Driving Under the Influence (DUI) charges can be life-altering. One of the most critical pieces of evidence often used against defendants in DUI cases is the blood alcohol concentration (BAC) test—typically administered through a breathalyzer or a blood test. DUI Blood tests are often viewed as more accurate than breath tests, but they are far from infallible.
If you’ve been charged with a DUI based on a blood test, challenging the results may be a crucial component of your defense strategy. Understanding how blood tests work, the potential for error, and the legal processes involved can help you or your attorney mount an effective defense.
What Is a DUI Blood Test?
A DUI blood test is a chemical analysis performed to measure the concentration of alcohol or drugs in a person’s bloodstream. After a traffic stop or arrest, law enforcement may require you to submit to chemical testing, especially if:
You refuse a breathalyzer or perform poorly on field sobriety tests.
You are suspected of being under the influence of drugs (DUID).
You are involved in an accident involving injury or death.
Blood is typically drawn at a hospital or medical facility and analyzed in a forensic laboratory.
Common Legal BAC Limits
Blood Alcohol Concentration (BAC) is measured as a percentage. Legal limits vary by jurisdiction, but the most common thresholds in the U.S. are:
0.08% for drivers over 21 (standard limit)
0.04% for commercial drivers
0.01% or 0.02% for underage drivers
If your BAC exceeds these limits, you may be presumed legally impaired—even if you don’t exhibit signs of intoxication.
How Blood Tests Can Be Challenged
Despite their reputation for accuracy, blood tests are not foolproof. Several scientific, procedural, and legal factors can lead to false positives or inflated BAC results. Below are the most common grounds on which DUI blood test results can be challenged:
1. Improper Blood Draw Procedures
Errors during the blood draw itself can compromise the sample:
Use of alcohol swabs before puncturing the skin can contaminate the sample and artificially elevate BAC.
Incorrect labeling can lead to mix-ups between different individuals’ samples.
Non-sterile equipment can introduce contaminants.
Your attorney may request the chain of custody documentation and medical protocols to identify any irregularities.
2. Storage and Handling Issues
Blood must be properly stored and refrigerated to prevent fermentation, which can cause alcohol to form in the sample, leading to a higher BAC. Problems include:
Delayed testing: If too much time passes before analysis, the sample may degrade.
Improper sealing: Containers must be airtight to prevent contamination.
Temperature fluctuations: Blood samples must be refrigerated. Exposure to warm temperatures can foster bacterial growth and fermentation.
These factors can compromise test integrity, making the results unreliable in court.
3. Laboratory Errors and Contamination
Lab technicians are human, and forensic labs are not immune to mistakes:
Calibration errors in analytical instruments like gas chromatographs.
Cross-contamination between samples during testing.
Clerical mistakes during data entry or reporting.
Defense attorneys often hire independent toxicologists to review lab procedures and results to identify any discrepancies or violations of standard protocol.
4. Fermentation and Blood Alcohol Formation
Blood samples contain sugars and microbes that can produce ethanol (alcohol) naturally if the sample is not properly preserved. This process, known as fermentation, can cause:
Artificially high BAC results
Presence of alcohol in the sample even if none was in the bloodstream at the time of the test
Adding a preservative (like sodium fluoride) and anticoagulant (like potassium oxalate) to the vial is standard practice. If these chemicals are missing or improperly mixed, fermentation becomes a real concern.
5. Timing of the Blood Draw
Alcohol levels change over time in the body. A test taken 30 minutes to 2 hours after driving may not accurately reflect your BAC at the time of the alleged offense. This is known as the “rising blood alcohol” defense.
For example, if you had several drinks just before driving, your BAC may have been under the legal limit while driving but exceeded it by the time your blood was drawn.
An experienced DUI attorney may bring in an expert witness to retrograde extrapolate your BAC to the time of driving.
6. Lack of a Proper Warrant
Under the Fourth Amendment, you are protected against unreasonable searches and seizures. In most DUI cases, police must obtain a warrant before drawing your blood, unless:
You give voluntary consent, or
There are exigent circumstances
If your blood was taken without a warrant and without valid consent, your defense lawyer can file a motion to suppress the blood test results. If successful, the results could be excluded from evidence, weakening the prosecution’s case.
Medical Conditions That Can Affect Results
Certain health conditions and medications can interfere with blood alcohol readings or mimic signs of intoxication:
Diabetes: Can cause high acetone levels, sometimes misread as alcohol.
Gastroesophageal Reflux Disease (GERD): Can affect breathalyzer results more than blood tests but still relevant.
If you have any of these conditions, your attorney can obtain medical records and expert testimony to support your case.
Requesting a Retest of the Blood Sample
In most jurisdictions, you have the right to independent testing of your blood sample. Your attorney can request that an independent lab re-analyze the sample to:
Confirm or dispute the BAC reported by the prosecution’s lab
Identify contamination or handling issues
Establish grounds for a motion to dismiss or reduce charges
Retesting can be a powerful tool, especially if the original results are close to the legal limit.
Legal Strategies for Challenging Blood Tests
Here are some key legal tactics used to challenge DUI blood tests:
– Motion to Suppress Evidence
Filed to exclude unlawfully obtained or mishandled evidence.
– Discovery Requests
Used to obtain lab records, calibration logs, technician credentials, and chain of custody documents.
– Cross-Examination of Lab Technicians
To identify inconsistencies, errors, or lack of proper training.
– Use of Expert Witnesses
Toxicologists or forensic experts can provide alternative interpretations or expose flaws in testing procedures.
What to Do If You’re Facing DUI Charges
If you’ve been charged with DUI based on a blood test:
Do not panic – blood tests can be challenged.
Hire a qualified DUI attorney – experience with forensic science is essential.
Preserve your rights – avoid discussing details with police without counsel.
Request all evidence – including lab reports and blood sample custody logs.
Act quickly – deadlines for retesting samples or filing motions can be short.
Conclusion
While DUI blood tests are considered one of the more accurate methods for detecting alcohol or drug use, they are not immune to error. From improper handling and storage to legal violations during collection, there are many ways to challenge the validity of blood test results in court.
A skilled DUI attorney with a solid understanding of both the scientific and legal aspects of DUI cases can uncover these weaknesses and potentially get the evidence thrown out—or your charges reduced or dismissed.
If you or someone you know is facing DUI charges and blood test results are a key piece of evidence, consult with a qualified DUI defense lawyer as soon as possible. Your future could depend on it.
At Southwest Legal, we specialize in defending DUI cases and understand the complexities of blood test evidence. Our experienced attorneys know how to challenge improper procedures, expose lab errors, and protect your rights every step of the way.
Whether your BAC was just above the limit or you believe the test was completely flawed, we’ll thoroughly investigate your case and build a strong defense tailored to your situation.
Don’t wait until it’s too late. Contact us today for a free, confidential consultation, and let us help you fight back against your DUI charge.
