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Blood Alcohol Test
A blood alcohol test measures the amount of alcohol (ethanol) in your body. Alcohol is quickly absorbed into the blood and can be measured within minutes of having an alcoholic drink. The amount of alcohol in the blood reaches its highest level about an hour after drinking. But food in the stomach may increase the amount of time it takes for the blood alcohol to reach its highest level. About 90% of alcohol is broken down in the liver. The rest of it is passed out of the body in urine and your exhaled breath.
Alcohol has a noticeable effect on the body, even when consumed in small amounts. In large amounts, alcohol acts as a sedative and depresses the central nervous system.
A blood alcohol test is often used to find out whether you are legally drunk or intoxicated. If this test is being done for legal reasons, a consent form may be required, but refusing to take the test may have legal consequences.
Why It Is Done
A test for blood alcohol level is done to:
- Check the amount of alcohol in the blood when a person is suspected of being legally drunk (intoxicated). Symptoms of alcohol intoxication include confusion, lack of coordination, unsteadiness that makes it hard to stand or walk, or erratic or unsafe driving.
- Find the cause of altered mental status, such as unclear thinking, confusion, or coma.
- Check to see whether alcohol is present in the blood at times when the consumption of alcohol is prohibited—for example, in underage people suspected of drinking or in people enrolled in an alcohol treatment program.
How To Prepare
In general, there's nothing you have to do before this test, unless your doctor tells you to.
How It Is Done
A health professional uses a needle to take a blood sample, usually from the arm.
How It Feels
When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.
There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.
All states consider an adult legally drunk when the blood alcohol concentration (BAC) is 0.08 or greater. But the legal BAC limit for people under age 18 may be lower, such as 0.02.
No alcohol is found in the blood.
Any alcohol is found in the blood.
Effects of drinking alcohol
Having any amount of alcohol in the blood can cause poor judgment and slowed reflexes. BAC and the effects of drinking alcohol vary from person to person and depend upon body weight, the amount of food eaten while drinking, and each person's ability to tolerate alcohol.
Estimated blood alcohol concentration (BAC)
Relaxation, slight body warmth
Sedation, slowed reaction time
Slurred speech, poor coordination, slowed thinking
Trouble walking, double vision, nausea, vomiting
May pass out, tremors, memory loss, cool body temperature
Trouble breathing, coma, possible death
0.50 and greater
Current as of: June 17, 2021
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Martin J. Gabica MD - Family Medicine
Kathleen Romito MD - Family Medicine
R. Steven Tharratt MD, MPVM, FACP, FCCP - Pulmonology, Critical Care Medicine, Medical Toxicology
Current as of: June 17, 2021
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