Up until recently, blood oxygen has been measured by means of an arterial blood gas (ABG), an invasive laboratory test. The test measures the acid-base balance (pH) and the levels of oxygen and carbon dioxide in the blood. Steps involved in the blood gas test include: 1. Taking a blood sample. Blood is taken by a nurse or therapist from an artery inside the wrist known as the radial artery. Sometimes the sample is taken from an artery in the arm (brachial artery) or groin (femoral artery). Unfortunately, taking a blood sample from an artery is usually more painful than getting a sample from a vein. This is because arteries are deeper in your body than veins and are surrounded by nerves. 2. Lab work: The sample is then sent off to the lab for examination, which can take hours.