was analyzed. Some studies indicate that plasma and serum alcohol levels are as much as 16 to 21 percent higher than whole blood levels.

After the blood is drawn, it is often centrifuged (spun in a mechanical device) and separated into its “solid” and “liquid” components for analysis. What is being analyzed by the toxicology lab may not be whole blood, but serum or plasma and, therefore, an analysis of plasma or serum will produce a higher percentage of alcohol than would actually be present in whole blood. Hence, the results must be mathematically “adjusted” or truncated to show the approximate true BAC.

Distorted Readings – Serum vs. Whole Blood

Some police departments and state forensic laboratories analyze whole blood. Many hospitals and clinical laboratories routinely analyze only serum. Evidence of a client’s blood alcohol level indicating a result of 0.10 percent (g/dL) BAC may, in fact, reflect a true BAC of 0.08 to 0.09 g/dL if serum was used.

In addition to the distorted reading, the testing may be challenged on the grounds that a serum alcohol concentration test is not a blood test for the purposes of the statute. Therefore, DWI defense attorneys should carefully review the chemical sobriety test statutes and definition sections of the state criminal code.


Due to variations in body chemistry plus the chemical changes that the blood undergoes while in the glass ampoules, the DWI blood specimens may become unrepresentative of the person accused. The difference is due to what occurs in vivo (those which occur inside the body) and in vitro (those which occur while the blood is in glass ampoules). In vivo changes occur because of normal fluctuations in the blood from moment to moment after the blood’s ability to hold and release alcohol.

Blood Specimen Neglected at the Lab

It is not an uncommon practice to let the blood specimen sit for days before analyzing it due to delay in getting it to the laboratory, or due to simple neglect. Because the carousels on most gas chromatographs hold fifty to 100 vials, many laboratories wait until a full carousel can be obtained before running the tests. Defense counsel may be surprised to learn how long a sample was not refrigerated.

Decomposing Blood Sample Produces Alcohol

Because blood is an organic material, it will decompose as a result of enzyme activity and bacterial action. One of the results of this decomposition is that ethyl alcohol is created from the breakdown of the blood. This is sometimes referred to as “endogenous” alcohol production. In a sample originally containing no alcohol, decomposition can cause an alcohol content reading of 0.25 g/dL or even higher depending on the stage of decay.

Failure to Add Sodium Fluoride or Potassium Oxalate

Usually the specimen will be refrigerated to prevent degradation of the sample. However, refrigeration will only slow down the decomposition process, not prevent it. To stop this decaying of the blood and the resultant formation of alcohol, a preservative such as sodium fluoride solution should be added. Blood without sodium fluoride is reliable at normal room temperatures for about two days. Without sodium fluoride, the BAC may rise to a maximum concentration in about fifteen days and then fall.

Failure to add sodium fluoride or potassium oxalate (and this is not uncommon) should provide DWI defense counsel with sufficient ammunition to discredit the state’s BAC test results, if not prevent their admission into evidence. In a criminal trial, raising reasonable doubt only requires that a sufficient factual basis for questioning the reliability and accuracy of the test results be raised by the defense.


Certain medications and the “disease process” can affect the level of BAC. Any medication that alters the rate of metabolism can affect blood alcohol levels.
For example, an article in the Canadian Society of Forensic Science Journal has reported that women taking oral contraceptives appear to eliminate ethanol significantly faster than women not taking oral contraceptives. (See, Papple, The Effect of Oral Contraceptive Steroids (O.C.S.) on the Rate of Post-Absorptive Phase Decline of Blood Alcohol Concentration in the Adult Woman, 15:1 Canadian Society of Forensic Science journal at 17 (1982).) Also, Candida albicans is a yeast-based medical problem that can cause alcohol to be created inside the vial, after being drawn from the subject’s arm.

The human body eliminates the amount of alcohol by oxidation of the “poison” (alcohol) in the liver. Like any other foreign compound, alcohol is broken down by enzymes in the liver and gradually reduced until 100 percent is eliminated.

Therefore, any disease process affecting the liver, such as hepatitis, will impair results. Also, any condition that causes “extracellular” water retention (heart disease or many forms of high blood pressure or diabetes, for example) will alter results.

Alcohol is “water miscible” meaning that it mixes easily with water. Alcohol also “migrates” to any water source, whether in the human body or in a test tube. Also, many women retain water on or around the time of menses. Therefore, defense counsel in the investigation of the case should obtain this information.

About the Author:

Michael Lowe is a Texas trial attorney practicing criminal defense law in the Dallas area for many years after first serving as a felony prosecutor for the Office of the District Attorney for Dallas County. He is Board Certified by the State Bar of Texas in Criminal Law. Mr. Lowe has tried to verdict over 150 criminal trials so far in the state and federal systems.



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