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

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.

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.

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.

The CO2 test is most often done as part of an electrolyte or basic metabolic panel. Changes in your CO2 level may suggest that you are losing or retaining fluid, which causes 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.

The normal range is 20-29 mEq/L (milliequivalent per liter).

Lower-than-normal levels may be due to:

• Addison disease
• Diarrhea
• Ethylene glycol poisoning
• Ketoacidosis
• Kidney disease
• Lactic acidosis
• Metabolic acidosis
• Methanol poisoning
• Salicylate toxicity (such as aspirin overdose)

Higher-than-normal levels may be due to:

• Breathing disorders
• Cushing syndrome
• Excessive vomiting
• Hyperaldosteronism

The following conditions may also alter bicarbonate levels:

• Alkalosis
• Delirium
• Dementia
• Renal tubular acidosis; distal
• Renal tubular acidosis; proximal

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Seifter JL. Acid-base disorders. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 119.