The various testing devices used by police in impaired driving cases have been determined to be generally reliable and accurate means of testing blood alcohol levels. Accordingly, the courts rightly place reliance on these test results in deciding guilt or innocence in a particular case. This does not mean that the equipment is infallible or cannot be challenged by your lawyer. The reliability of these machines assumes that all proper maintenance, calibration and operation procedures have been followed by the police. Only an experienced lawyer, often in consultation with an expert, can make this assessment on your behalf.

The main source of potential error in these machines, however, relates to the many assumptions made about human physiology when it comes to the absorption, distribution and elimination of alcohol in the human body. These machines test your breath, not your blood, to determine blood alcohol levels. The common basic assumption for all of these machines is that the amount of alcohol in the bottom of your lungs has a predictable and consistent relationship to the amount actually in your blood.