Blood tests are often considered the "gold standard" for measuring blood alcohol content, but they're not immune to errors and challenges. Understanding blood draw procedures, testing methodologies, and potential problems provides defense opportunities when facing DUI charges based on blood evidence.
Blood Draw Procedures
Proper blood collection requires following specific protocols. Blood draws must be performed by qualified personnel—typically phlebotomists, nurses, or trained medical technicians. Draws by unqualified individuals may be challengeable.
The draw site must be cleaned with a non-alcohol swab (alcohol wipes can contaminate samples). Proper collection tubes containing preservatives and anticoagulants must be used. Samples must be properly labeled, sealed, and stored.
Chain of Custody
Chain of custody documents everyone who handled your blood sample from collection through testing. Any gap in documentation—periods where the sample's location or handling is unknown—creates opportunities for contamination or tampering arguments.
Prosecutors must prove samples were properly handled throughout. Chain of custody documents should show every transfer, storage location, and person who accessed the sample.
Fermentation and Contamination
Improperly preserved blood samples can ferment, producing alcohol that wasn't present when collected. Fermentation occurs when bacteria or yeast in the sample convert sugars to alcohol, artificially elevating BAC readings.
Risk factors for fermentation include inadequate preservative in collection tubes, improper storage temperature, delayed testing, and contamination during collection.
Expert testimony can establish that fermentation affected results, particularly when samples show signs of microbial activity or weren't properly preserved.
Laboratory Testing Errors
Laboratories use gas chromatography or enzymatic methods to analyze blood samples. These techniques are generally reliable but not error-proof. Common lab issues include equipment calibration problems, failure to run required quality control samples, cross-contamination between samples, and analyst errors in procedures or calculations.
Laboratory records, quality control data, and analyst qualifications are all discoverable and may reveal problems affecting your results.
Warrant Requirements
After the Supreme Court's Birchfield v. North Dakota decision, warrantless blood draws generally violate the Fourth Amendment unless an exception applies (consent, exigent circumstances). Blood obtained without a warrant or valid consent may be suppressed.
Exceptions allowing warrantless draws are narrow. Mere dissipation of alcohol over time is not an exigent circumstance justifying warrantless blood collection. Officers must generally obtain a warrant before compelling blood draws.
Implied Consent Issues
States have implied consent laws deeming drivers to have consented to chemical testing by using public roads. However, implied consent doesn't eliminate warrant requirements for blood draws. Refusal to provide blood may trigger license suspension but doesn't authorize forcible draws without a warrant.
Challenges based on lack of valid consent or warrant can result in suppression of blood evidence.
Medical Conditions
Certain medical conditions affect blood test accuracy. Diabetes can produce ketones that some testing methods mistake for alcohol. Blood conditions may affect sample integrity. Medications may interfere with testing methodologies.
Medical records documenting relevant conditions support challenges to blood test reliability.
Independent Testing
You have the right to request independent testing of your blood sample. Having your sample tested by a separate laboratory can identify errors in the original results or provide evidence supporting your defense.
Request independent testing promptly—samples degrade over time, and delays may compromise the ability to obtain reliable independent results.
Building Your Defense
Challenging blood evidence requires analyzing the draw procedure (who performed it, what protocols were followed), reviewing chain of custody documentation, examining laboratory records and quality control data, and identifying medical or procedural factors affecting results.
An experienced DUI attorney knows what records to obtain and how to identify problems that undermine blood test evidence. Successful challenges can lead to evidence suppression, charge reduction, or acquittal.