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Also Known As:
Total CO2
TCO2
Carbon Dioxide Content
CO2 Content
Bicarb
HCO3-

This article was last modified on November 9, 2021

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Electrolytes and Anion Gap

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At a Glance

Why Get Tested?

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

When To Get Tested?

During a routine health exam or as recommended by your health care 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 (acidosis or alkalosis)

Sample Required?

A blood sample drawn from a vein

Test Preparation Needed?

None

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 your 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.

Common Questions

How is the test used?

The bicarbonate (or total CO₂) test is usually ordered along with sodium, potassium, and chloride as part of an electrolyte panel, which is included in a basic metabolic panel (BMP) and a comprehensive metabolic panel (CMP). A calculated bicarbonate level might also be reported as part of a blood gas panel.

The electrolyte panel may be used to help detect, evaluate, and monitor electrolyte imbalances and/or acid-base (pH) imbalances (acidosis or alkalosis). The tests may be used as part of a routine health exam or to help evaluate and monitor a variety of chronic or acute illnesses, such as:

Kidney disease
Liver disease
Lung disorders
High blood pressure (hypertension)

When acidosis or alkalosis is identified, bicarbonate tests 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 imbalance in the amount of acid produced by the body or ability of the kidneys to remove acid from the body). They also help monitor treatment until acid-base balance is restored.

When is it ordered?

Bicarbonate testing may be ordered when you have a routine health checkup.

This testing may also be ordered when acidosis or alkalosis is suspected or when you have an acute condition with symptoms such as:

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

Electrolytes may be ordered at regular intervals when you have a disease or condition or are taking a medication that can cause an electrolyte imbalance.

What does the test result mean?

Bicarbonate levels are typically interpreted along with results from other tests done at the same time, such as the other electrolytes.

A bicarbonate level that is higher or lower than normal may mean 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 conditions that can cause a low bicarbonate level include:

Addison disease
Chronic diarrhea
Diabetic ketoacidosis
Metabolic acidosis
Respiratory alkalosis, which can be caused by hyperventilation
Shock
Kidney disease
Ethylene glycol or methanol poisoning
Salicylate (aspirin) overdose

Examples of conditions that can cause a high bicarbonate level include:

Severe, prolonged vomiting and/or diarrhea
Lung diseases, including COPD
Cushing syndrome
Conn syndrome
Metabolic alkalosis

Bicarbonate 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.

My bicarbonate level is only slightly out of range. What does this mean?

Your bicarbonate result is interpreted by your health care practitioner within the context of other tests that you have had done as well as other factors, such as your medical history. A single high or low result may or may not have medical significance. Generally, this is the case when the test value is only slightly higher or lower than the reference range. This is why healthcare practitioners may repeat a test on you and why they may look at results from prior times when you had the same test performed.

On the other hand, a result outside the range may indicate a problem and warrant further investigation. Your healthcare practitioner will evaluate your test results and determine whether a result that falls outside of the reference range means something significant for you.

If I've had a bicarbonate (total CO2) test, why does my healthcare practitioner 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.

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, triamterene, calcium inhibitors, anti-viral/HIV drugs, and valproic acid.

View Sources

Sources Used in Current Review

2019 review by Patrick Sheehy, C(ASCP) and the Editorial Review Board.

(December 16, 2015) Drug-Induced Metabolic Acidosis. Pham, Amy Quynh Trang et al. Available online at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4754009/#. Accessed on 07/02/2019.

(May 17, 2002) Guidelines for Using Antiretroviral Agents Among HIV-Infected Adults and Adolescents. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/mmwr/preview/mmwrhtml/rr5107a1.htm. Accessed on 07/02/2019.

(November 20, 2017) Metabolic Acidosis. MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/ency/article/000335.htm. Accessed on 07/15/2019.

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.

Clarke, W and Dufour D R, Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pp 321-322.

Pagana K, Pagana T. Mosby’s Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006. Pp 157-159.

Pagana and Pagana. Mosby’s Manual of Diagnostic and Laboratory Tests. Fourth Edition. Pp 152-154.

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

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.

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.

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Bicarbonate (Total CO2)

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