What Is a Carbon Dioxide Blood Test?

What to Expect When Undergoing This Test

Carbon dioxide (CO2) blood tests evaluate the presence of the gas 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.

Your body produces 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.

This article explains why your healthcare provider might order a carbon dioxide blood test and the types of diseases and disorders the results may suggest. It offers a step-by-step approach of what to expect during a carbon dioxide blood test.

Possible Carbon Dioxide Blood Test Result Interpretations

Verywell / Laura Porter

Purpose of the 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 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.

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.

A carbon dioxide blood test may be known as other names, including:

  • Carbon dioxide content
  • CO2 content
  • Bicarbonate blood test
  • Bicarbonate test
  • Total CO2
  • TCO2
  • HCO3
  • CO2 test-serum

Risks and Contraindications

The carbon dioxide blood test should be similar to any blood tests you’ve had done in the past, and the risks associated with it are typically considered low. There may be some circumstances that make it more difficult for a technician or healthcare provider to obtain a blood sample on the first try.

For example, if your veins are difficult to locate or they shift their position in the process of getting a sample, the needle might have to be inserted more than once to draw blood.

The chances of other problems arising with a blood draw are small, but they could include:

  • A slight poke or stinging feeling at the insertion site of the needle
  • Bruising at the site
  • Feeling lightheaded or faint
  • The formation of a hematoma (when the blood pools under the skin)
  • Excessive bleeding
  • A throbbing feeling after the test, especially if the needle was inserted more than one time
  • Phlebitis (also known as a swollen vein)
  • An infection

In general, the risks and contraindications for the CO2 test are minimal.

Keep the bandage for the length of time the clinician instructs you—this will reduce the risk of bruising. Should you experience phlebitis, you should alert your health provider. They may recommend elevating the affected arm and warm compresses.

Before the Test

Be sure to tell your healthcare provider about any medications you’re taking, even over-the-counter ones. Drugs like corticosteroids or antacids can skew the lab results, and your practitioner will want to gather the most helpful information to figure out why you’re not feeling well.

If you have other tests done at the same time, you may be asked to fast for several hours before the blood sample is taken.

Your healthcare provider should provide you with any specific instructions you need to follow on the day of the test.

Make sure you have your insurance card with you and a form of identification so that there’s no delay in your ability to get the test. Most CO2 blood tests are often run as part of routine blood work, but you might want to speak with your insurance carrier regarding the out-of-pocket expenses you could be responsible for paying so that you’re not surprised by an unexpected bill.

During the Test

Your blood will be drawn for a CO2 blood test in the typical way that many blood tests are done. Most likely, you’ll be seated in a chair and your healthcare provider will wrap an elastic band around your arm, which will temporarily restrict blood flow and allow them to locate a vein.

Once they have found a vein, they’ll prepare the area with a disinfectant using an alcohol pad or an alcohol-moistened cotton ball prior to inserting the needle to draw your blood.

After the needle has entered your vein, the clinician or technician will place a vial at the end of the syringe to collect your blood sample. When they have drawn enough blood for the test, they’ll remove the elastic band and place a piece of gauze or a cotton ball over it.

They may ask you to put pressure on the site for a minute or two before covering it with an adhesive. Generally, the test takes only a few minutes to complete.

Interpreting the Results

When you receive your test results, remember that they are a guide to help your healthcare provider figure out what’s going on with you and why you may not be feeling well. Also, tests performed at different labs may yield different results.

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.

If you have too much CO2 in your blood, this could suggest:

If your blood levels indicate your CO2 is too low, this may point to:

  • Hyperventilation, which causes respiratory alkalosis and a compensatory metabolic acidosis
  • Excessive alcohol or drug consumption
  • Malnutrition
  • Hyperthyroidism (excessive thyroid activity)
  • Complications from type 1 or type 2 diabetes such as ketoacidosis
  • Impaired kidney function
  • Adrenal gland insufficiency like Addison’s disease


A carbon dioxide (CO2) blood test measures the amount of CO2 in your body. 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. Once you have the results, your healthcare provider will discuss them with you.

A Word From Verywell

If you have any questions or concerns regarding the carbon dioxide blood test, be sure to talk with your healthcare provider ahead of time so that you can feel informed about the procedure.

3 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. 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

  2. World Health Organization. Best Practices for Injections and Related Procedures Toolkit.

  3. Icahn School of Medicine at Mount Sinai. CO2 blood test.

By Jenny Lelwica Buttaccio, OTR/L
Jenny Lelwica Buttaccio, OTR/L, is a licensed occupational therapist and advocate for patients with Lyme disease.