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The Science of DWI Defense

Breath Testing

The Intoxilyzer 9000 is the breath testing instrument used in Texas. It measures alcohol in deep lung air and converts that measurement to an estimated blood alcohol concentration using a fixed partition ratio of 2100:1. The actual partition ratio varies across the population from approximately 1100:1 to 3400:1. A driver with a partition ratio higher than assumed will have a lower actual BAC than the instrument reports. The instrument’s calibration records, the 15-minute observation period compliance, and the presence of any interfering substances are all subject to independent examination.

Blood Testing

A blood draw provides a direct measurement but is only as reliable as the collection, storage, and analysis procedures that produced it. In vitro fermentation (the conversion of residual blood sugars to alcohol by bacteria after collection) can occur when specimens are improperly stored or when the sodium fluoride preservative is inadequate. GC-FID is the standard analytical methodology for blood alcohol. The raw laboratory data (the chromatogram, calibration curve, internal standard performance, and quality control records) is what the defense reviews, not just the final reported number.

Field Sobriety Tests

The three standardized field sobriety tests (HGN, Walk and Turn, and One Leg Stand) were validated by NHTSA under controlled conditions with trained officers. Their accuracy claims apply only when the tests are administered exactly as specified in the NHTSA manual, in comparable conditions, to subjects from the studied population. HGN has more than 47 documented causes other than alcohol. Physical conditions, age, weight, inner ear disorders, neurological conditions, medications, environmental factors, and protocol deviations all affect reliability. Deandra Grant is a certified SFST instructor who administers and grades these tests professionally.

Retrograde Extrapolation

When a blood or breath test is administered some time after the traffic stop, the prosecution may use retrograde extrapolation to project the BAC backward to the time of driving. This requires assumptions about absorption rate, elimination rate, and whether the driver was in the absorption or elimination phase at the time of the stop. Those assumptions are variable across individuals and are subject to challenge at the pharmacokinetic level.

The ALR Deadline: 15 Days

When a Texas driver is arrested for DWI and fails or refuses a breath or blood test, DPS initiates an Administrative License Revocation. The driver receives a notice of suspension and has 15 days from the date of receiving that notice to request an ALR hearing. Missing this deadline means the suspension takes effect automatically with no opportunity to contest it.

Requesting the ALR hearing delays the suspension. The arresting officer must appear and testify under oath. This provides the defense an early opportunity for sworn cross-examination and fact development before the criminal case is resolved. Every DWI defendant who received a notice of suspension should request the ALR hearing without exception.

DWI Practice Areas

Each of the following has dedicated page coverage with detailed analysis of the specific legal issues and forensic challenges involved:

• Boating While Intoxicated (BWI) — the sea legs phenomenon, NHTSA validation gap for marine environments, TPWD enforcement
• DWI with a CDL — 0.04% threshold, partition ratio challenge at near-threshold BAC, no occupational CDL, personal vehicle DWI triggers disqualification
• DWI with Injury (Intoxication Assault) — serious bodily injury definition, causation element, EDR evidence
• Felony DWI — DWI 3rd (2–10 years), pen packet challenges, §49.09(b) peace officer enhancement (first-degree felony), intoxication manslaughter, Bentley’s Law
• Hit and Run DWI — failure to stop and render aid, evidence preservation
• Multiple DWI — enhancement ladder, prior conviction proof requirements, mandatory minimums
• Underage DWI — §106.041 TABC Class C DUI vs. §49.04 DWI distinction, SFST validation gap for minors
• Driving Under the Influence of Drugs (DUID) — presence vs. impairment, LC-MS/MS biological specimen analysis, DRE protocol

Why Deandra Grant Law

• ACS-CHAL Forensic Lawyer-Scientist.  Both Deandra Grant and Douglas Huff. Demonstrated have obtained the designation.
• Master’s Degree in Pharmaceutical Science.  Blood alcohol pharmacokinetics, retrograde extrapolation, and GC-FID methodology.
• Graduate Certificate in Forensic Toxicology.  Forensic evidence interpretation applied to every case.
• Trained SFST Instructor.  
• 30+ years of DWI defense across North and Central Texas.  500+ trials to verdict.
• 17 published law books. Including The Texas DWI Manual.
• Texas Super Lawyer since 2011.  AV® Preeminent rated by Martindale-Hubbell®.
• Offices in Dallas, Fort Worth, Allen, Denton, Waco, and Rockwall.

If you or someone you know has been arrested for DWI in Texas, call (214) 225-7117 for a free, confidential consultation. If you received a notice of suspension, the 15-day ALR deadline is already running. Or schedule online at texasdwisite.com.