Carbon Dioxide

Test Overview

A carbon dioxide test measures the total amount of the three forms of carbon dioxide (bicarbonate, carbonic acid, and dissolved carbon dioxide) in your blood. This test is also called a total carbon dioxide or TCO2 test.

Carbon dioxide (CO2) is a gaseous waste product made from metabolism. The blood carries carbon dioxide to your lungs, where it is exhaled. More than 90% of carbon dioxide in your blood exists in the form of bicarbonate (HCO3). The remainder of the carbon dioxide 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 is usually done at the same time as an arterial blood gas test.

Why It Is Done

A carbon dioxide test is often done as part of a group of laboratory blood tests (chemistry screen) to help find the cause of many kinds of symptoms. It is often done if you are having breathing problems.

How To Prepare

You do not need to do anything before you have this 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.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?) .

How It Is Done

The health professional drawing 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.
  • 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 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.

Risks

There is very little chance of a problem from having 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. A warm compress can be used several times a day to treat this.
  • Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Results

A carbon dioxide test measures the total amount of the three forms of carbon dioxide (bicarbonate, carbonic acid, and dissolved carbon dioxide) in your blood. Results are usually available in 1 to 2 days.

Normal

Carbon dioxide
Adults:

23–29 millimoles per liter (mmol/L)

Children:

20–28 mmol/L

Babies:

13–22 mmol/L

High values

High values may be caused by:

Low values

Low values may be caused by:

  • Problems that increase blood pH (respiratory alkalosis), such as pneumonia, cirrhosis, liver failure, or hyperventilation.
  • Problems that decrease blood pH (metabolic acidosis), such as uncontrolled diabetes, kidney or heart failure, aspirin overdose, shock, frequent diarrhea, dehydration, long-term (chronic) starvation, and swallowing antifreeze (ethylene glycol) or wood alcohol (methanol).

What Affects the Test

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

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

What To Think About

Your doctor will often compare the results of a total carbon dioxide test with the results of an arterial blood gas test. For more information, see the medical test Arterial Blood Gases.

References

Other Works Consulted

  • Chernecky CC, Berger BJ, eds. (2004). Laboratory Tests and Diagnostic Procedures, 4th ed. Philadelphia: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
  • Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.

Credits

Author Jeannette Curtis
Editor Susan Van Houten, RN, BSN, MBA
Associate Editor Tracy Landauer
Primary Medical Reviewer Kathleen Romito, MD - Family Medicine
Specialist Medical Reviewer R. Steven Tharratt - Pulmonology, Critical Care Medicine, Medical Toxicology
Last Updated May 9, 2008

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