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Arterial Blood Gases

An arterial blood gas (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 check 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.
  • pH. 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 gas (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 receiving 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, severe infections, or after a drug overdose.

How To Prepare

Tell your doctor if you:

  • Have had bleeding problems or take blood thinners, such as aspirin or warfarin (Coumadin).
  • Are taking any medicines.
  • Are allergic to any medicines, such as those used to numb the skin (anesthetics).

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. If you can't breathe without the oxygen, the oxygen will not be turned off.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

WebMD Medical Reference from Healthwise

Last Updated: May 30, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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