Blood alcohol content could be determined if a person is taken to the hospital after an accident and their blood is drawn. An arrest warrant or a search warrant is made by the police department to test the blood for alcohol. Any time a police department investigates an arrest and it seems like a person is drunk, they would ask the person if they want to go to the hospital to have blood drawn and preserved so they are able to test it. A person does not need to go to the hospital.