Blood Alcohol Test

Test Overview

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.

Watch

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.

Risks

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.

Results

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.

Blood alcohol

Normal:

No alcohol is found in the blood.

Abnormal:

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.

Effects of drinking alcohol

Estimated blood alcohol concentration (BAC)

Observable effects

0.02

Relaxation, slight body warmth

0.05

Sedation, slowed reaction time

0.10

Slurred speech, poor coordination, slowed thinking

0.20

Trouble walking, double vision, nausea, vomiting

0.30

May pass out, tremors, memory loss, cool body temperature

0.40

Trouble breathing, coma, possible death

0.50 and greater

Death

Credits

Current as of: September 23, 2020

Author: Healthwise Staff
Medical Review:
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