Blood Sample Integrity in DWI Cases: Everything That Can Go Wrong Between the Draw and the Lab

By Deandra Grant, J.D., M.S. (Pharmaceutical Science), ACS-CHAL Forensic Lawyer-Scientist

In Texas DWI cases involving blood draws, the prosecution presents the lab result as if it is a fact: “The defendant’s blood alcohol concentration was 0.12.” But that number is only as reliable as every step in the chain from the moment the needle entered your arm to the moment the analyst reported the result. A failure at any point in that chain can produce a number that is inaccurate, inflated, or scientifically unreliable.

This article walks through the entire blood evidence process, from the draw to the lab, and identifies the specific points where errors occur. These are not theoretical vulnerabilities. They are documented problems that arise in real cases and that an attorney with forensic science training can identify and challenge.

Phase 1: The Blood Draw

The Phlebotomist

Texas law requires that blood draws in DWI cases be performed by qualified personnel: a physician, registered nurse, licensed vocational nurse, or qualified technician acting under the order of a physician. The person drawing the blood must follow accepted medical practices. If the draw was performed by someone who was not qualified or who deviated from standard procedures, the evidence may be challenged.

The Antiseptic: The Alcohol Swab Question

One of the most common questions defendants ask is: “They swabbed my arm with alcohol before the draw. Doesn’t that contaminate the sample?” The short answer is that peer-reviewed research has generally shown that standard isopropyl alcohol skin preparation does not meaningfully affect venous blood alcohol results. However, there are circumstances where non-alcohol antiseptics (such as betadine or chlorhexidine) should have been used, and the failure to do so can become part of a broader challenge to sample integrity. From the legal point of view the question may really be what are the standard operating procedures that are required and were they followed.

Tube Selection: The Gray-Top Tube

This is where many cases go wrong. Blood samples for forensic alcohol analysis must be collected in gray-top vacuum tubes containing two critical preservatives:

• Sodium fluoride (NaF): An enzyme inhibitor that prevents in vitro glycolysis and, critically, inhibits microbial activity that can produce ethanol in the tube after collection
• Potassium oxalate: An anticoagulant that prevents the blood from clotting, ensuring a homogeneous sample for analysis

If the wrong tube type is used (ex. a red-top tube (no preservative), a purple-top tube (EDTA anticoagulant but no NaF), a green-top tube (lithium heparin) or a gray top tube without the necessary amount and ratio of sodium fluoride and potassium oxalate) the sample lacks the sodium fluoride needed to prevent microbial fermentation. Without NaF, bacteria and yeast naturally present in the blood can metabolize glucose and produce ethanol inside the tube, artificially elevating the reported BAC.

This is not speculation. In vitro fermentation is a well-documented phenomenon in the forensic toxicology literature. Studies have shown that unpreserved blood samples can generate ethanol concentrations sufficient to push a below-limit sample above the legal threshold.

Phase 2: Transport and Storage

Refrigeration

After collection, blood samples must be refrigerated to slow microbial activity and preserve sample integrity. If samples are left at room temperature for extended period (ex. in a patrol car during a hot Texas summer, on a counter in a hospital, or in an unrefrigerated evidence locker) the risk of in vitro fermentation increases dramatically, even in samples collected in gray-top tubes. Sodium fluoride slows microbial activity but does not eliminate it entirely, particularly at elevated temperatures.

Chain of Custody

Every person who handles the blood sample from the moment of collection to the moment of analysis must be documented. Gaps in the chain of custody (periods where the sample was unaccounted for, transferred between facilities without documentation, or stored under unknown conditions) undermine the ability to establish that the sample tested in the lab is the same sample drawn from the defendant and in the same condition.

Phase 3: Laboratory Analysis

Headspace Gas Chromatography with Flame Ionization Detection (HS-GC-FID)

Most forensic crime labs in Texas analyze blood alcohol samples using HS-GC-FID. An aliquot of the blood sample is placed in a sealed headspace vial and heated. The ethanol volatilizes into the headspace above the liquid. A portion of the headspace gas is injected onto a chromatographic column, which separates ethanol from other volatile compounds. The separated ethanol passes through a flame ionization detector, which generates a signal proportional to the amount of ethanol present. The signal is compared to a calibration curve generated from known ethanol standards to calculate the BAC.

This process involves multiple potential sources of error:

• Calibration: If the calibration standards were improperly prepared, expired, or outside acceptable tolerance, every result generated against that calibration is suspect
• Internal standard failure: Most methods use an internal standard (commonly n-propanol or t-butanol) to account for variations in injection volume and instrument response. If the internal standard was improperly prepared or degraded, the quantitation is unreliable
• Coelution: If another volatile compound in the blood has a retention time similar to ethanol on the chromatographic column, it may co-elute with ethanol, artificially inflating the reported concentration. Compounds such as acetaldehyde, methanol, and isopropanol can potentially interfere depending on the column and conditions used
• Carryover: If the autosampler syringe or the sample path is not adequately cleaned between injections, residual ethanol from a previous high-concentration sample can carry over into the next analysis, producing a falsely elevated result
• Headspace temperature and equilibration time: The amount of ethanol that partitions into the headspace is temperature-dependent. If the headspace oven temperature was inconsistent or the sample was not equilibrated for the required time, the result may not be accurate

Phase 4: Reporting and Interpretation

• Measurement uncertainty: Every analytical measurement has an associated uncertainty. A reported BAC of 0.08 does not mean the true value is exactly 0.08. Rather, it means the true value falls within a range around that number. If the lab does not report measurement uncertainty, ask for it. A reported 0.08 with a measurement uncertainty of ±0.005 means the true value could be as low as 0.075 which is below the legal limit.
• Duplicate analysis: Standard forensic practice requires analyzing duplicate aliquots from the same sample. If the duplicates do not agree within acceptable limits, the result is unreliable.
• Whole blood vs. serum/plasma: Hospital blood draws for medical purposes typically analyze serum or plasma, which produces a BAC approximately 10–25% higher than whole blood. If a hospital draw was used for forensic purposes without correction for this difference, the reported BAC is artificially inflated.

What to Demand in Discovery

In every blood test DWI case, defense counsel should request:

• The type of collection tube used (manufacturer and lot number)
• The name and qualifications of the phlebotomist
• The time of collection and the time of analysis
• Refrigeration records and storage conditions from collection to analysis
• The lab’s standard operating procedure for blood alcohol analysis
• Calibration records, internal standard preparation logs, and quality control data for the analytical run that included the defendant’s sample
• Duplicate analysis results and the acceptance criteria
• The lab’s measurement of uncertainty for BAC analysis
• The chromatogram for the defendant’s sample (not just the reported number)
• Chain of custody documentation from draw to lab

At Deandra Grant Law, we review blood evidence at every stage of this process. Deandra Grant’s pharmaceutical science training and ACS-CHAL Forensic Lawyer-Scientist designation provide the foundation to identify errors that most attorneys, and even some experts, would miss. Call (214) 225-7117 or visit texasdwisite.com.