Alcohol breathalyzers work by testing the deep lung air sample for the concentration of alcohol. Alcohol is not digested when you consume it, it is absorbed. The alcohol enters your bloodstream and is present in every organ in your body, including your lungs. As the blood travels through the lungs some of the alcohol moves across the membrane of the lung’s alveoli (air sacks). When the alcohol is exposed to the air in your lungs, it evaporates and is mixed in with the air, waiting to be exhaled.
Using this ratio, a properly calibrated alcohol breathalyzer can accurately determine a person BAC by air sample alone, very seldom are blood tests done in order to test BAC.