There are several different reasons why a medical professional might do a blood alcohol test. If you are presenting in an emergency room, the ER staff needs to know your blood alcohol concentration to make sure not to give you any medicines that could interact with alcohol and to help guide your treatment. Legally a blood alcohol test result can be an important piece of evidence if someone is in a motor vehicle accident after drinking.
Each person's response to alcohol varies. The blood alcohol test measures the level of alcohol in your blood in terms of grams of alcohol per deciliter of blood. In the United States, someone is considered legally drunk if their blood alcohol test reveals a blood alcohol concentration of .08 g/dL. A blood alcohol test is accurate only within a six-to 12-hour window after drinking.