A CO2 blood test measures the amount of carbon dioxide (CO2) in the blood serum, or liquid part of the blood. Your doctor can use this test to learn whether there is an oxygen/carbon dioxide imbalance, or pH imbalance, in your blood. These imbalances may be a sign of a kidney, respiratory, or metabolic disorder.
The body contains two major forms of CO2:
- HCO3 (bicarbonate, the main form of CO2 in the body)
- PCO2 (carbon dioxide)
A CO2 test may be performed as part of metabolic panel (a group of tests that measure electrolytes and blood gases).
A CO2 test may also be called a carbon dioxide test, TCO2, total CO2, bicarbonate test, HCO3, or CO2 test–serum.
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
These symptoms may point to lung dysfunction involving the exchange between oxygen and carbon dioxide. If you are on oxygen therapy, or during certain surgeries, your blood’s oxygen and carbon dioxide levels will need to be measured frequently.
If only HCO3 is being measured, a simple venipuncture blood sample will be taken. Venipuncture is the term used to describe a basic blood sample taken from a vein.
To do this, a healthcare provider draws blood from a vein, usually on the underside of the elbow. Your provider will clean the site with a germ-killing antiseptic. Then, he or she will wrap an elastic band around your arm, above the vein. This causes the vein to swell with blood.
The healthcare provider gently inserts a needle into the vein. Blood collects in the syringe tube. When the tube is full, the needle is removed. The elastic band is then removed, and the puncture wound is covered with sterile gauze to stop bleeding.
Blood gas analysis is often done as part of the CO2 test. An arterial blood sample will be needed for this test. This is a more complicated procedure, done by a physician specially trained to access arteries safely. Unlike the venipuncture test, the arterial blood sample is taken from an artery.
Arterial blood is required because the gases and pH levels are different from venous blood (from a vein). The arteries carry oxygen to the body, and the veins carry waste to the lungs and kidneys.
Collection of arterial blood is usually done 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. Arterial blood may also be collected from the brachial artery in the elbow, or from the femoral artery in the groin.
The site is cleaned with a germ-killing antiseptic. The doctor gently inserts a needle into the artery and draws blood into a tube. When the tube is full, the needle is removed.
Because the arteries carry pumped blood, which doesn’t clot rapidly, the puncture site may take a few minutes to stop bleeding. After drawing blood, the physician will apply pressure firmly to the wound for at least five minutes to ensure the bleeding stops. A wrap will be placed tightly around the wound. It will need to remain in place for at least an hour.
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 are known to increase the concentration of bicarbonate in the body.
There are slight risks associated with both venipuncture and arterial blood tests. These include:
- excessive bleeding
- hematoma (a lump of blood under the skin)
- infection at the puncture site
After the blood draw, your doctor will ensure that you are feeling well and will tell you how to care for the puncture site to reduce the chances of infection.
The normal range for CO2 is 23 to 29 mEq/L (milliequivalent units per liter of blood).
Blood pH is often measured along with CO2 levels to further determine the cause of your symptoms. Blood pH is a measurement of acidity or alkalinity. Alkalosis is the term used when your bodily fluids are too alkaline. Acidosis, on the other hand, is the term used when your bodily 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, it is considered acidic. A substance is more alkaline as its pH measurement increases.
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
- 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:
A test result of high bicarbonate and low pH (less than 7.4) is a condition called respiratory acidosis. Common causes are lung diseases such as:
- COPD (chronic obstructive pulmonary disease)
- pulmonary fibrosis
- exposure to toxic chemicals
- drugs that suppress breathing (narcotics, especially when combined with alcohol)
- 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 (slowed breathing and decreased CO2 elimination)
If your doctor finds a CO2 imbalance suggesting acidosis or alkalosis, he or she will look into the cause of this imbalance and treat appropriately. Because the causes vary dramatically, treatment could mean anything from lifestyle changes to medications to surgery.