What Is a Blood Alcohol Test?

Medically Reviewed by Carol DerSarkissian, MD on February 21, 2024
4 min read

When you think of a blood alcohol test, you might picture a Breathalyzer at a traffic stop. But you might need one for other reasons, too. And there’s more than one type.

To check your blood for alcohol, your doctor uses a needle to take blood from your arm and measure the amount of alcohol. The other tests you might get for alcohol, like a breath or urine test, don’t use blood samples.

Each of these tests has the same goal: to check how much alcohol is in your body. Usually, you get a result called blood alcohol concentration (BAC). It’s a small number, like 0.05.

The higher your BAC, the more alcohol is in your system. And that affects your judgment, control, and lots of other things.

You might need one for legal or medical reasons.

A legal reason can be a police officer’s suspicion that you’re driving drunk or drinking underage. It’s also common after a car crash to see if alcohol played a role. Usually, you’d take a breath alcohol test on the spot. If you refuse, you may be required to get a blood alcohol test.

You may also get one of these alcohol tests at work during random drug checks. If you have an accident on the job, your company might check whether alcohol was involved. These tests can be done with urine, blood, saliva, or breath samples.

And life insurance companies may ask for it when you apply for insurance.

Medical reasons include if you come to a hospital or the ER passed out, confused, or showing other signs of heavy drinking. It helps doctors know what’s happening and how best to care for you. Doctors often do a breath test, but sometimes will draw your blood instead.

When you have a drink, your stomach and small intestine soak it up and send the alcohol into your blood. From there, it’s your liver’s job to process it.

But your liver can only handle so much alcohol each hour. Whatever’s left over stays in your blood, so the more you drink, the higher your blood alcohol concentration.

As your BAC goes up, alcohol affects you more. How quickly that happens depends on things including your age, gender, and weight. A small woman will feel the effects faster than a big man, for instance.

0.03. You’re a bit buzzed. You still feel in control, but in reality, your judgment and vision aren’t as good as when you’re sober, and it’s harder to do two things at the same time.

0.05. You may feel loose and less self-conscious, but you’re not as alert as when your BAC is lower. You would have trouble steering, focusing on moving objects, and reacting quickly to emergencies.

0.08. You’re legally drunk. You have a hard time with balance, talking, and seeing straight. Your reaction time slows, and your judgment and self-control have slipped quite a bit.

0.10. Your balance, reaction time, and judgment keep getting worse. You’ll slur your words and have trouble thinking straight.

0.20. You might be confused, stagger, blackout, or throw up.

0.40. This level is life-threatening. It could put you in a coma and you could die. It’s an emergency.

Never try to guess your blood alcohol concentration based on how you feel. The stakes are too high.

For adults 21 and over, all U.S. states have the same BAC limit: 0.08. At this point, you’re driving drunk and breaking the law.

Even if your BAC is below the legal limit, no amount of alcohol is safe when you drive. If you’re going to drink, don’t drive!

Your state may also have other laws based on your job. For example, in some states, school bus drivers need a BAC below 0.02 to be legal.

And if you’re under 21, you can’t drive with any alcohol in your blood.

Not if you’re driving. You might have heard the old rule of thumb that you’re fine if you’ve had at most a drink an hour because that’s what your liver can process. Don’t trust that. A lot of things affect your BAC, such as:

Age. As you get older, the same drink boosts your BAC faster than in a younger person.

Drink strength. Even different drinks in the same family, like two different beers, can have different amounts of alcohol.

Food. When you eat before and while you drink, your BAC goes up more slowly.

Sex. BAC generally goes up more quickly in women than in men.

Weight. Usually, the less you weigh, the faster your BAC ramps up.

Medicines and drugs. Legal and illegal drugs can raise your BAC more quickly or create dangerous side effects.

Race and ethnicity. Your genes affect how your liver handles alcohol, which means your race or ethnicity can also play a role. Asians and Native Americans tend to process alcohol more slowly, so their BAC goes up more quickly.

Don’t trust your judgment in the moment. When you’re drinking, you’re not able to make good decisions like you would when you’re sober. The only rule to remember is that no matter what, you won’t drink and drive.