Police officers in Maryland and across the country regularly use breath analysis testing devices (commonly known as "Breathalyzers") to determine the blood alcohol content of suspected drunk drivers. If the result is .08 percent or above (or just .01 for a driver under the legal drinking age), that driver can be charged with a DUI. The driver must have two measurable readings within .02 of one another for the Breathalyzer test to be admissible as evidence in court.
Like all machines, Breathalyzers are fallable. Attorneys can challenge them -- many times successfully. A common strategy used by defense attorneys is to determine whether the Breathalyzer was properly maintained and regularly calibrated as directed by state law at the time that it was used to test a driver's BAC. An improperly maintained or calibrated machine can potentially show a higher BAC level than a person actually had.
There are other methods for challenging Breathalyzer results. For example, the officer administering it needs to be certified to use the specific testing device and do so as he or she has been trained to. The device must also be one that conforms to the state's legal standards.
An experienced DUI defense attorney can subpoena the records for the Breathalyzer's calibration and maintenance and information about how the test was performed and by whom. That doesn't necessarily mean that there will be no evidence. If someone failed roadside field sobriety tests or exhibited other signs of intoxication, he or she could still be convicted of DUI. However, being able to place doubt on Breathalyzer results can help people avoid a DUI conviction.
Source: FindLaw, "Breathalyzer Calibration," accessed March 27, 2017