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 carbon dioxide blood test and find out what can make your levels too high or too low.
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 your cells convert sugars and fats into energy, they create CO2 as a byproduct. CO2 then travels through the blood and into the lungs, where it is exhaled .
Most of the CO2 in your body, usually over 90%, is actually in the form of bicarbonate (HCO3-). The rest is dissolved in the blood or found in 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].
Carbon dioxide, especially in the form of bicarbonate, plays an important role in maintaining the pH of the blood. Bicarbonate acts as a buffer, preventing your blood from becoming too acidic or basic .
Your breathing rate is also regulated by CO2. A high level of CO2 in your blood tells your body that you’re not getting enough oxygen, which will increase your urge to breathe .
Why test CO2 levels?
CO2 levels are most commonly used to evaluate the acid-base balance in the blood, which may be caused by issues in your lungs or kidneys. Three pieces of information are needed to get a full picture of acid-base status, which include :
- 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.
Carbon Dioxide (CO2) Blood Test
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 your health.
Total Carbon Dioxide Test
The serum total carbon dioxide test (TCO2 test) measures the amount of all the forms of CO2 in the blood, including CO2 bound to other compounds, HCO3-, and H2CO3. This test requires a blood sample, which a medical professional will take from a vein in your arm .
The TCO2 test does not measure the components typically used to evaluate your acid-base balance (blood pH, HCO3-, and pCO2 are what is needed). However, since so much of your CO2 is in the form of HCO3-, your total CO2 is often used as a surrogate for your HCO3- levels. In fact, the terms CO2 and HCO3- are sometimes used interchangeably in terms of these tests .
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 .
Arterial Blood Gas Test
The arterial blood gas (ABG) test is also fairly common. It measures pCO2, pO2, and blood pH. In addition, it is possible to calculate your HCO3- level using the results of this test. This means the ABG test provides all the information needed to fully evaluate your acid-base balance .
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].
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].
Carbon Dioxide (CO2) Normal Levels
By Test Type
For total CO2 in the blood, the normal range is about 23 to 30 mEq/L .
For HCO3- in the blood, the normal range is about 22 to 26 mEq/L .
For pCO2 in the arteries, the normal range is about 35 to 45 mmHg 
However, these ranges can vary depending on many factors, such as :
- 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
Normal ranges can also vary depending on the laboratory. Different hospitals tend to have their own reference ranges for what they consider normal .
What Does a Carbon Dioxide Level of 33 Mean?
You’ve just had your carbon dioxide levels checked and the results come back as 33 mEq/L. What exactly does this mean for you?
In most cases, this value would be considered slightly high. This could mean you have a breathing issue, causing a buildup of CO2. This could also be a sign that your blood pH is becoming too high. Luckily, mild elevations like this usually don’t cause symptoms .
On the other hand, it could be that your CO2 is not elevated at all. Some laboratories consider ranges as high as 33 to 35 mEq/L to be normal. In the end, your doctor will have to evaluate many other factors, such as the health of your lungs and kidneys, before deciding if this test result is concerning .
This post is part of a three-part series about carbon dioxide. Read the other parts to learn about:
- Carbon dioxide poisoning and the causes & risks of high CO2 levels
- The causes & risks of low CO2 levels
Irregular CO2 Levels?
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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.