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Carbon Dioxide (CO2) in Blood
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Carbon Dioxide (CO2) in Blood

A carbon dioxide blood test measures the amount of CO2 in your blood to help evaluate your body's electrolyte balance and acid-base status.

What is a carbon dioxide (CO2) blood test?

This test measures the amount of carbon dioxide in your blood. Carbon dioxide is a waste product of metabolism—the process your body uses to convert food into energy. Most of this CO2 exists as bicarbonate (HCO3), an electrolyte that helps control fluid levels and the body's pH balance. Your blood carries CO2 to the lungs, where it is exhaled. Abnormal levels can indicate underlying health issues.

What is it used for?

The test is often part of an electrolyte panel during routine checkups. It is also used to diagnose or monitor conditions related to electrolyte imbalances, including high blood pressure and diseases of the kidneys, lungs, or liver.

Why do I need a CO2 in blood test?

Your provider may order this test if you show symptoms of an electrolyte imbalance, such as:

  • Trouble breathing
  • Confusion
  • Weakness or fatigue
  • Prolonged vomiting or diarrhea

It may also be used to check for side effects of certain medications.

What happens during a CO2 blood test?

A health care professional will collect a blood sample from a vein in your arm using a small needle. The procedure usually takes less than five minutes.

Preparation and Risks

Tell your provider about all medications you take, as some can affect CO2 levels. Do not stop taking any medication unless instructed. You may need to fast for several hours if other blood tests are ordered simultaneously. Risks are minimal, consisting of slight pain or bruising at the puncture site.

What do the results mean?

Abnormal results may indicate that your lungs or kidneys are struggling to maintain the proper pH balance.

Too much CO2 (hypercapnia/hypercarbia) may be a sign of:

  • Lung diseases
  • Cushing's syndrome
  • Kidney failure
  • Metabolic alkalosis (blood is not acidic enough, often due to vomiting or dehydration)

Too little CO2 (hypocarbia) may be a sign of:

  • Addison disease
  • Diabetic ketoacidosis
  • Shock
  • Metabolic acidosis (blood is too acidic, often due to kidney/liver disease or diarrhea)
  • Respiratory alkalosis (blood is not acidic enough due to breathing problems or hyperventilation)

Because other factors, including medications, can influence these levels, your provider may order follow-up tests to confirm a diagnosis.

Frequently Asked Questions

low CO2 level, known as metabolic acidosis, means your blood is more acidic than normal. This can happen if your body produces too much acid, such as in diabetic ketoacidosis (a serious complication of diabetes), or if your kidneys are not removing acid effectively due to kidney disease. It can also occur with severe diarrhea, which leads to a loss of bicarbonate. While mild fluctuations are common, significantly low levels require medical attention to address the underlying cause.
Fasting is not strictly required for a CO2 test alone. However, because this test is frequently ordered as part of a comprehensive metabolic panel (CMP) or basic metabolic panel (BMP), which includes tests for glucose and lipids, your provider may ask you to fast for eight to twelve hours. Always follow the specific instructions given by your healthcare provider or the lab, as they will clarify if fasting is necessary based on the full panel being ordered.
Yes, several medications can influence CO2 results. Diuretics, such as furosemide or hydrochlorothiazide, can raise CO2 levels by altering electrolyte balance. Corticosteroids and antacids containing bicarbonate can also increase levels. Conversely, some medications used for kidney stones or acid-base disorders can lower levels. It is crucial to provide a complete list of all prescription drugs, over-the-counter medicines, and supplements to your provider before the test to help them interpret the results accurately.
high CO2 level, often called metabolic alkalosis, can result from conditions that cause the body to lose acid or gain base. Common causes include prolonged vomiting, which depletes stomach acid, or the use of diuretics (water pills) that affect kidney function. It can also be seen in severe dehydration or certain lung conditions where the body retains too much carbon dioxide. Your provider will interpret this in the context of your other electrolytes and medical history.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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