With most DUIs, the prosecution’s case usually rests on the results of a blood alcohol content (BAC) test. Breathalyzer tests and blood screening are the most common methods used by law enforcement, though it is also possible to measure an individual’s BAC using samples of hair follicles, urine or saliva.

In most cases, police will administer a breath test if they suspect that you are over the legal drinking limit. (If you refuse, officers can take you to a local clinic to have your blood sampled for testing.) Understanding how breath tests work is vital for any DUI defense.

The testing equipment used by law enforcement works by analyzing the alcohol content of exhaled vapor using spectroscopic analysis. In simple terms, the vapor absorbs certain frequencies of light depending on how much alcohol it contains. The breathalyzer records this frequency and outputs the resulting BAC measurement as a percentage of alcohol in the individual’s bloodstream.

While this may sound impressively technical, breath testing devices are not completely accurate. A breath test result can differ as much as 15 percent from the true BAC level as reported by a blood test. Improper calibration and usage can greatly affect the accuracy of a breath test, as can the temperature of the unit and the body temperature of the person being tested. These issues, as well as design flaws identified in the units themselves, have led to breath test results being thrown out of certain courts.

If you have been charged with a DUI on the basis of a breathalyzer test, a knowledgeable DUI defense attorney will be able to examine the accuracy of the equipment and potentially challenge the results. If issues are found with the way the equipment was calibrated or operated, the evidence obtained through its use may be excluded and the case dismissed.