Definition of CO2 test - serum:

CO2 is carbon dioxide. A serum CO2 test measures the amount of carbon dioxide in the liquid part of your blood.

In the body, most of the CO2 is in the form of a substance called bicarbonate (HCO3-). Therefore, the CO2 blood test is really a measure of your bicarbonate level.

See also: Blood gases

How the test is performed:

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How to prepare for the test:

Your health care provider may tell you to stop taking any drugs that may affect test results. Corticosteroids and excessive use of antacids can increase bicarbonate levels.

How the test will feel:

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

The CO2 test is most often done as part of an electrolyte panel. Changes in your CO2 level suggest you may be losing or retaining fluid, cause an imbalance in your body's electrolytes. Electrolytes are minerals in your blood and other body fluids.

CO2 levels in the blood are influenced by kidney and lung function. The kidneys are mainly responsible for maintaining the normal bicarbonate levels.