Challenging DUI Evidence: Field Sobriety Tests, Breathalyzers, and Blood Tests

Introduction to DUI Testing Challenges

The prosecution’s case in a DUI or DWI charge often relies heavily on chemical test results and field observations. However, these tests are far from infallible. A good DUI defense lawyer in Mississippi knows how to challenge testing methods and results. This can help get evidence thrown out and cases dismissed.

Whether you’re facing charges based on a failed breath test, questionable field sobriety performance, or blood test results, understanding how to challenge this evidence is essential. A breathalyzer defense lawyer or field sobriety test defense specialist can identify numerous procedural and technical problems that undermine the prosecution’s evidence.

Field Sobriety Tests: Understanding and Challenging Them

What Are Field Sobriety Tests?

Field sobriety tests are roadside exercises that law enforcement uses to evaluate driver impairment. The three standardized tests are:

Horizontal Gaze Nystagmus (HGN) Test:

• Officer observes involuntary eye jerking as you follow a moving object
• Supposed to detect nystagmus (eye jerking) associated with alcohol impairment
• Highly subjective and affected by many non-alcohol-related factors

Walk-and-Turn (WAT) Test:

• You walk heel-to-toe along a line, turn, and return
• Evaluates balance, coordination, and ability to follow instructions
• Often administered on uneven road surfaces under poor lighting

One-Leg Stand (OLS) Test:

• Stand on one leg for 30 seconds without swaying or putting foot down
• Requires balance and coordination
• Affected by road conditions, fatigue, medical conditions, and nervousness

Are Field Sobriety Tests Mandatory?

Field sobriety tests are not mandatory in Mississippi. You have the right to refuse them. This is an important point to remember when defending against these tests.

These tests are voluntary, though officers often don’t clearly communicate this fact. You have the legal right to politely decline.

Challenging Field Sobriety Test Results

A DUI defense attorney will challenge field sobriety tests by highlighting numerous factors that affect performance:

Environmental Factors:

• Uneven or slippery road surfaces
• Poor lighting conditions
• Weather (rain, wind, extreme heat or cold)
• Passing traffic creating distraction and vibrations
• Officer positioning obstructing your view

Medical and Physical Conditions:

• Inner ear problems or vertigo
• Injuries or arthritis affecting balance
• Neurological conditions
• Fatigue or drowsiness
• Medications affecting balance or coordination
• Age-related balance difficulties

Procedural Errors:

• Improper instructions or demonstration
• Failure to administer standardized procedure
• Officer bias or predetermined conclusion
• Improper scoring or interpretation
• Video recording unavailability to verify performance

Officer Training and Certification:

• Insufficient training in proper administration
• Outdated certification or training
• Failure to follow National Highway Traffic Safety Administration (NHTSA) guidelines
• Officer bias and expectancy effects

Why Field Sobriety Tests Are Unreliable

Research demonstrates that field sobriety tests are not accurate measures of impairment:

• HGN test accuracy is estimated at 77-88% even under ideal conditions
• WAT and OLS tests show similar margins of error
• Non-impaired individuals often fail these tests
• Nervousness, embarrassment, and stress cause poor performance
• The tests do not measure actual impairment levels or BAC

The Right to Refuse Field Sobriety Tests

One of the strongest DUI defense strategy elements is exercising your right to refuse field sobriety tests. While refusal can serve as circumstantial evidence, refusing removes the physical evidence the prosecution can use against you.

Breathalyzer Tests: Understanding How They Work and Challenging Results

What Is a Breathalyzer Test?

A breathalyzer is a machine that measures alcohol in your breath to estimate blood alcohol concentration (BAC). The device analyzes breath samples to calculate your BAC.

Problems with Breathalyzer Testing

A breathalyzer defense lawyer will challenge these devices on multiple grounds:

Calibration and Maintenance Issues:

• Machines must be calibrated regularly using certified reference gases
• Missing or incomplete calibration records can be fatal to prosecution evidence
• Out-of-tolerance calibrations mean unreliable readings
• Many machines are never properly maintained

Operator Errors:

• Officers often lack proper training in breathalyzer administration
• Improper breath sample collection procedures
• Incorrect machine operation
• Failure to observe the required observation period before testing

Physiological Factors Affecting Results:

• Rising BAC Defense: Your BAC continues rising after you stop drinking. If you’re tested soon after driving, your BAC at the time of driving may have been below the legal limit
• GERD (acid reflux) can cause artificially elevated readings
• Breath temperatures affect readings
• Mouth alcohol from mouthwash, cough syrup, or dental work elevates results
• Certain foods (fermented foods, bread, energy bars) can cause false positives

Device Limitations:

• Breathalyzers estimate BAC based on breath samples—not direct blood measurement
• Individual variations in breath-to-blood alcohol ratios mean results may overestimate BAC
• Machines have known reliability issues and are subject to manufacturer recalls
• The partition ratio (the relationship between blood and breath alcohol) varies significantly among individuals

The Chain of Custody Problem

Blood test DUI cases often involve chain of custody issues:

• Blood samples must be properly collected, labeled, stored, and transported
• Any break in this chain can render results unreliable or inadmissible
• Improper collection procedures (using wrong tubes, inadequate preservative)
• Storage temperature problems
• Lab documentation gaps
• Cross-contamination possibilities

Challenging Breath Test Results

Specific challenges to breath test evidence include:

Request Calibration Records:

• Courts have ruled that missing calibration records can result in breath test suppression
• Request all maintenance logs and service records
• Identify any periods where the device was out of service
• Look for calibration failures or adjustments

Timing and Administration:

• Officers must observe you for 15-20 minutes before testing (no eating, drinking, smoking, or vomiting)
• Failure to follow this protocol means results are unreliable
• Officers must administer two breath samples
• Results must be within 0.02% of each other or test is invalid

Medical Conditions and Medications:

• GERD, diabetes, and other conditions can produce false positives
• Many medications affect breath test results
• Certain religious or medical practices may affect readings

Mouth Alcohol:

• Mouthwash, cough syrup, breath sprays, and dental work cause inflated readings
• Burping or regurgitation within 20 minutes before test elevates results artificially

Blood Tests and Blood Test Challenges

Blood test DUI lawyers represent drivers charged based on blood test results. While blood tests are generally more accurate than breathalyzers, they’re still subject to serious challenges.

Probable Cause for Blood Draw:

• Officer must have probable cause to request blood testing
• Warrant is typically required for blood draws (with limited exceptions)
• Challenge whether probable cause existed

Improper Blood Collection:

• Wrong collection tubes or improper labeling
• Insufficient preservative or anticoagulant
• Collection by untrained phlebotomist
• Improper handling after collection

Lab Analysis Problems:

• Chain of custody breaks
• Lab contamination or cross-contamination
• Improper testing procedures
• Lab technician certification or training issues
• Lack of quality control documentation

Blood Test Accuracy Issues:

• Blood samples can ferment, creating false elevated readings
• Improper storage temperatures
• Sample degradation over time
• Lab equipment maintenance and calibration problems

Suppressing Chemical Test Evidence

The most powerful DUI defense often involves filing a motion to suppress evidence:

• If the stop was unlawful, all subsequent evidence can be suppressed
• If the arrest was made without probable cause, evidence is tainted
• If chemical testing violated your constitutional rights, results are inadmissible
• If testing procedures violated your implied consent rights, evidence may be excluded

Challenging Test Accuracy Through Expert Testimony

An experienced DUI defense attorney in Mississippi will retain forensic experts to:

• Challenge the scientific reliability of testing methods
• Identify calibration or maintenance issues
• Testify about rising BAC defenses
• Explain physiological factors affecting results
• Demonstrate operator errors or training deficiencies

The Margin of Error Problem

All breath and blood tests have margins of error:

• Breath tests typically have ±0.01-0.05% margin of error
• Blood tests have smaller but still significant margins
• If your BAC was 0.08% and the margin of error is 0.01%, you could have actually been at 0.07%
• The prosecution must prove guilt “beyond a reasonable doubt”—margins of error create that doubt

Refusing Chemical Testing: Implied Consent Implications

DUI refusal attorneys in Mississippi handles cases where drivers refused breath or blood tests:

• Refusal results in automatic license suspension (90 days for first offense, 1 year if prior convictions)
• Refusal can be used as evidence of consciousness of guilt at trial
• However, without test results, prosecution’s case is significantly weaker
• You can challenge the legality of the refusal demand itself

Implications of Test Refusal

• Administrative consequences: License suspension and ignition interlock requirements
• Criminal consequences: Refusal admissible as consciousness of guilt
• Strategic benefit: No chemical evidence for prosecution to present
• Your right: You have the right to speak with an attorney before deciding whether to submit

Conclusion

Chemical tests and field sobriety tests are central to DUI prosecutions, but they’re far from foolproof. A Mississippi DUI defense attorney can use strong challenge strategies and technical skills.

They can investigate thoroughly to suppress or discredit evidence. This can lead to case dismissal or lower charges. Don’t assume failed tests guarantee conviction—contact a DUI specialist immediately. Call us at (601) 948-4444 and set up a free initial consultation.