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An arterial blood gases (ABG) test measures the acidity (pH) and the levels of oxygen and carbon dioxide in the blood from an artery. This test is used to find out how well your lungs are able to move oxygen into the blood and remove carbon dioxide from the blood.
As blood passes through your lungs, oxygen moves into the blood while carbon dioxide moves out of the blood into the lungs. An ABG test uses blood drawn from an artery, where the oxygen and carbon dioxide levels can be measured before they enter body tissues. An ABG measures:
This measures the pressure of oxygen dissolved in the blood and how well oxygen is able to move from the airspace of the lungs into the blood.
This measures the pressure of carbon dioxide dissolved in the blood and how well carbon dioxide is able to move out of the body.
The pH measures hydrogen ions (H+) in blood. The pH of blood is usually between 7.35 and 7.45. A pH of less than 7.0 is called acid and a pH greater than 7.0 is called basic (alkaline). So blood is slightly basic.
Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.
O2 content measures the amount of oxygen in the blood. Oxygen saturation measures how much of the hemoglobin in the red blood cells is carrying oxygen (O2).
Some labs may also measure other factors, such as carboxyhemoglobin, methemoglobin, hematocrit, sodium, potassium, chloride, ionized calcium, glucose, lactate, or base excess.
Blood for an ABG test is taken from an artery. Most other blood tests are done on a sample of blood taken from a vein, after the blood has already passed through the body's tissues where the oxygen is used up and carbon dioxide is produced.
An arterial blood gases (ABG) test is done to:
If you are on oxygen therapy, the oxygen may be turned off for 20 minutes before the blood test. This is called a "room air" test. But if you can't breathe without the oxygen, the oxygen won't be turned off.
A sample of blood from an artery is usually taken from the inside of the wrist (radial artery). But it can also be taken from an artery in the groin (femoral artery) or on the inside of the arm above the elbow crease (brachial artery).
If blood is taken from the wrist, you will be seated with your arm extended and your wrist resting on a small pillow. The health professional taking the blood may rotate your hand back and forth and feel for a pulse in your wrist.
A procedure called the Allen test may be done to make sure that the blood flow to your hand is normal. An ABG test will not be done on an arm used for dialysis or if there is an infection or inflammation in the area of the puncture site.
Collecting blood from an artery is more painful than collecting it from a vein. That's because the arteries are deeper and are surrounded by nerves.
Most people feel a brief, sharp pain as the needle to collect the blood sample enters the artery. If you get a local anesthetic, you may feel nothing at all from the needle puncture. Or you may feel a brief sting or pinch as the needle goes through the skin.
There is little chance of a problem from having a blood sample taken from an artery.
Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.
The concentration of oxygen being breathed, called the fraction of inhaled oxygen (FiO2), is also usually reported. This is only useful if you are receiving oxygen therapy from a tank or are on a ventilator.
Many conditions can change blood gas levels. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.
Adaptation Date: 2/24/2022
Adapted By: Alberta Health Services
Adaptation Reviewed By: Alberta Health Services
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