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CO2 blood test

Definition

CO2 is carbon dioxide. This article discusses the laboratory test to measures the amount of carbon dioxide in the liquid part of your blood, called the serum.

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 blood bicarbonate level.

See also: Blood gases

Alternative Names

Bicarbonate test; HCO3-; Carbon dioxide test; TCO2; Total CO2; CO2 test - serum

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 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.

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

Normal Values

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

What abnormal results mean

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

What the risks are

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)

References

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