How Do DUI Breath Tests Work?


The breath test is another testing option used by law enforcement to determine if there is alcohol in your system and to measure your Breath Alcohol Concentration. These machines measure the alcohol concentration in the air that is exhaled from a person’s lungs. If required to do a breath test, you will be asked by a police officer to blow into a breathalyzer or intoxilyzer, which will then provide the operator with an allegedly correct reading. Arizona law requires that the machine used to capture your breath be properly calibrated and serviced with Arizona Department of Health Services guidelines, the test be given by a certified operator, and the test be administered within Department of Health Services guidelines.

DUI Breath Testing Inaccuracies


These machines provide results based on assumptions for various physiological factors, including a certain body temperature, the temperature of the breath at the time of testing, and a postabsorptive partition ratio (meaning that it is assumed that the subject’s body is no longer absorbing any more alcohol and is in the process of eliminating it). These machines produce results that are supposed to be accurate, plus or minus 10%. However, an individual’s true values for these factors often fluctuate from the standard assumptions used—this means that the Breath Alcohol Concentration obtained may be inaccurate by up to 10-15% more than the standard 10% allotted. Complicating this result is the fact that hyperventilation will generally result in an inaccurately lowered reading, and holding one’s breath prior to testing will result in an artificially increased reading. This can add yet another 10-15% variation to the “results” obtained.

For certain DUI charges in Arizona, you are allowed to introduce evidence in each of the above areas to dispute the standard partition ratio used by the machine in computing a person’s breath alcohol concentration (State v. Cooperman, 2 CA-CV 2011-0197(Ariz. Ct. App. 2012)). These partition ratios are not static. They “vary from person to person, and even from moment to moment for the same person.”

Combating DUI Breath Test Results in Arizona


Your DUI attorney must be able to review and evaluate the test results, machine and calibration records, and police operator records to determine how to proceed in defending your case.