Breath alcohol testing devices, commonly known by the term for one specific type of device (the “Breathalyzer”), measure how much alcohol is currently in the blood. Blood alcohol tests also show current levels of impairment or intoxication, but not past use. This is in contrast to tests for illegal drugs which do show past use.
The person being tested for alcohol use blows into a breath alcohol device, and the results are given as a number. The number, known as the blood alcohol concentration (BAC), shows the level of alcohol in the person’s blood at the time the test was taken. It does not measure past use of alcohol.