Also Known As
Total CO2
TCO2
Carbon Dioxide Content
CO2 Content
Bicarb
HCO3-
Formal Name
Bicarbonate
This article was last reviewed on
This article waslast modified on
May 25, 2018.
At a Glance
Why Get Tested?

As part of an electrolyte panel to identify or monitor an electrolyte imbalance or acid-base (pH) imbalance

When To Get Tested?

During a routine physical or as recommended by your healthcare practitioner if you are experiencing symptoms such as weakness, confusion, prolonged vomiting, or breathing problems that could indicate an electrolyte imbalance or an acid-base imbalance (commonly described as acidosis or alkalosis)

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

None

You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

Bicarbonate (Total CO2) Reference Range

The reference ranges1 provided here represent a theoretical guideline that should not be used to interpret your test results. Some variation is likely between these numbers and the reference range reported by the lab that ran your test. Please consult your healthcare provider. 

Age Conventional Units2 SI Units3
0-18 years Not available due to wide variability. See child's lab report for reference range.
Adult 23-29 mEq/L 23-29 mmol/L
>60 years 23-31 mEq/L 23-31 mmol/L
>90 years 20-29 mEq/L 20-29 mmol/L

1 from Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Burtis CA, Ashwood ER, Bruns DE, eds. 5th edition, St. Louis: Elsevier Saunders; 2011.

2 Conventional Units are typically used for reporting results in U.S. labs

3 SI Units are used to report lab results outside of the U.S.

What is being tested?

Bicarbonate is an electrolyte, a negatively charged ion that is used by the body to help maintain the body's acid-base (pH) balance. It also works with the other electrolytes (sodium, potassium, and chloride) to maintain electrical neutrality at the cellular level. This test measures the total amount of carbon dioxide (CO2) in the blood, which occurs mostly in the form of bicarbonate (HCO3-). The CO2 is mainly a by-product of various metabolic processes.

Measuring bicarbonate as part of an electrolyte or metabolic panel may help diagnose an electrolyte imbalance or acidosis or alkalosis. Acidosis and alkalosis describe the abnormal conditions that result from an imbalance in the pH of the blood caused by an excess of acid or alkali (base). This imbalance is typically caused by some underlying condition or disease.

The lungs and kidneys are the major organs involved in regulating blood pH through the removal of excess bicarbonate.

  • The lungs flush acid out of the body by exhaling CO2. Raising and lowering the respiratory rate alters the amount of CO2 that is breathed out, and this can affect blood pH within minutes.
  • The kidneys eliminate acids in the urine and they regulate the concentration of bicarbonate (HCO3-, a base) in blood. Acid-base changes due to increases or decreases in HCO3- concentration occur more slowly than changes in CO2, taking hours or days.

Any disease or condition that affects the lungs, kidneys, metabolism, or breathing has the potential to cause acidosis or alkalosis.

The bicarbonate test gives a healthcare practitioner a rough estimate of a patient's acid-base balance. This is usually sufficient, but measurements of gases dissolved in the blood (blood gases) may be done if more information is needed. Bicarbonate is typically measured along with sodium, potassium, and possibly chloride in an electrolyte panel as it is the balance of these molecules that gives the healthcare practitioner the most information.

How is the sample collected for testing?

A blood sample is drawn by needle from a vein in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

Accordion Title
Common Questions
  • How is it used?

    The bicarbonate (or total CO2) test is usually ordered along with sodium, potassium, and chloride as part of an electrolyte panel. The electrolyte panel is used to help detect, evaluate, and monitor electrolyte imbalances and/or acid-base (pH) imbalances (acidosis or alkalosis). It may be ordered as part of a routine exam or to help evaluate a variety of chronic or acute illnesses.

    An electrolyte panel may be used to help monitor conditions, such as kidney disease, lung disorders, and high blood pressure (hypertension). When acidosis or alkalosis is identified, bicarbonate (as part of the electrolyte panel) and blood gases may be ordered to evaluate the severity of the pH imbalance. These tests help determine whether it is primarily respiratory (due to an imbalance between the amount of oxygen coming in and CO2 being released) or metabolic (due to increased or decreased amounts of bicarbonate in the blood). They also help monitor treatment until acid-base balance is restored.

  • When is it ordered?

    Bicarbonate testing may be ordered, usually as part of an electrolyte panel, a basic metabolic panel (BMP), or a comprehensive metabolic panel (CMP), when a person has a routine health checkup.

    This testing may be ordered when acidosis or alkalosis is suspected or when someone has an acute condition with symptoms that may include the following:

    • Prolonged vomiting and/or diarrhea
    • Weakness, fatigue
    • Difficulty breathing (respiratory distress)

    Electrolytes may be ordered at regular intervals when a person has a disease or condition or is taking a medication that can cause an electrolyte imbalance. Electrolyte panels or basic metabolic panels are commonly used to monitor treatment of certain problems, including high blood pressure (hypertension), heart failure, and liver and kidney disease.

  • What does the test result mean?

    When bicarbonate levels are higher or lower than normal, it suggests that the body is having trouble maintaining its acid-base balance, either by failing to remove carbon dioxide through the lungs or the kidneys or perhaps because of an electrolyte imbalance, particularly a deficiency of potassium. Both of these imbalances may be due to a wide range of conditions.

    Examples of causes of a low bicarbonate level include:

    Examples of causes of a high bicarbonate level include:

  • Is there anything else I should know?

    Some drugs may increase bicarbonate levels including fludrocortisone, barbiturates, bicarbonates, hydrocortisone, loop diuretics, and steroids.

    Drugs that may decrease bicarbonate levels include methicillin, nitrofurantoin, tetracycline, thiazide diuretics, and triamterene.

  • If I've had a bicarbonate (total CO2) test, why does my doctor want to test my blood gases?

    Blood gas tests, in which blood is drawn from an artery instead of a vein, can give your healthcare practitioner more information about your acid-base balance. They can tell your provider whether your lungs are working properly to keep oxygen and carbon dioxide at healthy levels.

  • If bicarbonate levels are too high or low, what treatments can help?

    If your bicarbonate is high or low, your healthcare practitioner will identify and treat the underlying cause. For example, high bicarbonate may be caused by emphysema, which may be treated with oxygen therapy and medications, or by severe diarrhea or vomiting, which would be addressed by treating the cause of the diarrhea or vomiting. Low bicarbonate may be caused by diabetic ketoacidosis, for example, which can be addressed in part by fluid and electrolyte replacement and insulin therapy.

View Sources

Sources Used in Current Review

Dugdale, D. (2013 April 29, Updated). CO2 blood test. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003469.htm. Accessed 10/15/15.

(© 1995–2015). Bicarbonate, Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/876. Accessed 10/15/15.

Genzen, J. et. al. (2015 March, Updated). Metabolic Acidosis. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/MetabolicAcidosis.html. Accessed 10/15/15.

Byrd, R. and Roy, T. (2015 July 31). Respiratory Acidosis. Medscape Drugs & Diseases [On-line information]. Available online at http://emedicine.medscape.com/article/301574-overview. Accessed 10/15/15.

Lechtzin, N. (@ 2015). Exchanging Oxygen and Carbon Dioxide. Merck Manual Consumer Edition. [On-line information]. Available online at http://www.merckmanuals.com/home/lung-and-airway-disorders/biology-of-the-lungs-and-airways/exchanging-oxygen-and-carbon-dioxide. Accessed 10/15/15.

Quinn, A. and Sinert, R. (2015 July 27, Updated). Metabolic Acidosis in Emergency Medicine. Medscape Drugs & Diseases. [On-line information]. Available online at http://emedicine.medscape.com/article/768268-overview. Accessed 10/15/15.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(1995-2004). Minerals and Electrolytes. The Merck Manual of Medical Information – Second Home Edition [On-line information]. Available online at http://www.merck.com/mmhe/sec12/ch155/ch155a.html?qt=electrolytes&alt=sh.

Ben-Joseph, E., Reviewed (2004 July). Dehydration. Familydoctor.org Information for Parents [On-line information]. Available online at http://www.kidshealth.org/PageManager.jsp?dn=familydoctor&lic=44&article_set=21646.

Webner, D., Updated (2003 August 18). CO2. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003469.htm.

A.D.A.M. editorial, Updated (2003 October 15). Electrolytes. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/002350.htm.

Voorhees, B (Updated May 17, 2007). MedlinePlus Medical Encyclopedia: CO2 test, serum. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003469.htm. Accessed July 2008.

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MedlinePlus Medical Encyclopedia. CO2 blood test. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003469.htm. Accessed September 2011.

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