Challenging the Blood Test: How a DWI Defense Attacks a Blood Alcohol Result

A blood result looks final, but it is not

Jurors tend to treat a blood result as objective, precise, scientific proof, and most defense attorneys lack the training to question it. But a blood number is the end product of a long chain of human steps, the warrant, the draw, the storage, the transport, the lab analysis, and an error at any point can make it wrong. Challenging it means working through that chain and attacking the weak links, on both the law and the science. For how the analysis works, see the DWI blood test page; this page is about how the result gets dismantled.

Start with the warrant

Before the science, there is the law. Under Missouri v. McNeely, police generally need a warrant to draw your blood, and the natural dissipation of alcohol is not by itself an emergency that excuses getting one. A warrantless draw without consent or a true exigency is subject to suppression. And even a warrant can fall: under Franks v. Delaware, a warrant built on a false statement made knowingly or recklessly, or on a material omission, can be invalidated, taking the blood result with it. If the draw was unlawful, the science never has to be reached. See the motion to suppress and the blood search warrant.

Attack the draw

The result is only as good as the sample, and the collection is the first scientific battleground.

• Alcohol-based antiseptic. A forensic draw should use a non-alcohol antiseptic. An alcohol swab violates standard procedure and is challengeable.
• The wrong tube or too little preservative. Blood must go in a gray-top tube with sodium fluoride and potassium oxalate in the proper amounts. The wrong tube or insufficient preservative allows chemical changes that skew the result.
• Failure to mix. The tube must be inverted several times to distribute the preservative; poor mixing risks uneven preservation and an invalid result.
• Unqualified personnel. Texas law specifies who may draw blood for DWI testing, and a draw by an unauthorized person can be challenged.

Attack the storage and the lab

Two of the most powerful scientific challenges live between the draw and the report.

• In vitro fermentation. If the sample is under-preserved, stored warm, or delayed, microorganisms such as the yeast Candida albicans can ferment the blood’s glucose into ethanol, raising the reported BAC above the person’s actual level at the time of the draw. Diabetic and pre-diabetic individuals are especially susceptible.
• Coelution. In the gas chromatography analysis, two compounds can exit the column at the same time and overlap. If something like acetone (elevated in diabetics) or isopropanol coelutes with ethanol, the instrument can overcount the alcohol and report a falsely high BAC. Catching this requires reading the chromatogram, not just the final number.

Beyond these, the lab must follow its own validated protocols, use calibrated equipment and in-range reference standards, run quality controls, and document everything, all of which is discoverable and challengeable, along with the analyst’s qualifications.

Hospital blood and the margin of error

Two more attacks often decide close blood cases:

• Hospital serum is not whole blood. When blood is drawn at a hospital after a crash, the lab usually tests serum or plasma, which reads higher than the whole blood the DWI statute is written around. To compare it to the limit it has to be converted down, typically by dividing by about 1.16 to 1.18, so a 0.10 hospital reading is closer to 0.084 to 0.086 as whole blood. A hospital number put in front of a jury without that conversion overstates the result.
• Measurement uncertainty. A lab result is not a single exact value; it carries a margin of error. A reported 0.082 with an uncertainty of plus or minus 0.006 could actually be below 0.08. If the lab cannot produce its calculated uncertainty, that gap is itself a weakness.

Attack the chain of custody

A blood sample must be documented, stored, and transported under controlled conditions from the draw to the analysis. Any gap raises the question of whether the sample was properly preserved, whether it was contaminated, or whether it is even the right sample. Chain-of-custody gaps are not technicalities; they go to whether the number the state is relying on belongs to the defendant at all.

Why reading the chromatogram changes everything

Knowing that a blood test can be challenged and knowing how to challenge it are different things. Managing Partner Deandra Grant holds the ACS-CHAL Forensic Lawyer-Scientist designation, is an Assistant Chromatography Instructor at Axion Labs in Chicago where she teaches gas chromatography four weeks a year, and devotes the largest chapter of her book, the Texas DWI Manual, to blood alcohol analysis. Partner Douglas Huff holds the same designation. That training lets the firm read the chromatogram, identify coelution and contamination, cross-examine the analyst on methodology and quality control, and work with independent experts, rather than simply asserting the number might be off.

Frequently Asked Questions

Can a DWI blood test be challenged in Texas?

Yes. A blood result can be attacked on the warrant, the draw, the tube and preservative, storage and fermentation, coelution and analyst error in the lab, and gaps in the chain of custody. It is the end of a long chain, and any link can fail.

Can a blood draw be suppressed?

Yes. Under Missouri v. McNeely a warrantless draw is generally unlawful without consent or a true emergency, and under Franks v. Delaware even a warrant can be invalidated if its affidavit contained false statements or reckless omissions.

What is in vitro fermentation?

It is the creation of alcohol inside the collection tube after the draw, when yeast such as Candida albicans ferments the blood’s glucose into ethanol. It makes the reported BAC higher than the person’s actual level, and diabetics are especially susceptible.

What is coelution?

Coelution is when another compound exits the lab’s chromatography column at the same time as ethanol, causing the instrument to overcount the alcohol and report a falsely high BAC. Reading the chromatogram is required to catch it.

Why does the chain of custody matter?

A blood sample must be tracked continuously from the draw to the analysis. A gap raises doubt about whether the sample was preserved, whether it was contaminated, or whether it is even the right sample.

Is a hospital blood test the same as a police blood test?

No. Hospitals usually test serum or plasma, which reads higher than whole blood and must be converted (roughly dividing by 1.16 to 1.18) to compare it to the 0.08 limit. A 0.10 hospital serum result is closer to 0.084 to 0.086 as whole blood.

A Blood Number Is the End of a Long Chain. Any Link Can Break.

From the warrant to the chromatogram, a DWI blood test has failure points at every step. Deandra Grant Law teaches gas chromatography and challenges blood evidence at the level of the science. Call (214) 225-7117 for a free, confidential consultation.

Related Defenses Topics

• Challenging the Breath Test — Attacking breath evidence on procedure and science.
• Motion to Suppress — Getting unlawfully obtained evidence thrown out.
• Medical Conditions Defense — How diabetes and fermentation skew a blood result.
• DWI Defense Framework — How a strong defense works through the whole case.