Arterial Blood Gases (ABG) Test
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:
- Partial pressure of oxygen (PaO2).
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.
- Partial pressure of carbon dioxide (PaCO2).
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 (HCO3).
Bicarbonate is a chemical (buffer) that keeps the pH of blood from becoming too acidic or too basic.
- Oxygen content (O2CT) and oxygen saturation (O2Sat) values.
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).
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.
Why It Is Done
An arterial blood gases (ABG) test is done to:
- Check for severe breathing problems and lung diseases, such as asthma, cystic fibrosis, or chronic obstructive pulmonary disease (COPD).
- See how well treatment for lung diseases is working.
- Find out if you need extra oxygen or help with breathing (mechanical ventilation).
- Find out if you are getting the right amount of oxygen when you are using oxygen in the hospital.
- Measure the acid-base level in the blood of people who have heart failure, kidney failure, uncontrolled diabetes, sleep disorders, or severe infections or who have had a drug overdose.
How To Prepare
- If you take a medicine that prevents blood clots, your doctor may tell you to stop taking it before your test. Or your doctor may tell you to keep taking it. (These medicines include aspirin and other blood thinners.) Make sure that you understand exactly what your doctor wants you to do.
- Tell your doctor ALL the medicines, vitamins, supplements, and herbal remedies you take. Some may increase the risk of problems during your test. Your doctor will tell you if you should stop taking any of them before the test and how soon to do it.
- Do not smoke just before the test or breathe secondhand smoke.
How It Is Done
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.
How It Feels
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.
- You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for at least 10 minutes after the needle is removed (longer if you have bleeding problems or take blood thinners).
- In rare cases, the needle may damage a nerve or the artery. This can cause the artery to become blocked.
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.