CO2

Alternative names 
Bicarbonate test; HCO3-; Carbon dioxide

Definition

This is a blood test that measures the amount of carbon dioxide (CO2) in serum (the liquid portion of the blood).

Serum CO2 is really a measure of serum HCO3-, also called bicarbonate. The procedure used to measure HCO3- in the laboratory first converts it to CO2. In the body, 95% of the CO2 is present as HCO3-, so most of what is measured in the laboratory represents HCO3-.

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

CO2 and bicarbonate are also measured as part of an arterial blood gas analysis.

How to prepare for the test
The health care provider may advise you to discontinue drugs that may affect the test (see “Special Considerations”).

For Infants and children:
The preparation you can provide for this test depends on your child’s age and previous experience. For specific information regarding how you can prepare your child, see the following topics:

     
  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed
The CO2 levels in the blood are influenced by kidney and respiratory (lung) function.

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

What abnormal results mean
Lower-than-normal levels of HCO3- may indicate:

     
  • ketoacidosis  
  • lactic acidosis  
  • kidney disease  
  • diarrhea  
  • methanol poisoning  
  • salicylate toxicity (such as aspirin overdose)  
  • ethylene glycol poisoning  
  • Addison’s disease (adrenal gland insufficiency)

Higher-than-normal levels of HCO3- may indicate:

     
  • excessive vomiting  
  • respiratory dysfunction (breathing disorders)  
  • hyperaldosteronism  
  • Cushing’s syndrome

Note: The kidneys are the organ primarily responsible for maintaining the normal levels of HCO3-. Metabolic acidosis (pH less than 7.35 and bicarbonate less than 20 meq/l) is a symptom of kidney disease. However, HCO3- levels are also decreased in the presence of organic acids (ketone bodies in diabetic ketoacidosis) because HCO3- is used to neutralize the acids.

Additional conditions under which the test may be performed:

     
  • alkalosis  
  • delirium  
  • dementia  
  • renal tubular acidosis; distal  
  • renal tubular acidosis; proximal  
  • any medical condition in which the examiner feels there might be a metabolic derangement

What the risks are

     
  • excessive bleeding  
  • fainting or feeling light-headed  
  • hematoma (blood accumulating under the skin)  
  • infection (a slight risk any time the skin is broken)  
  • multiple punctures to locate veins

Special considerations
Drugs that can increase HCO3- measurements include corticosteroids and excessive use of antacids.

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.

Johns Hopkins patient information

Last revised: December 4, 2012
by Harutyun Medina, M.D.

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.