arterial blood gas

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The concentration of oxygen and carbon dioxide in the blood, whose partial pressures are measured along with other factors such as blood pH in order to assess oxygen saturation and other metabolic indicators in patients, especially those with respiratory disorders.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A blood test in which a syringe is used to draw a small blood sample from an artery.