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.

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

Most states consider a person 21 or older legally drunk when the blood alcohol concentration (BAC) is 0.08 or greater. But the legal BAC limit for people under age 21 may be lower, such as 0.02.

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.

Blood alcohol concentration (BAC) is a measurement of how much alcohol you have in your body. This can be used to determine when someone is legally intoxicated (drunk).

Most states consider a person 21 or older legally drunk when the BAC is 0.08 or greater. But the legal BAC limit for people under age 21 may be lower, such as 0.02.

If alcohol is found in your blood, the BAC can range from very little, such as a BAC of 0.02, to a BAC of more than 0.50. A BAC of 0.50 is very high and can cause death.

Here are some examples of BAC and its effect:

• 0.05: Slowed reaction time
• 0.10: Slurred speech
• 0.20: Hard to walk, nausea, vomiting
• 0.40: Possible coma and death

What affects BAC

BAC depends on the number of drinks you have and the strength of alcohol (proof or percentage) in the drinks. Other things that affect blood alcohol levels include:

How fast you drink.

As you increase the number of drinks per hour, your blood alcohol level steadily increases.

Your weight.

The more you weigh, the more water you have in your body. The water dilutes the alcohol and lowers the blood alcohol level.

Your sex.

Women's bodies usually have less water and more fat than men's bodies. Alcohol does not go into fat cells as well as other cells, so women tend to keep more alcohol in their blood than men do. Also, a woman's hormones may affect the breakdown of alcohol.

Your metabolism.

This is how fast your body breaks down the alcohol. Different people metabolize alcohol at different rates.

Your age.

One drink raises the blood alcohol level of an older adult more than it does for a young adult.

Food.

Food in the stomach absorbs some of the alcohol. The blood alcohol level will be higher if you do not eat before or while you drink.

Medicines or drugs.

A person who drinks alcohol and takes certain medicines, such as antihistamines or sedatives (tranquilizers), may feel more of the effects of alcohol. Also, a person who uses other drugs, such as marijuana, will feel the effects of both drugs more than if the drugs were used separately.