What is a carbon dioxide (CO2) blood test?

A carbon dioxide (CO2) blood test measures the amount of carbon dioxide in your blood. Carbon dioxide is an odorless, colorless gas. It is a waste product from metabolism, the process your body uses to make energy from food you eat.

Most of the carbon dioxide in your body is in the form of bicarbonate (HCO3), which is a type of electrolyte. Electrolytes are electrically charged minerals that help control the amount of fluid and the balance of acids and bases (pH balance) in your body.

Your blood carries carbon dioxide to your lungs, and then you breathe it out. Too much or too little carbon dioxide in your blood can be a sign of a health problem.

Other names: carbon dioxide content, CO2 content, carbon dioxide blood test, bicarbonate blood test, bicarbonate test, total CO2; TCO2; carbon dioxide content; CO2 content; bicarb; HCO3

What is it used for?

A CO2 blood test is often part of a group of tests called an electrolyte panel, which may be part of a routine checkup. The test may also help diagnose or monitor conditions related to an electrolyte imbalance. These may include high blood pressure and diseases of the kidneys, lungs, or liver.

Why do I need a CO2 in blood test?

Your health care provider may order a CO2 blood test as part of your routine checkup or if you have symptoms of an electrolyte imbalance. These symptoms may include:

• Trouble breathing
• Confusion
• Weakness
• Fatigue
• Vomiting and/or diarrhea over a long period

Your provider may also order a CO2 blood test to check for side effects of certain medicines that can cause electrolyte imbalances.

What happens during a CO2 blood 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 usually takes less than five minutes.

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

Some prescription and over-the-counter medicines can increase or decrease the amount of carbon dioxide in your blood. You may need to stop taking certain medicines before this test, so tell your provider about everything you take. But don't stop taking any medicines unless your provider tells you to.

If your provider has ordered more tests on your blood sample, you may need to fast (not eat or drink) for several hours before the test. Your provider will let you know if there are any special instructions to follow.

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?

Abnormal test results may be a sign that your body is having a hard time keeping the right acid-base balance (pH balance). This may be because your lungs or kidneys are having a problem removing carbon dioxide or because you have an electrolyte imbalance. Many different types of conditions can cause these problems.

Too much CO2 in the blood (called hypercapnia or hypercarbia) can be a sign of many conditions, including:

• Lung diseases.
• Cushing's syndrome.
• Kidney failure.
• Metabolic alkalosis, a condition in which your blood is not acidic enough. You may lose acid from conditions such as vomiting, dehydration, and anorexia.

Too little CO2 in the blood (hypocarbia) may be a sign of:

• Addison disease.
• Diabetic ketoacidosis, a complication of type 1 and type 2 diabetes.
• Shock.
• Metabolic acidosis, a condition in which your blood is too acidic. It can have many causes, including kidney or liver disease or long-lasting diarrhea.
• Respiratory alkalosis, a condition in which your blood is not acidic enough because of a lung or breathing problem, including hyperventilation (rapid, deep breathing).

If your test results are not in the normal range, it doesn't necessarily mean you have a medical condition that needs treatment. Your provider may order other tests to confirm a diagnosis. Other factors, including certain medicines, can affect the level of CO2 in your blood. To learn what your results mean, talk with your provider.

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

Is there anything else I need to know about a CO2 blood test?

A CO2 blood test may also be completed as part of a basic or comprehensive metabolic panel. These tests measure different substances in your blood to learn about your body's chemical balance and metabolism.