An arterial blood gas test measures the levels of oxygen and carbon dioxide in the blood as well as blood pH. It is performed by drawing blood from an artery, usually in the wrist, groin or arm, and analyzing the sample to evaluate respiratory conditions and determine the effectiveness of oxygen therapy. The test involves inserting a small needle into the artery to collect blood, after which pressure is applied to stop any bleeding. Special preparation is generally not needed other than maintaining consistent oxygen levels if the patient uses oxygen therapy. Risks are low but can include bleeding, bruising or fainting at the puncture site.