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Bicarbonate Test

(CO2 Test; HCO3- Carbon Dioxide Test; TCO2; Total CO2)

What Is Bicarbonate?

Carbon dioxide (CO2) takes the form of bicarbonate (HCO3-) when it is dissolved in the blood. Most of the CO2 in the body is in the form of bicarbonate. Normal levels of bicarbonate are controlled by the kidneys and by breathing.

Reason for the Test

This test measures the amount of CO2 in the blood. It is usually done as part of an electrolyte panel or a basic metabolic profile.

The test is done to monitor acid-base balance and electrolyte levels to check for an imbalance in people who have a health problem or are having a treatment that may cause this.

Type of Sample Taken

A sample will be taken from a vein in the arm or the back of the hand.

Prior to Collecting the Sample

You do not need to do anything special before the test. Some medicines may affect test results. Your doctor may want you to stop taking some medicines for a few days before the test.

During the Sample Collection

You will be asked to sit. An area inside your elbow will be cleaned with a wipe. A large band will be tied around your arm. The needle will be put in a vein. A tube will collect the blood from the needle. The band on your arm will be taken off. After the blood is collected, the needle will be removed. Gauze will be held on the site to help stop bleeding. You may also be given a bandage. This test takes about 5-10 minutes.

After Collecting the Sample

After the blood sample is taken, you may need to stay seated for 10 to 15 minutes. If you are lightheaded, you may need to stay seated longer. When you feel better, you can leave.

A bit of blood may ooze from the vein beneath the skin. It will cause a bruise. A bruise will usually fade in a day or two.

Call your doctor right away if you have redness, swelling, lasting bleeding, or pain.


It may take several hours to days to get your results. It depends on the lab.

A low level may be a sign of:

  • Addison disease, a hormone deficiency
  • Chronic diarrhea
  • Poisoning by ethylene glycol, a chemical often used in anti-freeze
  • Ketoacidosis, a problem that happens in people with diabetes
  • Kidney disease
  • A buildup of lactic acid
  • Too much acid in the blood—metabolic acidosis
  • Hyperventilation
  • Poisoning from methanol—wood alcohol
  • An overdose of aspirin

A higher than normal level may be a sign of:

  • Problems with poor breathing
  • Cushing syndrome—excess cortisol
  • Excessive vomiting
  • Excess production of the hormone aldosterone

Talk to your doctor about your test results. A test may point to an illness that you do not have. It can also miss an illness that you may have. The doctor will check your symptoms and all test results before making a diagnosis.


Bicarbonate. Lab Tests Online—American Association for Clinical Chemistry website. Available at: http://www.labtestsonline.org/understanding/analytes/co2/test.html. Updated January 28, 2019. Accessed May 13, 2019.

Carbon dioxide content measurement. EBSCO DynaMed website. Available at:http://www.dynamed.com/topics/dmp~AN~T906779/Carbon-dioxide-content-measurement. Updated October 8, 2018. Accessed May 13, 2019.

Last reviewed March 2019 by EBSCO Medical Review Board James P. Cornell, MD  Last Updated: 5/13/2019