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

  • 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
  • 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

How To Prepare

No special preparation is needed before
having a blood alcohol test.

Many medicines may change the results
of this test. Be sure to tell your doctor about all the nonprescription and
prescription medicines you take.

How It Is Done

The health professional drawing blood

  • Wrap an elastic band around your upper arm to
    stop the flow of blood. This makes the veins below the band larger so it is
    easier to put a needle into the vein.
  • Clean the needle site with a
    non-alcohol solution such as povidone-iodine or antiseptic
  • Put the needle into the vein. More than one needle stick may
    be needed.
  • Attach a tube to the needle to fill it with
  • Remove the band from your arm when enough blood is
  • Put a gauze pad or cotton ball over the needle site as
    the needle is removed.
  • Put pressure on the site and then put on a

How It Feels

The blood sample is taken from a vein in
your arm. An elastic band is wrapped around your upper arm. It may feel tight.
You may feel nothing at all from the needle, or you may feel a quick sting or


There is very little chance of a problem from
having a blood sample taken from a vein.

  • You may get a small bruise at the site. You
    can lower the chance of bruising by keeping pressure on the site for several
  • In rare cases, the vein may become swollen after the blood
    sample is taken. This problem is called phlebitis. A warm compress can be used
    several times a day to treat this.


A blood alcohol test measures the amount
of alcohol (ethanol) in your body. Some states have no set limit for legal
intoxication. But the National Highway Traffic Safety Administration (NHTSA)
recommends that all states set the legal definition of intoxication as the
point when the blood alcohol concentration (BAC) exceeds 0.08 (which is
equivalent to 80
mg/dL or 17

Blood alcohol

No alcohol is found in the


Any alcohol is found in the

Legal intoxication is defined as having a blood
alcohol concentration (BAC) of 0.08 or greater. But the legal blood alcohol concentration (BAC) limit for people
under age 18 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

Effects of drinking alcohol
Estimated blood alcohol
concentration (BAC)
Observable effects


Relaxation, slight body warmth


Sedation, slowed reaction time


Slurred speech, poor coordination, slowed


Trouble walking, double vision, nausea,


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


Trouble breathing, coma, possible

0.50 and


What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

  • Using rubbing alcohol to clean the skin
    before inserting a needle to draw blood.
  • You have high blood
    ketones, as in
    diabetic ketoacidosis.
  • Taking cough
    medicines that contain alcohol or herbal supplements, such as kava or
  • Drinking other alcohols, such as isopropyl alcohol or

Many medicines may change the results of this test. Be sure
to tell your doctor about all the nonprescription and prescription medicines
you take.

Things that affect how quickly the blood
alcohol level rises in the body include:

  • The number of drinks per hour. As you
    increase the number of drinks per hour, your blood alcohol level steadily
  • The strength of alcohol (proof or percentage) in the
  • Your weight. The more you weigh, the more water is present
    in your body, which dilutes the alcohol and lowers the blood alcohol
  • 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
    hormones may affect the breakdown of
  • Your age. One drink raises the blood alcohol level of an
    older adult more than it does for a young adult.
  • Eating. Food in
    the stomach absorbs some of the alcohol. The blood alcohol level will be higher
    if you do not eat before or during drinking.

What To Think About

  • The blood alcohol test measures only the amount
    of alcohol in the blood at the time the sample is taken. It does not show how
    long you have been drinking or whether you have an alcohol use
  • Highway patrol officers in most states now have devices
    (toximeters) that measure the breath alcohol levels of drivers they think are
    drunk. A person charged with drunken driving who does not think the breath
    analysis is accurate may ask for a blood alcohol test.
  • The time
    that passes between drinking alcohol and collecting the blood or breath sample
    affects test results. The body continues to break down alcohol at a steady rate
    after drinking. So the amount of alcohol you drink can be estimated by knowing
    how much alcohol is present in your blood or breath and how much time has
    passed since you had a drink. In general, your body is able to break down about
    one drink per hour.
  • A person who drinks alcohol and takes
    certain medicines, such as
    sedatives (tranquilizers), or opioids, 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
  • A breath
    alcohol self-test can estimate your blood alcohol concentration. The handheld
    device to measure breath alcohol is similar to, though not as precise as, the
    type of test used by police for suspected drunk drivers. The breath alcohol
    self-test offers one piece of information to help you make a safe decision
    about drinking and driving. The safest decision is always not to drive if you
    have been drinking. To learn more, see the topic
    Self-Test for Breath Alcohol.


Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.


ByHealthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Martin J. Gabica, MD – Family Medicine
Kathleen Romito, MD – Family Medicine
Specialist Medical Reviewer R. Steven Tharratt, MD, MPVM, FACP, FCCP – Pulmonology, Critical Care Medicine, Medical Toxicology

Current as ofOctober 9, 2017