A DWI blood case can appear simple:
- Police obtained blood.
- The laboratory reported an alcohol concentration.
- Therefore, the case is proven.
But a blood result is actually the end product of a much longer chain.
Before a number appears on a laboratory report, police must have legal authority to obtain the blood, someone must collect it, the specimen must be identified and preserved, it must reach a laboratory, an analyst must test it, and the resulting data must be interpreted.
At Taylor Defense Firm, we may examine the entire process:
traffic stop → DWI investigation → arrest → request for specimen → warrant or other claimed legal authority → blood draw → collection tubes → labeling → storage → transportation → laboratory receipt → testing → quality-control data → reported result → interpretation.
A reported BAC is important evidence.
It is not the beginning of the analysis.
Why Do Police Obtain Blood in a DWI Case?
Texas DWI investigations may involve breath or blood evidence.
Blood testing is particularly common when:
- a driver refuses a requested specimen and police seek a warrant;
- a serious collision occurs;
- a person is transported to a hospital;
- alcohol and drugs are both suspected;
- circumstances make breath testing impractical; or
- investigators believe blood evidence is appropriate under the applicable law.
The legal authority for obtaining the specimen should be identified in every case.
Blood Is Different From Ordinary Physical Evidence
Taking blood involves physically penetrating a person's body to obtain biological evidence.
That implicates constitutional protections against unreasonable searches and seizures.
The fact that police are investigating DWI does not eliminate those protections.
Depending upon the circumstances, the State may rely upon:
- a search warrant,
- consent,
- or another legally recognized basis for obtaining the specimen.
The validity of that basis depends upon the actual facts and applicable law.
What Is a DWI Blood Warrant?
A blood warrant is a search warrant authorizing officers to obtain a biological specimen from a person for evidentiary testing.
An officer ordinarily submits a sworn affidavit to a magistrate setting out the facts the officer contends establish probable cause.
The magistrate determines whether the affidavit provides a lawful basis for issuance of the warrant.
The defense can examine both:
- the warrant
- and
- the affidavit supporting it.
What Should a Blood-Warrant Affidavit Show?
The affidavit should provide facts supporting probable cause for the search requested.
In a DWI investigation, that may include allegations concerning:
- driving behavior;
- a traffic violation;
- a collision;
- odor of alcohol;
- appearance;
- speech;
- balance;
- admissions;
- field sobriety testing;
- witness information;
- open containers;
- drug evidence;
- refusal of a requested specimen; or
- other circumstances known to the officer.
The legal question is not simply whether the affidavit uses the word "intoxicated."
The underlying facts matter.
The Affidavit Should Be Compared With the Video
Modern DWI investigations frequently generate body-camera or dash-camera footage.
That creates an opportunity to compare what the officer told the magistrate with what was actually recorded.
We may compare:
- claimed driving behavior with dash-camera footage;
- claimed physical symptoms with body-camera footage;
- claimed field sobriety performance with the recorded tests; and
- the affidavit's chronology with timestamps and other records.
Material differences deserve examination.
A Warrant Does Not End the Constitutional Analysis
The existence of a signed warrant does not mean every legal issue disappears.
Depending upon the case, questions may concern:
- whether probable cause supported issuance;
- whether material information in the affidavit was inaccurate;
- whether material facts were omitted under circumstances recognized by law;
- whether the warrant sufficiently identified the person and evidence sought;
- whether the warrant was properly executed;
- whether the specimen was obtained within lawful parameters; and
- whether another constitutional or statutory issue exists.
The validity of a warrant challenge depends upon the specific facts.
Consent Is a Different Legal Theory
In some cases, the State may contend that the driver voluntarily consented to providing blood.
Consent and a warrant are different legal bases for a search.
The defense may examine:
- What exactly did the officer ask?
- What did the driver say?
- Was the conversation recorded?
- Was consent actually given?
- Was it voluntary?
- Was it limited in any way?
The complete interaction matters.
Texas Has Statutes Addressing Required Specimens in Certain Circumstances
Texas law contains implied-consent and specimen provisions applicable to intoxication investigations.
Those statutes can be important.
But statutory authority and constitutional authority are related questions that should not simply be assumed to be identical in every situation.
Whether a warrant is required, whether an exception applies, and what remedies may exist depend upon the facts and governing law.
Who Draws the Blood Matters
Once legal authority exists, the specimen still has to be collected.
Texas law regulates the taking of blood specimens in intoxication investigations.
The defense may examine:
- who performed the draw;
- that person's qualifications;
- where the draw occurred;
- what collection equipment was used;
- how the site was prepared;
- how the tubes were handled;
- whether required information was recorded; and
- what happened to the specimen immediately afterward.
These facts become part of the evidentiary foundation.
The Blood Kit Creates a Chain of Evidence
A typical forensic blood collection may involve tubes, labels, seals, packaging, paperwork, and transportation.
From the moment the blood enters the tube, the defense can ask:
- Whose blood is this?
- How do we know?
- Who possessed it?
- Was it sealed?
- Where was it stored?
- When was it transferred?
- When did the laboratory receive it?
- Was there any unexplained gap?
This is the beginning of chain-of-custody analysis.
Chain of Custody Does Not Mean Saying "It Went to the Lab"
Biological evidence may pass through several hands and locations before testing.
Records can include:
- collection forms;
- evidence labels;
- property-room records;
- shipping information;
- laboratory intake records;
- accession numbers;
- storage records;
- analyst worksheets; and
- disposition records.
The defense should be able to trace the specimen from the person to the reported laboratory result.
What Does the Laboratory Actually Test?
Forensic alcohol laboratories commonly use analytical procedures designed to measure ethanol in a blood specimen.
The final laboratory report may contain only a small fraction of the information generated during testing.
The underlying materials may include:
- instrument data;
- chromatograms;
- calibration information;
- controls;
- standards;
- blanks;
- replicate testing;
- analyst notes;
- batch information;
- maintenance records; and
- quality-control documentation.
Those materials can help determine how the reported result was produced.
Headspace Gas Chromatography
Forensic laboratories commonly use headspace gas chromatography to analyze blood for ethanol.
The technique separates and measures volatile compounds in a sample.
The instrument produces data that an analyst evaluates under laboratory procedures before reporting a result.
The phrase "gas chromatography" can sound conclusive to a jury.
But scientific evidence still depends upon proper:
- sample identification,
- preparation,
- instrument operation,
- calibration,
- quality control,
- data review,
- and
- interpretation.
Quality Controls Matter
Laboratories use controls and other quality-assurance procedures to determine whether an analytical run performed acceptably.
Depending upon the laboratory and method, the defense may examine:
- Were required controls within acceptable limits?
- Were calibration requirements satisfied?
- Were samples tested consistently?
- Were any irregularities documented?
- Was testing repeated?
- Were instrument problems identified?
- Did the analyst follow the laboratory's procedure?
Scientific reliability is demonstrated through data and procedure, not simply by the existence of a final report.
Measurement Is Not Infinitely Precise
Laboratory measurements have limitations.
The reported number should be understood within the laboratory's validated analytical process and applicable uncertainty.
That becomes particularly important when a reported concentration is near a legally significant threshold.
A result should not be made more precise in argument than the underlying science supports.
Timing Can Be Critical
A blood sample measures alcohol concentration when the blood is collected.
But the legally relevant question in a DWI prosecution concerns intoxication while operating the motor vehicle.
Those times may not be the same.
Consider the chronology:
driving → stop → investigation → field sobriety tests → arrest → transport → warrant process → blood draw.
That sequence can take substantial time.
The laboratory result therefore must be considered together with the timeline.
Alcohol Concentration Changes Over Time
Alcohol is absorbed, distributed, and eventually eliminated from the body.
Depending upon drinking history and timing, a person's alcohol concentration may have been:
- rising,
- near a peak,
- or
- falling
during the relevant period.
A later blood result does not by itself establish the precise concentration at every earlier moment.
What Is Retrograde Extrapolation?
In some DWI cases, a party may attempt to estimate an earlier alcohol concentration from a later test result.
This is often called retrograde extrapolation.
The reliability of such an estimate can depend upon the information available concerning matters such as:
- time of driving;
- time of the blood draw;
- drinking pattern;
- time of last drink;
- amount consumed;
- food consumption;
- absorption status; and
- other relevant physiological assumptions.
A mathematical calculation is only as reliable as the information and assumptions supporting it.
The .15 Enhancement Uses the Time of Analysis
Texas DWI law contains an important distinction concerning the 0.15 enhancement.
Under Texas Penal Code §49.04, the offense can be elevated when an analysis of the person's blood, breath, or urine shows an alcohol concentration of 0.15 or more at the time the analysis was performed.
That statutory language matters.
The enhancement question is not phrased identically to the basic question of whether the person was intoxicated while operating.
What If the Blood Result Is Below .08?
A result below .08 does not automatically end a DWI prosecution.
Texas also permits the State to attempt to prove intoxication through loss of normal mental or physical faculties.
The prosecution may therefore rely upon other evidence, including:
- driving;
- field sobriety testing;
- officer observations;
- statements;
- drugs;
- combinations of substances; and
- other evidence.
Likewise, the defense may use a lower result together with video and other evidence to challenge the State's theory.
Drug Testing Raises Different Questions
Blood may also be tested for drugs.
A toxicology report may identify prescription medication, controlled substances, THC-related compounds, or other substances.
The presence of a drug does not automatically establish that the person lacked normal use of mental or physical faculties while driving.
Depending upon the substance, relevant questions can include:
- Was the compound active or inactive?
- What concentration was detected?
- When was it likely consumed?
- What effects are associated with that concentration, if scientifically supportable?
- Were multiple substances present?
- Was alcohol also present?
- What did the person actually look and act like?
Toxicology should be connected to the legal issue of intoxication rather than treated as guilt merely because a drug name appears on a report.
Hospital Blood and Forensic Blood Are Not Necessarily the Same Evidence
After a collision, a hospital may obtain blood for medical purposes.
Police may separately obtain a forensic specimen.
Those samples can involve different:
- purposes;
- collection procedures;
- testing methods;
- records;
- units;
- legal issues; and
- evidentiary foundations.
A hospital laboratory result should not automatically be treated as though it were generated through the same procedure as a law-enforcement forensic blood test.
Blood Evidence Can Be Strong—and Still Deserves Examination
The purpose of examining blood evidence is not to assume the laboratory is wrong.
Sometimes the laboratory evidence is exceptionally strong.
The purpose is to understand exactly what it proves.
A defense lawyer should know:
- where the specimen came from,
- why police were legally permitted to obtain it,
- how it was collected,
- how it reached the laboratory,
- how it was tested,
- what the underlying data show,
- and
- how the result relates to the time of driving.
What Punishment Can a DWI Blood Case Carry?
Blood evidence does not create a separate DWI punishment range.
Punishment depends upon the offense charged and applicable enhancements.
A basic first DWI is generally a Class B misdemeanor, with:
- 72 hours to 180 days in county jail; and
- a possible fine up to $2,000.
A qualifying 0.15 or greater allegation can elevate the offense to a Class A misdemeanor, generally carrying:
- up to one year in county jail; and
- a possible fine up to $4,000.
Qualifying prior intoxication convictions can further increase punishment, including felony exposure.
If intoxication allegedly causes serious bodily injury or death, the prosecution may involve Intoxication Assault or Intoxication Manslaughter and substantially greater punishment.
Build the Blood Case From the Warrant to the Number
At Taylor Defense Firm, we may reconstruct blood evidence as:
traffic stop → observations → field sobriety investigation → arrest → specimen request → warrant/consent/other claimed authority → blood draw → identification → sealing → storage → transportation → laboratory accession → preparation → instrument testing → controls → data review → reported result → interpretation at time of driving.
We then ask:
- Was the stop lawful?
- What established probable cause?
- What did the warrant affidavit actually say?
- Does the video support it?
- Was the blood obtained under lawful authority?
- Who drew it?
- Can the specimen be traced through the chain of custody?
- What testing method was used?
- Did the analytical data satisfy the laboratory's requirements?
- What limitations or uncertainty exist?
- How long after driving was the specimen collected?
- What does the result actually establish about intoxication while operating?
- And does the chemical evidence, viewed with all the other evidence, prove the charged DWI beyond a reasonable doubt?
Blood evidence can be among the strongest evidence in a Texas DWI prosecution.
That is precisely why the warrant, collection, chain of custody, laboratory data, and interpretation should be understood rather than reducing the entire case to one number printed on a report.