How Can You Challenge Evidence in a DWI Case?
Sept. 22, 2025
Facing a DWI charge can be one of the most stressful and uncertain experiences in a person’s life. The fear of potential penalties, loss of driving privileges, and impact on personal reputation can be overwhelming. Many individuals in this position feel like the odds are stacked against them. This is where the right representation can make a meaningful difference.
In St. Joseph, Missouri, Johnson Law Firm helps clients evaluate the evidence in their case and identify possible weaknesses in the prosecution’s argument. Those facing DWI charges shouldn’t assume that all evidence is airtight. There may be legal grounds to question its reliability and validity.
If you’re in St. Joseph or surrounding areas like Country Club, Platte City, Plattsburg, Stewartsville, Savannah, Cameron, Lathrop, Maryville, or anywhere in Northwest Missouri, reach out to us today to discuss your situation and start building your criminal defense.
Common Types of Evidence in DWI Cases
When someone is charged with a DWI, the prosecution often relies on specific types of evidence to support its case. Understanding these categories can help identify where potential challenges might arise.
Typical forms of DWI evidence include:
Breath test results: Measurements of blood alcohol concentration (BAC) using a breathalyzer device.
Blood test results: Laboratory analysis of a person’s blood to determine alcohol or drug content.
Field sobriety tests: Physical coordination and balance tests conducted roadside by an officer.
Officer observations: Statements about the driver’s appearance, behavior, speech, and odor of alcohol.
Video or audio recordings: Footage from dashcams, bodycams, or police station interview rooms.
Witness testimony: Statements from passengers, bystanders, or other officers.
Each type of evidence has its strengths and weaknesses. A successful criminal defense often focuses on identifying and exposing any issues with the collection, handling, or interpretation of these materials before they’re used against the accused.
Challenging Breath Test Accuracy
Breath test results are among the most common pieces of evidence in DWI cases, but they’re not infallible. Various factors can lead to inaccurate readings, and these issues can be raised in court.
Key points that may be questioned include:
Device calibration: Whether the breathalyzer was properly maintained and calibrated at the required intervals.
Operator training: Whether the officer administering the test had proper training and certification.
Medical conditions: Whether health issues such as acid reflux or diabetes may have impacted results.
Environmental factors: Whether nearby substances, temperature changes, or electronic interference affected the device.
If the breath test results can be shown to be unreliable, this could significantly weaken the prosecution’s case. In many situations, demonstrating a lack of reliability may lead to reduced charges or even dismissal.
Questioning Blood Test Procedures
Blood tests are often perceived as more accurate than breath tests, but they’re not immune to error. In fact, the process of collecting, storing, and analyzing blood samples is subject to strict protocols.
Potential grounds for challenging blood test evidence include:
Chain of custody issues: Whether the sample was handled by authorized personnel at all times.
Contamination risks: Whether improper storage or handling led to sample degradation.
Testing methodology: Whether the laboratory followed proper scientific standards.
Lab technician qualifications: Whether the person conducting the test was properly certified.
Raising doubts about the validity of a blood test can be an effective strategy for an experienced criminal defense lawyer in DWI cases, especially when supported by testimony or documented procedural errors.
Field Sobriety Test Limitations
Field sobriety tests, such as the walk-and-turn or one-leg stand, are often used by officers to establish probable cause for an arrest. While these tests are standardized, they’re still subjective and can be influenced by many factors.
Reasons these tests may be challenged include:
Environmental conditions: Uneven pavement, poor lighting, or bad weather affecting performance.
Physical limitations: Pre-existing medical conditions, injuries, or age-related factors impacting balance.
Officer instructions: Lack of clarity or improper explanation of how to perform the tests.
Subjective interpretation: Differences in how officers assess the same performance.
Challenging field sobriety tests can be especially effective if video evidence contradicts the officer’s written report.
Disputing Officer Observations
An officer’s personal observations can play a major role in building probable cause for a DWI arrest. However, human perception is imperfect, and memories can be influenced by bias or assumptions.
Common issues with officer observations include:
Misinterpretation of symptoms: Confusing fatigue, illness, or allergies with signs of intoxication.
Limited vantage points: Inability to clearly observe behavior or driving patterns.
Inconsistent statements: Differences between initial reports and later testimony.
Influence of preconceived notions: Letting assumptions affect the interpretation of behavior.
When credibility issues arise, it can cast doubt on the prosecution’s narrative and bolster the defense’s position.
Reviewing Video and Audio Evidence
Video or audio recordings can be powerful in a DWI case—either supporting the prosecution’s claims or helping the defense. Reviewing this evidence closely is critical.
Key factors to examine include:
Completeness of footage: Whether any portions of the recording are missing or unclear.
Consistency with reports: Whether the video matches the officer’s written statements.
Audio clarity: Whether spoken words are clearly heard and understood.
Potential tampering: Whether there’s any indication of edits or alterations.
If video evidence contradicts the official account, it can be a persuasive tool for the criminal defense.
Challenging Witness Testimony
Witnesses, such as passengers or bystanders, may provide testimony that supports the prosecution’s version of events. But these statements aren’t immune to scrutiny.
Factors that may weaken witness testimony include:
Inconsistent recollections: Differences between earlier statements and courtroom testimony.
Bias or motive: Personal relationships, grudges, or incentives to testify.
Limited perception: Lack of a clear vantage point to observe the incident.
Memory reliability: The time elapsed between the incident and the testimony affects accuracy.
By highlighting weaknesses in witness credibility, the defense can shift the focus away from unverified claims.
Exploring Constitutional Violations
If law enforcement officers violated a defendant’s constitutional rights during the stop, arrest, or evidence collection, this could lead to suppression of evidence.
Possible violations include:
Unlawful traffic stops: Pulling over a vehicle without reasonable suspicion.
Illegal searches: Conducting a search without probable cause or a warrant.
Failure to read Miranda rights: Not advising the defendant of their right to remain silent.
Prolonged detention: Holding someone for an unreasonable length of time before arrest.
Constitutional arguments can be among the strongest defenses in a DWI case, as they may render critical evidence inadmissible.
Building a Comprehensive Criminal Defense Strategy
Challenging evidence in a DWI case isn’t about focusing on just one detail. It often involves a combination of approaches, carefully tailored to the specific circumstances.
A thorough criminal defense strategy may include:
Evaluating all available evidence: Reviewing every piece of documentation, test result, and recording.
Consulting qualified witnesses: Bringing in professionals to question scientific methods.
Filing pretrial motions: Requesting that questionable evidence be excluded from trial.
Preparing for cross-examination: Anticipating the prosecution’s questions and responses.
When these methods are combined effectively, they can create reasonable doubt about the prosecution’s case.
Contact an Experienced Lawyer
Those charged with a DWI don’t have to face the process alone. Johnson Law Firm has experience helping in these situations, offering tailored criminal defense strategies. Call today to discuss your case and start building your defense. Located in St. Joseph, Missouri, Johnson Law Firm serves clients in Country Club, Platte City, Plattsburg, Stewartsville, Savannah, Cameron, Lathrop, Maryville, or anywhere in Northwest Missouri.