This glossary serves as a central reference for key terms used in DWI and DUI blood and breath testing.
Blood Alcohol Concentration (BAC)
The amount of alcohol in a person’s bloodstream, expressed as a percentage. In DWI cases, BAC is typically measured through breath or blood testing.
Calibration Curve
A reference graph created using known concentrations to calculate the BAC of an unknown sample. Calibration errors can invalidate results.
Carryover Contamination
Residual material from a previous sample that affects the next analysis, potentially causing falsely elevated readings.
Chain of Custody
The documented process tracking who handled a blood sample, when, and under what conditions. Breaks in chain of custody can undermine evidence reliability or admissibility.
Evidentiary Weight
The degree of credibility or persuasiveness a piece of evidence carries before a judge or jury, even if technically admissible.
Fermentation
The post-collection production of alcohol in a blood sample is often caused by improper preservation or storage.
A laboratory technique that separates chemical components in a blood sample based on how they move through a column when vaporized.
GC/MS
A combined analytical method using gas chromatography for separation and mass spectrometry for identification. It is the primary method used in forensic blood alcohol testing.
Horizontal Gaze Nystagmus (HGN)
A standardized eye test used in DWI investigations to observe involuntary jerking of the eyes as they track a moving stimulus. Officers are trained to look for six specific “clues” across both eyes: lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus before 45 degrees. HGN is considered the most scientifically supported SFST, but its reliability depends entirely on proper administration, correct timing, accurate positioning, and exclusion of medical or neurological causes unrelated to alcohol.
Internal Standard
A known substance added to a blood sample during testing to ensure accurate measurement and compensate for variations during analysis.
Laboratory Accreditation
Certification indicating a lab meets certain quality standards. Accreditation does not guarantee error-free testing.
Mass Spectrometry (MS)
A method used to identify substances by measuring their mass-to-charge ratio. In DWI testing, MS confirms the identity of alcohol or drugs detected by GC.
A substance produced when the body breaks down drugs or alcohol. Presence does not always equate to impairment.
One-Leg Stand Test
A balance and divided-attention test requiring the subject to stand on one leg while counting aloud for approximately 30 seconds. Officers are trained to observe four standardized clues: swaying, using arms for balance, hopping, and putting the foot down. This test is particularly sensitive to non-alcohol-related factors such as weight, age, leg or back injuries, footwear, and environmental conditions, all of which can impact performance regardless of intoxication.
Preservative (Sodium Fluoride)
A chemical added to blood vials to prevent clotting and fermentation. Incorrect amounts or poor mixing reduce effectiveness.
Quality Control (QC)
Procedures used by labs to verify instrument accuracy and reliability during testing.
Retrograde Extrapolation
A scientific method used to estimate BAC at an earlier time based on a later test result. It relies on assumptions and is often disputed.
Split Sample
A portion of the blood sample that's retained for independent testing by the defense. Split samples are critical for verification.
Suppression
A legal remedy excluding evidence obtained or processed in violation of constitutional or statutory requirements.
Toxicology Report
This official document summarizes test results, methods used, and conclusions drawn by the lab.
Walk-and-Turn Test
A divided-attention test designed to assess a person’s ability to follow instructions while performing physical tasks. The subject is instructed to take nine heel-to-toe steps along a straight line, turn in a prescribed manner, and return with nine additional steps. Officers are trained to score eight standardized clues, including loss of balance, stepping off the line, improper turn, and incorrect step count. Performance can be affected by footwear, surface conditions, injuries, fatigue, age, or anxiety, which may reduce the test’s reliability.
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DWI Science Glossary FAQs
What does BAC mean in a DWI case?
BAC, or Blood Alcohol Concentration, is the percentage of alcohol in a person’s blood. In DWI cases, BAC is measured using breath or blood tests to estimate intoxication at the time of driving.
Is a blood test always more accurate than a breath test?
Not always. Blood tests involve more steps than breath testing, including collection, storage, transport, and lab analysis. Each of these steps introduces potential errors that can affect reliability and admissibility.
What is GC/MS, and why is it used in DWI blood testing?
GC/MS stands for gas chromatography–mass spectrometry. It is used to separate substances in a blood sample and then identify them, making it the primary method for forensic blood alcohol analysis.
What is chain of custody and why does it matter?
Chain of custody documents who handled a blood sample and when. Breaks in the chain can undermine the reliability of the evidence and may support suppression or reduced evidentiary weight.
Can blood samples produce alcohol after collection?
Yes. Improper storage or preservation can cause fermentation, which may produce alcohol inside the vial and falsely elevate BAC results.
What is sodium fluoride and why is it important?
Sodium fluoride is a preservative added to blood vials to prevent fermentation. Incorrect amounts or poor mixing reduce its effectiveness and can compromise test results.
What is carryover contamination in forensic testing?
Carryover contamination occurs when residue from a previous sample affects the next test, potentially causing falsely elevated or inaccurate readings.
What does “split sample” mean in a DWI case?
A split sample is a portion of the blood sample preserved for independent testing by the defense. It allows verification of the State’s results.
What is retrograde extrapolation?
Retrograde extrapolation is a scientific method used to estimate a person’s BAC at an earlier time based on a later test result. It relies on assumptions and is frequently challenged.
Does lab accreditation guarantee accurate results?
No. Accreditation indicates a lab meets minimum standards, but it does not prevent errors in collection, analysis, or interpretation.
What is evidentiary weight?
Evidentiary weight refers to how persuasive or credible evidence is to a judge or jury, even if the evidence is technically admissible.
Can DWI scientific evidence be suppressed?
Yes. Evidence may be suppressed if it was obtained or processed in violation of constitutional, statutory, or scientific reliability requirements.
What is an HGN test?
The Horizontal Gaze Nystagmus (HGN) test observes involuntary eye movements during a roadside evaluation. Its reliability depends on strict adherence to standardized procedures.
Are field sobriety tests scientific?
Field sobriety tests are standardized observational tools, not medical tests. Their reliability depends on proper administration and the exclusion of non-alcohol-related factors.
Sources: Michael & Associates research, DWI and DUI Glossary, American Chemical Society, National Highway Traffic Safety Administration (NHTSA) DWI Detection and SFST manuals, forensic toxicology textbooks, the Texas Forensic Science Commission, Texas statutory law, and Texas Court of Criminal Appeals.
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