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Carbon dioxide (CO2) is a gaseous waste product from metabolism. The blood carries carbon dioxide to your lungs, where it is exhaled. More than 90% of it in your blood exists in the form of bicarbonate (HCO3). The rest of it is either dissolved carbon dioxide gas (CO2) or carbonic acid (H2CO3). Your kidneys and lungs balance the levels of carbon dioxide, bicarbonate, and carbonic acid in the blood.
This test measures the level of bicarbonate in a sample of blood from a vein. Bicarbonate is a chemical that acts as a buffer. It keeps the pH of blood from becoming too acidic or too basic.
Bicarbonate is not usually tested by itself. The test may be done on a blood sample taken from a vein as part of a panel of tests that looks at other electrolytes. These may include items such as sodium, potassium, and chloride. It can also be done as part of an arterial blood gas (ABG) test. For this blood gas study, the blood sample comes from an artery.
Why It Is Done
A carbon dioxide test helps find and checks conditions that affect blood bicarbonate levels. These include many kidney diseases, some lung diseases, and metabolic problems.
How To Prepare
You do not need to do anything special to prepare for this test.
Be sure to tell your doctor about all the medicines you take, even over-the-counter ones. Many medicines can change the results of this test.
Talk to your doctor if you have any concerns about the need for the test, its risks, how it will be done, or what the results will mean. To help you learn about this test and how important it is, fill out the medical test information form .
How It Is Done
The health professional taking a sample of your blood will:
- 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 alcohol.
- Put the needle into the vein. More than one needle stick may be needed.
- Attach a tube to the needle to fill it with blood.
- Remove the band from your arm when enough blood is collected.
- Apply 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 bandage.
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 pinch.
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 minutes.
- In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. You can use a warm compress several times a day to treat this.
A carbon dioxide test measures the level of bicarbonate in the blood.
These numbers are just a guide. The range for "normal" varies from lab to lab. Your lab may have a different range. Your lab report should show what range your lab uses for "normal." Also, your doctor will evaluate your results based on your health and other factors. So a number that is outside the normal range here may still be normal for you.
Results are usually ready in 1 to 2 days.
A high level may be caused by:
- Vomiting, dehydration, blood transfusions, or overuse of medicines that contain bicarbonate (especially antacids).
- Conditions such as anorexia, chronic obstructive pulmonary disease (COPD), fluid in the lungs (pulmonary edema), heart disease, Cushing's disease, or Conn's syndrome.
A low level may be caused by:
What Affects the Test
You may not be able to have the test, or the results may not be helpful, if:
- You drink liquids that are very acidic, such as orange juice or some types of fizzy drinks, shortly before the test.
- You take certain medicines, such as diuretics, some antibiotics, glaucoma medicine, or corticosteroids.
- The person taking the blood sample leaves the elastic band on your arm for too long before taking the blood sample.
What To Think About
- The carbon dioxide test also can be done on a blood sample taken from an artery for an arterial blood gas (ABG) test. To learn more, see the topic Arterial Blood Gases.
- Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.
Current as of:
February 24, 2020
Author: Healthwise Staff
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Elizabeth T. Russo MD - Internal Medicine
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