"Per se" DUI laws establish a legal presumption that a driver was intoxicated if he or she has a blood alcohol concentration (BAC) above a certain limit usually .08 percent. In order to establish a DUI suspect's BAC, police officers will employ a chemical test of some kind. While a blood test produces the most exact results, police officers regularly use portable breath analyzers to estimate a suspect's BAC while at the scene of the initial DUI traffic stop.
Commonly known as "breathalyzers," breath analysis testing devices offer strong evidence concerning a suspect's BAC, but they are not infallible. Courts regularly accept breathalyzer evidence as sufficient proof of a defendant's BAC, but the police department must demonstrate that the testing device in question can produce reliable results. Many DUI defendants have successfully challenged the breathalyzer evidence brought against them, thus putting a serious dent in the prosecution's case.
The most frequent challenges to breathalyzer evidence concern whether or not a testing device was in proper working order at the time of the test. Breathalyzers require regular calibration and maintenance to ensure that they deliver results with a sufficient level of accuracy. An improperly calibrated or poorly maintained machine will produce unreliable results that cannot form the basis of a presumption of intoxication under per se DUI laws.
If a defendant can show that the police department did not follow proper calibration procedures, or that the device in question consistently provided erroneous readings, then a court will likely declare the results of the breathalyzer test inadmissible as evidence of the defendant's intoxication. At that point, the onus will shift back to the government to prove the defendant's intoxication through some other type of evidence.