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What is Carbon Dioxide (CO2)? Blood Tests & Normal Levels

Written by Mathew Eng, PharmD | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Mathew Eng, PharmD | Last updated:
Carbon Dioxide

Carbon dioxide is in the air, in the ocean, and even in soda. You can also find carbon dioxide in your blood, where it plays an important role in regulating your breathing and maintaining the pH of your blood. Learn about the different ways to test carbon dioxide levels and what the results mean.

What is Carbon Dioxide?

Carbon dioxide, which has the chemical formula CO2, is a gas that is naturally found in the Earth’s atmosphere and the human body. When cells convert sugars and fats into energy, they create CO2 as a byproduct. This CO2 then travels through the blood and into the lungs, where it is exhaled out of the body [1].

Most of the CO2 in the body (usually over 90%) is actually in the form of bicarbonate (HCO3-). The rest is dissolved in the blood or found in the form of carbonic acid (H2CO3). CO2 naturally switches between these different forms depending on how acidic or basic the blood is, otherwise known as blood pH [1, 2].

CO2, especially in the form of bicarbonate, plays an important role in maintaining the pH of the blood. Bicarbonate acts as a buffer, preventing blood from becoming too acidic or basic [3].

The breathing rate is also regulated by CO2. A high level of CO2 in the blood tells the body that it’s not getting enough oxygen, which will trigger the urge to breathe [4].

The body creates CO2 as a byproduct of energy production. It helps maintain blood pH and regulates breathing.

Why test CO2 levels?

CO2 levels are most commonly used to evaluate the acid-base balance in the blood, which may be disturbed by issues in the lungs or kidneys. Three pieces of information are needed to get a full picture of acid-base status, which include [1]:

  • Blood pH, which is based on the concentration of hydrogen ions in the blood. Hydrogen ions are released when an acid reacts with water and is a marker for acidity.
  • Bicarbonate (HCO3-), the most common form of CO2 in your body. It plays an important role in maintaining blood pH.
  • Partial pressure of CO2 (pCO2), which represents the pressure that CO2 gas exerts in the blood.

Besides acid-base balance, CO2 levels are also helpful in monitoring lung diseases and the effectiveness of oxygen therapy [1].

Testing CO2 levels can reveal acid-base imbalances in the blood that may be due to lung or kidney problems.

Carbon Dioxide (CO2) Blood Test

Test Types

There are several different tests that measure CO2 levels in the blood. Each test measures a different form or aspect of CO2 and sometimes multiple tests are needed to get a full picture of an individual’s health.

Total Carbon Dioxide Test

The serum total carbon dioxide test (TCO2 test) measures the total amount of CO2 (in all forms) in the blood, including [1]:

  • CO2 bound to other compounds
  • HCO3-
  • H2CO3.

This test requires a blood sample, which a medical professional will usually take from a vein in the arm [1].

The TCO2 test does not measure the components typically used to evaluate acid-base balance (blood pH, HCO3-, and pCO2 are what is needed) [1].

However, since so much of a person’s CO2 is in the form of HCO3-, the total CO2 is often used as a surrogate for HCO3- levels. In fact, the terms CO2 and HCO3- are sometimes used interchangeably in terms of these tests [1].

Because the TCO2 test is fairly quick and easy to perform, it is usually the first test used to screen for acid-base disorders. If results are abnormal, additional tests may be performed [1].

The total carbon dioxide test estimates HCO3- levels and is used to screen for acid-base imbalances.

Arterial Blood Gas Test

The arterial blood gas (ABG) test is also commonly performed. It measures pCO2, pO2, and blood pH. In addition, it is possible to calculate HCO3- levels using the results of this test. This means the ABG test provides all the information needed to fully evaluate acid-base balance [5].

The ABG test is also a little more difficult to perform compared to other tests. This test requires a blood sample from the artery, which can be more painful and potentially has more complications compared to drawing blood from the veins [1, 6].

Despite being harder to perform, the ABG test is still one of the best ways to evaluate blood pH imbalances and is commonly performed whenever an acid-base disorder is suspected [1, 6].

The arterial blood gas gives the most complete picture of acid-base status in a patient.

Bicarbonate Test

The bicarbonate test measures only the amount of HCO3- in the blood using samples from the veins. It is less commonly used than the ABG test because it provides less information. However, a venous HCO3- test is also easier and cheaper to perform than an ABG test, making it sometimes more popular in rural areas or less developed countries [1, 6].

CO2 tests are used to evaluate the acid-base balance in the blood or lung issues.

Carbon Dioxide (CO2) Normal Levels

Normal ranges can vary between laboratories due to differences in equipment, techniques, and chemicals used. If your results are outside of the normal range, it may not necessarily mean there is something wrong. However, a normal result also doesn’t mean a particular medical condition is absent. Always talk with your doctor to learn more about your test results.

By Test Type

For total CO2 in the blood, the normal range is about 23 to 30 mEq/L [2].

For HCO3- in the blood, the normal range is about 22 to 26 mEq/L [7].

For pCO2 in the arteries, the normal range is about 35 to 45 mmHg [7].

However, these ranges can vary depending on many factors, such as [1]:

  • Gender: men usually have slightly higher CO2 levels than women
  • Blood sample source: venous blood has slightly higher CO2 levels than arterial blood
  • Altitude: higher elevations may lower CO2 levels

Learn More

This post is part of a three-part series about carbon dioxide. Read the other parts to learn about:


Carbon dioxide (CO2) blood tests are normally used to evaluate issues with your breathing or imbalances in your blood pH. Many factors can affect your CO2 levels, most of which stem from kidney and lung disorders. Correcting an abnormal CO2 level usually involves treating the underlying condition that is causing the imbalance.

About the Author

Mathew Eng

Mathew Eng

Mathew received his PharmD from the University of Hawaii and an undergraduate degree in Biology from the University of Washington.
Mathew is a licensed pharmacist with clinical experience in oncology, infectious disease, and diabetes management. He has a passion for personalized patient care and believes that education is essential to living a healthy life. His goal is to motivate individuals to find ways to manage their chronic conditions.


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