Carbon Dioxide (CO2) Test

Test Overview

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

CO2 testing is often done as part of a group of blood tests (chemistry screen) to help find the cause of many kinds of symptoms. These causes may include many kidney diseases, some lung diseases, and metabolic problems.

How To Prepare

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How It Is Done

• A health professional uses a needle to take a blood sample, usually from the arm.

How long the test takes

The test will take a few minutes.

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

High values

A high level may be caused by:

• Severe vomiting or diarrhea.
• Starvation.
• Overuse of medicines that contain bicarbonate (especially antacids).
• Lung, liver, or thyroid disease.

Low values

A low level may be caused by:

• Hyperventilation.
• Aspirin or alcohol overdose.
• Severe diarrhea, dehydration, or malnutrition.
• Liver or kidney disease.
• Uncontrolled diabetes.