CO2 Blood Test
A CO2 blood test measures the amount of carbon dioxide (CO2) in the blood serum. You may receive a CO2 test as a part of a metabolic panel.

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What Is a CO2 Blood Test?

A CO2 blood test measures the amount of carbon dioxide (CO2) in the blood serum, or liquid part of your blood. A CO2 test may also be called:

  • a carbon dioxide test
  • a TCO2 test
  • a total CO2 test
  • bicarbonate test
  • an HCO3 test
  • a CO2 test-serum

You may receive a CO2 test as a part of a metabolic panel. A metabolic panel is a group of tests that measures electrolytes and blood gases.

The body contains two major forms of CO2:

  • HCO3 (bicarbonate, the main form of CO2 in the body)
  • PCO2 (carbon dioxide)

Your doctor can use this test to determine if there’s an imbalance between the oxygen and carbon dioxide in your blood or a pH imbalance in your blood. These imbalances can be signs of a kidney, respiratory, or metabolic disorder.

Why the CO2 Blood Test Is Ordered

Your doctor will order a CO2 blood test based on your symptoms. Signs of an imbalance of oxygen and carbon dioxide or a pH imbalance include:

  • shortness of breath
  • other breathing difficulties
  • nausea
  • vomiting

These symptoms may point to lung dysfunction involving the exchange between oxygen and carbon dioxide. Your blood’s oxygen and carbon dioxide levels will need to be measured frequently if you’re on oxygen therapy or having certain surgeries.

How a Venipuncture Blood Sample Is Taken

Venipuncture is the term used to describe a basic blood sample taken from a vein. Your doctor will order a simple venipuncture blood sample if they only want to measure HCO3.

A healthcare provider will take a venipuncture sample by drawing blood from a vein, usually on the underside of the elbow. Your provider will clean the site with a germ-killing antiseptic. Then, they will wrap an elastic band around your arm, above the vein. This causes the vein to swell with blood.

The healthcare provider will then gently insert a needle into the vein. Blood will collect in the syringe tube. When the tube is full, the elastic band is released and then the needle is removed. They will then cover the puncture wound with sterile gauze to stop bleeding.

How an Arterial Blood Sample Is Taken

Blood gas analysis is often a part of the CO2 test. An arterial blood sample will be necessary for this test. This is a more complicated procedure, which is done by a practitioner trained to access arteries safely. Unlike the venipuncture test, the arterial blood sample is taken from an artery.

A blood gas analysis requires arterial blood because the gases and pH levels in arterial blood are different from venous blood (blood from a vein). The arteries carry oxygen to the body, and the veins carry waste to the lungs and kidneys.

The practitioner usually collects arterial blood through an artery in the wrist called the radial artery. This is the major artery just below the thumb, where you can feel your pulse. They can also collect arterial blood from the brachial artery in the elbow or the femoral artery in the groin.

The practitioner will clean the site with a germ-killing antiseptic. The doctor will then gently insert a needle into the artery and draw blood into a tube. When the tube is full, they will remove the needle.

The puncture site may take a few minutes to stop bleeding because the arteries carry pumped blood, which doesn’t clot rapidly. After drawing blood, the will apply pressure firmly to the wound for at least five minutes to ensure the bleeding stops. They will place a wrap tightly around the puncture site. It will need to remain in place for at least an hour.

How to Prepare for Your Blood Test

Your doctor may ask you to fast, or stop eating and drinking, before the blood test. If you take corticosteroids or use antacids, your doctor will probably ask you to stop taking them. These drugs increase the concentration of bicarbonate in the body.

Risks of a CO2 Blood Test

There are slight risks associated with both venipuncture and arterial blood tests. These include:

  • excessive bleeding
  • fainting
  • lightheadedness
  • hematoma, which is a lump of blood under the skin
  • infection at the puncture site

After the blood draw, your practitioner will enusre that you’re feeling well and will tell you how to care for the puncture site to reduce the chances of infection.

Test Results

The normal range for CO2 is 23 to 29 mEq/L (milliequivalent units per liter of blood).

The blood test often measures blood pH along with CO2 levels to further determine the cause of your symptoms. Blood pH is a measurement of acidity or alkalinity. Alkalosis is when your body fluids are too alkaline. Acidosis, on the other hand, is when your body fluids are too acidic.

A measurement of 7.0 is considered neutral, and is halfway between acidic and alkaline on the scale. If your pH measurement is less than 7.0 on the scale, your pH is acidic. A substance is more alkaline as its pH measurement increases.

Low Bicarbonate

A test result of low bicarbonate and low pH (less than 7.4) is a condition called metabolic acidosis. Common causes are:

  • kidney failure
  • severe diarrhea
  • lactic acidosis
  • seizures
  • cancer
  • prolonged lack of oxygen from severe anemia, heart failure, or shock
  • diabetic ketoacidosis (diabetic acidosis)

A test result of low bicarbonate and high pH (more than 7.4) is a condition called respiratory alkalosis. Common causes are:

  • hyperventilation
  • fever
  • pain
  • anxiety

High Bicarbonate

A test result of high bicarbonate and low pH (less than 7.4) is a condition called respiratory acidosis. Common causes are:

  • pneumonia
  • chronic obstructive pulmonary disease (COPD)
  • asthma
  • pulmonary fibrosis
  • exposure to toxic chemicals
  • drugs that suppress breathing, especially when they’re combined with alcohol
  • tuberculosis
  • lung cancer
  • pulmonary hypertension
  • severe obesity

A test result of high bicarbonate and high pH (more than 7.4) is a condition called metabolic alkalosis. Common causes are:

  • chronic vomiting
  • low potassium levels
  • hypoventilation, which involves slowed breathing and decreased CO2 elimination

Long-Term Outlook

If your doctor finds a CO2 imbalance suggesting acidosis or alkalosis, they will look into the cause of this imbalance and treat it appropriately. Because the causes vary, treatment could mean anything from lifestyle changes to medications to surgery.

Written by: Sandy Calhoun Rice
Edited by:
Medically Reviewed by: Deborah Weatherspoon, PhD, RN, CRANA, COI
Published: Oct 13, 2015
Published By: Healthline Networks, Inc.
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