Test Overview

An arterial blood gas (ABG) test measures the levels of oxygen and carbon dioxide in the blood to determine how well your lungs are working. It evaluates the ability of your lungs to move oxygen into the blood and to 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 airspace of 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 and become changed. An ABG measures:

• Partial pressure of oxygen (PaO₂). The oxygen level indicates how well oxygen is able to move from the airspace of the lungs into the blood.
• Partial pressure of carbon dioxide (PaCO₂). The carbon dioxide level indicates how well carbon dioxide is able to move out of the blood into the airspace of the lungs and out with exhaled air.
• pH. The pH is a measure of hydrogen ion (H+) in blood which indicates the acid or base (alkaline) nature of blood. A pH of less than 7 is acidic, and a pH greater than 7 is called basic (alkaline). The pH of blood is usually close to 7.4.
• Bicarbonate (HCO₃). Buffers are chemical substances that keep the pH of blood within a normal range. Bicarbonate is the most important buffer in the blood.
• Oxygen content (O₂CT) and oxygen saturation (O₂Sat) values. Like the PaO₂, these values provide information about the amount of oxygen in the blood.

Abnormal values for oxygen, carbon dioxide, and pH can be caused by changes in:

• Lung function.
• Heart and circulation function.
• Kidney function.
• The ability of the body to use food for energy (metabolism).
• The use of some medications.

Most 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 ABG test is usually taken from the radial artery in the wrist. Arteries in the elbow (brachial) and groin (femoral) may also be used.

Medical review	Author	Last updated
Caroline S. Rhoads, MD - Internal Medicine	Jan Nissl, RN, BS	July 16, 2004

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