Carbon Dioxide (CO2)

What is Carbon Dioxide?

Your body produces Carbon Dioxide (CO2) gas as a byproduct. It's carried by the bloodstream to your lungs, primarily in a bicarbonate (HCO3) form, and then exhaled out while breathing. In a healthy individual, the presence of CO2 in the blood stays within a normal range and doesn’t present any problems.

Carbon Dioxide (CO2) is one of the most important buffer systems in maintaining normal blood and body fluid acid-base balance (pH). Carbon Dioxide (CO2) is an electrolyte, a negatively charged ion used by the body to help maintain the acid-base balance in the body. It also works with other electrolytes (sodium, potassium, and chloride) to maintain electrical neutrality at the cellular level.

In the body (as mentioned above), 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.

What is the Carbon Dioxide (CO2) blood test?

A Carbon Dioxide (CO2) blood test evaluates the presence of the gas Carbon Dioxide (CO2) in your blood. If your CO2 levels rise too high or fall too low, the test results may be an indication that you have a health condition that needs diagnosis and treatment.

This test is also known as: Carbon dioxide content, CO2 content, Bicarbonate blood test, Bicarbonate test, Total CO2, TCO2, HCO3, CO2 test-serum.

A Carbon Dioxide (CO2) test gives a health care practitioner a rough estimate of your acid-base balance. This is usually sufficient, but measurements of gasses dissolved in the blood (blood gasses) may be done if more information is needed.

Your CO2 level needs to stay within a certain range, but when it's too high (or too low) the blood test results can help your healthcare provider to identify and diagnose a health condition.

There are a number of diseases and disorders that can cause changes in CO2 levels, but keep in mind that not all findings outside of the normal ranges on a carbon dioxide blood test indicate a serious illness. Dehydration, for example, can cause high CO2 levels.

The blood test itself is straightforward and rarely leads to complications.

You may receive a CO2 test as a part of a metabolic panel. A metabolic panel is a group of tests that measure electrolytes and blood gasses.

Why do I need a Carbon Dioxide (CO2) test?

Typically, a CO2 blood test is done in conjunction with an electrolyte panel, which measures sodium, potassium, and chloride levels, or as part of a Comprehensive Metabolic Panel. Electrolytes are an integral part of the way your body regulates its fluid balance and maintains appropriate acid-base (pH) levels.

Additionally, your healthcare provider may use this test to monitor other health conditions, such as those that affect the kidneys, liver, blood pressure, and more. It may also be a helpful test in monitoring the effects of some medications.

Your doctor will order a CO2 blood test based on your symptoms. Signs of an imbalance of oxygen and carbon dioxide or a pH imbalance include:

- shortness of breath

- other breathing difficulties

- nausea

- vomiting

These symptoms may point to lung dysfunction involving the exchange between oxygen and carbon dioxide.

You will need to have your blood’s oxygen and carbon dioxide levels measured frequently if you’re on oxygen therapy or having certain surgeries.

The blood test often measures blood pH along with CO2 levels to further determine the cause of your symptoms. Blood pH is a measurement of acidity or alkalinity. Alkalosis is when your body fluids are too alkaline. Acidosis, on the other hand, is when your body fluids are too acidic.

What is the difference between Bicarbonate and Carbon Dioxide?

A bicarbonate (HCO3-) test is part of an electrolyte panel, or metabolic panel used to identify or monitor an electrolyte imbalance or acid-base (pH) imbalance. This test measures the total amount of carbon dioxide (CO2) in the blood, which occurs mostly in the form of HCO3–. Measuring HCO3– as part of an electrolyte or metabolic panel may also help diagnose acidosis or alkalosis.

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.

Bicarbonate (HCO3) serves a vital purpose in your blood—it helps keep the body’s acids and bases in check. The purpose of the carbon dioxide (CO2) blood test is to confirm whether or not there’s a fluctuation in your CO2 levels and an electrolyte imbalance in your body. 

Sometimes it is useful to measure the amount of carbon dioxide in the arteries along with the HCO3 in your veins. This is most often helpful in people with a lung disorder, used to determine how well the lungs are functioning. It's called an arterial blood gases (ABG) test and the blood is taken from an artery rather than a vein.

Bicarbonate is a form of carbon dioxide (CO2), a gas waste left when your body burns food for energy. Bicarbonate belongs to a group of electrolytes, which help keep your body hydrated and make sure your blood has the right amount of acidity. Too much or too little bicarbonate can be a sign of a number of conditions, including diarrhea, liver failure, kidney disease, and anorexia.

Normal Carbon Dioxide (CO2) results:

In general, adult CO2 levels should be between 20 - 29  milliequivalents per liter (mEq/L).

Keep in mind that a test that falls outside the normal values of the reference range doesn’t automatically indicate that you have a medical condition. There can be other elements, such as medications you might be taking, that contribute to your results.

CO2 levels are typically interpreted along with results from other tests done at the same time, such as the other electrolytes.

Sources:

Acidosis and alkalosis -- https://labtestsonline.org.uk/conditions/acidosis-and-alkalosis

Carbon dioxide (blood) -- https://www.urmc.rochester.edu/encyclopedia/content.aspx?ContentTypeID=167&ContentID=carbon_dioxide_blood

Icahn School of Medicine at Mount Sinai. CO2 blood test -- www.mountsinai.org/health-library/tests/co2-blood-test

References:

Csaba P. Kovesdy, Metabolic acidosis and kidney disease: does bicarbonate therapy slow the progression of CKD?, Nephrology Dialysis Transplantation, Volume 27, Issue 8, August 2012, Pages 3056–3062, https://doi.org/10.1093/ndt/gfs291

Bijapur MB, Kudligi NA, Asma S. Central Venous Blood Gas Analysis: An Alternative to Arterial Blood Gas Analysis for pH, PCO2, Bicarbonate, Sodium, Potassium and Chloride in the Intensive Care Unit Patients. Indian J Crit Care Med. 2019 Jun;23(6):258-262. doi: 10.5005/jp-journals-10071-23176. PMID: 31435143; PMCID: PMC6698350.

Seifter JL, Chang HY. Disorders of Acid-Base Balance: New Perspectives. Kidney Dis (Basel). 2017 Jan;2(4):170-186. doi: 10.1159/000453028. Epub 2016 Dec 10. PMID: 28232934; PMCID: PMC5260542.