Blood alcohol content, or BAC, is one of the most commonly used ways of determining not only if a person has been drinking alcohol, but also how much alcohol a person has consumed. Although a Breathalyzer test can estimate a person's BAC, to determine actual blood alcohol content, a blood sample must be used.
A blood sample for alcohol content is taken in the same way as any other blood sample, using a needle. The needle is placed into a vein and a collection tube is inserted into the back of the needle. The needle is withdrawn, and the collection tube is sent to a lab for testing. Lab tests calculate the level of ethanol in the bloodstream, giving a result back as a percentage. Any blood alcohol content over 0.08 is considered over the legal limit.
When alcohol is consumed, ethanol is released into the bloodstream. Ethanol is the substance that most blood alcohol tests search for. According to Dr. David J. Hanson, Ph.D, up to 10 percent of ethanol leaves the body through sweat, breath and urine. The rest has to be metabolized out of the body by the liver. The time that it takes for alcohol to leave the bloodstream is directly dependent on how fast the body can metabolize the alcohol.
The metabolism of alcohol occurs in the liver. Once the ethanol in an alcoholic beverage reaches the liver, special enzymes called alcohol dehydrogenase are used to break down the ethanol into acetaldehyde, which is further broken down by enzymes into acetic acid, or vinegar. This process takes about an hour to break down a one half ounce of ethanol. This means that your blood alcohol content will drop an estimated 0.015 percent ever hour. For example, a blood alcohol content of 0.05 would take approximately three hours and twenty minutes to metabolize out of your body.