DUI Testing & Evidence
The evidence in a DUI case is not as ironclad as the prosecution would have you believe. Breath tests, blood tests, and field sobriety tests all have known weaknesses, limitations, and failure points.
Breath Tests
Arizona uses the Intoxilyzer 8000 as its certified breath testing instrument. The machine estimates BAC by measuring alcohol in breath and applying a mathematical conversion that assumes a standard partition ratio that does not apply to every person.
We had a case where the Intoxilyzer’s calibration logs showed a drift pattern across several weeks. The state argued the machine was “within acceptable range,” but our expert testified about what that drift meant for the reliability of the 0.153 reading our client received. The extreme DUI charge was reduced.
For a breath test to be admissible, the machine must be calibrated and certified, the operator must hold a valid permit, a 15-minute continuous observation period must be performed, and two separate breath samples must agree within an acceptable range. Deviation from any of these protocols can compromise the result.
Common issues include lapses in the observation period, overdue calibration, operator errors, environmental factors, and medical conditions including GERD, acid reflux, and diabetes that cause mouth alcohol.
Blood Tests
Blood testing is generally more accurate than breath testing but is not immune to challenge. The chain of custody, qualifications of the person who drew the blood, the type of kit, mixing procedure, and laboratory analysis are all targets for defense challenges. Fermentation of improperly preserved samples can artificially raise the reported BAC.
Arizona law gives defendants the right to have their blood sample independently tested. Discrepancies between the state’s results and the independent lab’s results can create reasonable doubt.
Field Sobriety Tests
The three standardized tests (HGN, Walk-and-Turn, One-Leg Stand) were developed by NHTSA and validated through research that acknowledges significant error rates. The HGN test can be affected by neurological conditions, fatigue, medications, and contact lenses. The Walk-and-Turn and One-Leg Stand are affected by age, weight, footwear, road surface, weather, and anxiety.
Non-standardized tests like reciting the alphabet backward have even less scientific support.
Body Camera and Dashcam Footage
Most Maricopa County agencies now use body-worn cameras, and many patrol vehicles have dashcams. This footage often reveals discrepancies between the officer’s report and what actually happened. Requesting, reviewing, and using this footage is standard practice at Arizona DUI Guys