What is a blood alcohol test?

A blood alcohol test measures the level of Alcohol in your blood. Most people are more familiar with the breathalyzer, a test often used by police officers on people suspected of drunk driving. While a breathalyzer gives fast results, it is not as accurate as measuring alcohol in the blood.

Alcohol, also known as ethanol, is the main ingredient of alcoholic drinks such as beer, wine, and liquor. When you have an alcoholic drink, it is absorbed into your bloodstream and processed by the liver. Your liver can process about one drink an hour. One drink is usually defined as 12 ounces of beer, 5 ounces of wine, or 1.5 ounces of whiskey.

If you are drinking faster than your liver can process the alcohol, you may feel the effects of drunkenness, also called intoxication. These include behavioral changes and impaired judgment. The effects of alcohol can vary from person to person, depending on a variety of factors such as age, weight, gender, and how much food you ate before drinking.

Other names: blood alcohol level test, ethanol test, ethyl alcohol, blood alcohol content

What is it used for?

A blood alcohol test may be used to find out if you:

• Have been drinking and driving. In the United States, .08 percent blood alcohol level is the legal alcohol limit for drivers who are aged 21 and over. Drivers younger than 21 are not allowed to have any alcohol in their system when driving.
• Are legally drunk. The legal alcohol limit for drinking in public varies from state to state.
• Have been drinking while in a treatment program that prohibits drinking.
• Have alcohol poisoning, a life-threatening condition that happens when your blood alcohol level gets very high. Alcohol poisoning can seriously affect basic body functions, including breathing, heart rate, and temperature.

Teens and young adults are at higher risk for binge drinking, which can cause alcohol poisoning. Binge drinking is a pattern of drinking that raises the blood alcohol level within a short period of time. Though it varies from person to person, binge drinking is usually defined as four drinks for women and five drinks for men in a two-hour period.

Young children may get alcohol poisoning from drinking household products that contain alcohol, such as mouthwash, hand sanitizer, and certain cold medicines.

Why do I need a blood alcohol test?

You may need a blood alcohol test if you are suspected of drunk driving and/or have symptoms of intoxication. These include:

• Difficulty with balance and coordination
• Slurred speech
• Slowed reflexes
• Nausea and vomiting
• Mood changes
• Poor judgment

You or your child may also need this test if there are symptoms of alcohol poisoning. In addition to the above symptoms, alcohol poisoning can cause:

• Confusion
• Irregular breathing
• Seizures
• Low body temperature

What happens during a blood alcohol test?

A health care professional will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This process usually takes less than five minutes.

Will I need to do anything to prepare for the test?

You don't need any special preparations for a blood alcohol test.

Are there any risks to the test?

There is very little risk to having a blood test. You may have slight pain or bruising at the spot where the needle was put in, but most symptoms go away quickly.

What do the results mean?

Blood alcohol level results may be given in different ways, including percentage of blood alcohol content (BAC). Typical results are below.

• Sober: 0.0 percent BAC
• Legally intoxicated: .08 percent BAC
• Very impaired: .08–0.40 percent BAC. At this blood alcohol level, you may have difficulty walking and speaking. Other symptoms may include confusion, nausea, and drowsiness.
• At risk for serious complications: Above .40 percent BAC. At this blood alcohol level, you may be at risk for coma or death.

The timing of this test can affect the accuracy of the results. A blood alcohol test is only accurate within 6–12 hours after your last drink. If you have questions or concerns about your results, you may want to talk to a health care provider and/or a lawyer.

Learn more about laboratory tests, reference ranges, and understanding results.

Is there anything else I need to know about a blood alcohol test?

A police officer may ask you to take a breathalyzer test if you are suspected of drunk driving. If you refuse to take a breathalyzer, or think the test wasn't accurate, you may ask for or be asked to take a blood alcohol test.