A CO2 blood test measures the amount of carbon dioxide (CO2) in the blood serum, which is the liquid part of blood. It’s typically used to screen for kidney, respiratory, or metabolic disorders.
You may receive a CO2 test as a part of a metabolic panel. A metabolic panel is a group of tests that measure electrolytes and blood gases.
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
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.
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.
You will need to have your blood’s oxygen and carbon dioxide levels measured frequently if you’re on oxygen therapy or having certain surgeries.
Blood samples for a CO2 blood test may be taken from either a vein or an artery.
Venipuncture blood sample
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.
To get a venipuncture blood sample, a healthcare provider:
- cleans the site (often the inside of the elbow) with a germ-killing antiseptic
- wraps an elastic band around your upper arm to cause the vein to swell with blood
- gently inserts a needle into the vein and collect blood in the attached tube until it is full
- removes the elastic band and the needle
- covers the puncture wound with sterile gauze to stop any bleeding
Arterial blood sample
Blood gas analysis is often a part of the CO2 test. A blood gas analysis requires arterial blood because the gases and pH levels in the arteries different from venous blood (blood from a vein).
Arteries carry oxygen throughout the body. Veins carry metabolic waste and deoxygenated blood to the lungs to be exhaled as carbon dioxide and to the kidneys to be passed in urine.
This more complicated procedure is done by a practitioner trained to safely access arteries. Arterial blood is usually taken from an artery in the wrist called the radial artery. This is the major artery in line with the thumb, where you can feel your pulse.
Or, blood can be collected from the brachial artery in the elbow or the femoral artery in the groin. To get an arterial blood sample, the practitioner:
- cleans the site with a germ-killing antiseptic
- gently inserts a needle into the artery and draws blood into an attached tube until it is full
- removes the needle
- applies pressure firmly to the wound for at least five minutes to ensure the bleeding stops. (Arteries carry blood at higher pressures than veins, so it takes more time for the blood to form a clot.)
- puts a tight wrap around the puncture site that will need to stay in place for at least an hour
Your doctor may ask you to fast, or stop eating and drinking, before the blood test. Your doctor may also ask you to stop taking certain medications before the test such as corticosteroids or antacids. These drugs 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, which is a lump of blood under the skin
- infection at the puncture site
After the blood draw, your practitioner will ensure that you’re feeling well and will tell you how to care for the puncture site to reduce the chance of infection.
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.
Typically, a blood is slightly basic with pH measurement of close to 7.4 in maintained by the body. Normal range from 7.35 to 7.45 is considered neutral. A blood pH measurement less than 7.35 is considered acidic. A substance is more alkaline when its blood pH measurement is greater than 7.45.
Low bicarbonate (HCO3)
A test result of low bicarbonate and low pH (less than 7.35) 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 7.45) is a condition called respiratory alkalosis. Common causes are:
High bicarbonate (HCO3)
A test result of high bicarbonate and low pH (less than 7.35) is a condition called respiratory acidosis. Common causes are:
- chronic obstructive pulmonary disease (COPD)
- pulmonary fibrosis
- exposure to toxic chemicals
- drugs that suppress breathing, especially when they’re combined with alcohol
- lung cancer
- pulmonary hypertension
- severe obesity
A test result of high bicarbonate and high pH (more than 7.45) is a condition called metabolic alkalosis. Common causes are:
- chronic vomiting
- low potassium levels
- hypoventilation, which involves slowed breathing and decreased CO2 elimination
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 involve a combination of lifestyle changes, medications, and surgery.