A carbon dioxide (CO2) blood test measures the amount of carbon dioxide in the liquid part of your blood, called the serum. Carbon dioxide is a waste product produced by the body when it burns food for energy, and it is carried by the blood to the lungs where it is exhaled. Most of the carbon dioxide in the body is in the form of bicarbonate, which is a type of electrolyte that helps control the amount of fluid and the balance of acids and bases (pH balance) in the body. Therefore, the CO2 blood test is really a measure of the blood bicarbonate level.

The CO2 blood test is most often done as part of an electrolyte or basic metabolic panel, which is a group of blood tests that measures different chemicals in the blood. The test may be ordered as part of a regular check-up or if there are symptoms of an electrolyte imbalance, such as vomiting or diarrhea that won’t go away, trouble breathing, weakness, or tiredness. The test may also help diagnose or monitor conditions related to an electrolyte imbalance, such as high blood pressure, kidney diseases, lung diseases, Cushing disease, or Addison disease.

During the test, a healthcare professional will take a blood sample from a vein in the arm using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. The test usually takes less than five minutes. The normal range for CO2 levels in the blood is 23 to 29 milliequivalents per liter (mEq/L) or 23 to 29 millimoles per liter (mmol/L) for adults, but this may vary by lab. Abnormal test results may be a sign that the body is having a hard time keeping the right acid-base balance (pH balance) .