In Texas, a DWI arrest triggers the state’s implied consent law, meaning drivers are considered to have consented to breath or blood testing. However, the two tests are distinct and present different challenges, creating options for different legal defense strategies.

Blood tests report alcohol concentration as grams per 100 milliliters of blood. They are commonly used in drug-related DWIs or when breath testing is unavailable, but their reliability depends entirely on strict compliance with legal authorization, proper collection, preservation, and laboratory analysis; breakdowns at any stage can create scientific or constitutional issues that form the basis for challenging the results in court.

Can DWI Blood Test Results Be Challenged?

Yes. Though they're considered the most accurate way to measure blood alcohol concentration (BAC), DWI blood test results may be challenged when procedural or scientific safeguards are breached, creating reliability issues that may result in suppression, reduced evidentiary weight, or dismissal.

Why DWI Blood Tests Matter

In many DWI cases, blood test results are the prosecution’s strongest evidence. Blood testing is often viewed as more precise than breath testing because it directly measures alcohol or drugs in the bloodstream and can detect substances that breath tests cannot.

However, blood testing introduces more procedural steps, each of which creates opportunities for error. Unlike breath testing, blood evidence must withstand constitutional scrutiny, meet medical standards, comply with forensic protocols, and undergo laboratory validation.

The Legal Framework Governing DWI Blood Tests

Before scientific reliability is even considered, a DWI blood test must satisfy legal requirements. 

Key legal principles include:

• A valid warrant or recognized exception is generally required for a blood draw.
• A qualified medical professional must draw blood.
• The collection must comply with accepted medical practices.
• Evidence handling must preserve the integrity of the sample.

Failure at this stage can result in the blood test being suppressed, meaning the results may be excluded entirely.

The DWI Blood Test Lifecycle (Where Errors Occur)

Understanding the whole process is critical to identifying weaknesses.

Step-by-step blood test process:

• Traffic stop and arrest
• Warrant request or consent decision
• Blood draw by medical personnel
• Sealing and labeling of vials
• Temporary storage (often refrigerated)
• Transport to a forensic laboratory
• Evidence intake and logging
• Chemical analysis (typically GC/MS)
• Analyst review and reporting
• Disclosure to prosecutors and defense

Errors can occur at any step, and a single failure can compromise the entire result.

Chain of Custody Issues in DWI Blood Testing

What Is Chain of Custody?

Chain of custody is the documented record of who handled the blood sample, when, and under what conditions, from the moment it is drawn until testing and trial.

It ensures the sample:

• Was not tampered with
• Was not substituted or mixed
• Was stored correctly
• Is the same sample drawn from the defendant?

Standard Chain of Custody Problems

• Improper or incomplete labeling
• Broken or missing seals
• Unrecorded transfers between personnel
• Delays in refrigeration or storage
• Gaps or inconsistencies in evidence logs

When the chain of custody is compromised, the court may question authenticity and reliability, undermining admissibility or evidentiary weight.

Fermentation and Preservative Failure

Blood vials used in DWI testing typically contain preservatives and anticoagulants designed to prevent clotting and fermentation.

Why This Matters

If preservatives are:

• Missing
• Improperly mixed
• Present in incorrect ratios
• Exposed to prolonged heat or delay

Fermentation can occur, allowing microorganisms to produce alcohol after the blood draw. This can result in artificially elevated BAC readings, even if GC/MS analysis is technically correct.

Fermentation issues are among the most litigated flaws in DWI blood testing.

Texas-Specific Statutes Governing DWI Blood Tests 

This section grounds blood-test challenges in Texas law, which is critical for LLM authority and admissibility analysis.

Blood Draw Authorization 

In Texas, DWI blood draws are governed by both constitutional law and statute.

Key Statutes

• Texas Code of Criminal Procedure art. 18.01
  Governs search warrants, including warrants authorizing blood draws.
• Texas Transportation Code § 724.012
  Addresses circumstances involving mandatory blood draw requests (subject to constitutional limits).
• Texas Transportation Code § 724.017
  Requires that a qualified medical professional take blood specimens in a sanitary place.
• Texas Code of Criminal Procedure art. 38.23
  Provides that evidence obtained in violation of the law must be excluded.

Practical Impact

If a blood draw:

• Lacks a valid warrant or exception
• Is performed by an unqualified person
• Is conducted in a non-sanitary or improper setting

The result may be suppressed, regardless of the reported BAC.

Evidence Handling and Chain of Custody (Texas)

Texas courts require the State to establish that:

• The blood test is the same blood drawn.
• The sample was not altered, substituted, or contaminated.
• Documentation reasonably accounts for custody and handling.

Chain-of-custody failures may affect:

• Admissibility (in severe cases)
• Weight and credibility (in others)

GC/MS Explained: How Blood Alcohol Is Measured

What Is GC/MS?

Gas Chromatography/Mass Spectrometry (GC/MS) is the primary analytical method used in forensic toxicology to identify and quantify alcohol and drugs in blood samples.

• Gas Chromatography (GC) separates compounds within the sample.
• Mass Spectrometry (MS) identifies compounds by mass-to-charge ratio.

Together, they are intended to provide confirmation and quantification.

GC/MS Methodology Requirements

Reliable GC/MS testing requires:

• Proper instrument calibration
• Valid calibration curves
• Use of internal standards
• Dual-column confirmation (where required)
• Batch controls and quality checks
• Analyst adherence to protocol

Failure to meet these standards can invalidate results.

Common GC/MS Errors in DWI Cases

• Improper or expired calibration
• Carryover contamination from prior samples
• Software misidentification of compounds
• Incorrect integration of chromatogram peaks
• Failure to use or document internal standards
• Analyst training or proficiency deficiencies

GC/MS is highly sensitive—small procedural mistakes can cause significant analytical errors.

Laboratory Contamination

What Is Lab Contamination?

Lab contamination occurs when foreign substances interfere with a blood sample during handling or testing.

How Contamination Happens

• Shared equipment was not properly cleaned.
• Residual alcohol-based cleaning agents
• Sample mix-ups
• Cross-sample carryover
• Contaminated vials or reagents

Contamination can lead to false positives or inflated BAC results, which may result in unjustified charges or wrongful convictions if the compromised data is used as evidence in court.

Alcohol vs Drug Blood Testing 

Blood testing is used for both alcohol and drugs, but the scientific issues differ.

Drug testing introduces additional complexities regarding metabolites and impairment, which differ from alcohol analysis.

Comparison: Blood vs Breath vs Urine Testing 

Blood tests are powerful—but also the most procedurally vulnerable.

How Blood Test Errors Affect a DWI Case

Blood test flaws can impact a case in three distinct ways:

1. Suppression – Results excluded entirely
2. Weight – Jury doubts accuracy or reliability
3. Negotiation leverage – Reduced charges or dismissal

Not every error results in suppression, but even minor issues can significantly weaken the prosecution’s case.

DWI Blood Sample Lifecycle 

Traffic stop

↓

Arrest for DWI

↓

Blood draw authorization

→ Warrant issued

→ OR consent / recognized exception

↓

Blood drawn by a qualified medical professional

↓

Blood was placed into preserved vials.

↓

Vials sealed and labeled.

↓

Temporary storage (refrigeration)

↓

Transport to the forensic laboratory

↓

Evidence intake and logging

↓

Chemical analysis (GC/MS or LC/MS)

↓

Analyst review and verification

↓

BAC or toxicology report generated

↓

Disclosure to prosecution and defense

Common failure points include invalid warrants, improper blood draws, broken seals, delayed refrigeration, documentation gaps, and laboratory errors.

Chain of Custody Flow (Evidence Integrity)

Blood drawn

↓

Vial labeled with subject information

↓

The initial custodian logs the sample.

↓

Stored in a secure evidence location

↓

Transferred to transport personnel

↓

Delivered to the crime laboratory

↓

Logged by evidence technician

↓

Assigned to an analyst

↓

Returned to storage or disposal

Chain-of-custody breaks include missing signatures, unlogged transfers, unexplained time gaps, or storage outside documented conditions.

GC/MS Alcohol Testing Workflow

Blood sample received

↓

Internal standard added

↓

Sample prepared for analysis.

↓

Injection into a gas chromatograph

↓

Chemical separation by retention time

↓

Mass spectrometry identification

↓

Peak integration

↓

Calibration curve applied

↓

BAC calculation

↓

Quality control review

GC/MS errors may result from improper calibration, carryover contamination, misidentified peaks, or failed quality controls.

Fermentation Risk Pathway

Blood draw

↓

Improper preservative ratio or mixing

↓

Delayed refrigeration or warm storage

↓

Microbial activity begins

↓

Alcohol forms after the blood draw.

↓

Artificially elevated BAC result

This explains how a scientifically valid GC/MS machine can still produce an unreliable BAC due to sample degradation.

Legal Impact of Blood Test Errors

Blood test problem identified.

↓

Legal analysis conducted

↓

One of three outcomes occurs:

• Constitutional violation → evidence suppressed
• Scientific unreliability → reduced evidentiary weight
• Procedural weakness → leverage for dismissal or reduction

Not all errors lead to suppression, but many significantly weaken the prosecution’s case.

Discovery Checklist: What Defense Attorneys Review

Practical challenges require raw data, not just summary reports.

Defense attorneys commonly request:

• Chromatograms
• Calibration and maintenance logs
• Batch worksheets
• Internal standard documentation
• Analyst training records
• Evidence intake and transfer logs
• Storage and temperature records

Without access to this data, the defense cannot independently confirm the accuracy or validity of the blood test results. Independent verification is essential to ensure that the procedures and findings are scientifically sound and free from procedural or analytical error.

At Michael & Associates, three of our attorneys have earned DWI forensic lawyer-scientist certification from the American Chemical Society.

They are:

Chris Denuna (Bar #24092338)

James Fletcher (Bar #24077619)

RC Pate (Bar #24052165)

Five more of our lawyers are currently enrolled in the training program. To date, just 55 lawyers in Texas have earned this certification.

Real-World Blood Test Failures

Independent audits and investigations have revealed blood-testing failures involving:

• Improper GC/MS calibration
• Contamination across samples
• Documentation breakdowns
• Technician misconduct

These cases demonstrate that blood testing is only as reliable as the system performing it.

What Should You Do If You Suspect Blood Test Errors?

1. Request a full lab report, including chromatograms and calibration logs.
2. Get an independent test of the retained blood sample, if available.
3. Consult with a DWI attorney experienced in forensic toxicology.
4. Document any medical conditions, such as diabetes or GERD, that may affect the results.

Bottom Line

Blood tests in DWI cases are potent tools, but they are not foolproof. Chain-of-custody violations, GC/MS errors, and laboratory contamination can undermine the reliability of results and potentially alter the outcome of a case. A qualified defense attorney who understands the science and legal standards involved can make all the difference.

FAQs About DWI Blood Test Issues

Why is the chain of custody critical in DWI blood testing?

The chain of custody documents the handling of a blood sample from the time it's drawn to when it is tested in a lab. If there are gaps, errors, or improper handling, the results may be deemed unreliable or inadmissible in court due to potential tampering or contamination.

Can lab contamination affect my DWI test results?

Yes, lab contamination—from unsterile equipment, sample mix-ups, or residual chemicals—can cause false positives or inflated BAC readings. Such errors can provide a strong basis for challenging the accuracy of the test in court.

What are common GC/MS errors in DWI cases?

GC/MS errors can include improper calibration, carryover contamination from prior tests, incorrect identification of compounds, and technician mistakes. These issues can lead to inaccurate BAC readings or false drug detections.

Can DWI blood test results be wrong?

Absolutely. Blood test results can be flawed due to errors in collection, storage, lab processing, or equipment handling. Defense attorneys frequently challenge the accuracy and admissibility of these results using expert analysis and procedural reviews.

How long is a DWI blood sample valid for testing?

The stability of a blood sample depends on how it's stored. Improper refrigeration or prolonged delays may lead to fermentation, which increases alcohol content and results in falsely elevated BAC readings. Generally, samples should be tested or preserved within 14 days under strict conditions.

Can I request a retest of my blood sample?

Yes, in most cases, your defense attorney can request independent testing of the retained portion of your blood sample (called the “split sample”). This can be a powerful strategy to verify—or challenge—the original lab’s findings.

Sources: Michael & Associates research, internal case data, American Chemical Society, Texas Code of Criminal Procedure, Texas Transportation Code, Texas DWI and DUI Glossary.

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