The blood supply to your hand normally comes from 2 arteries, the radial artery and the ulnar artery. Before drawing blood for an arterial blood gas test, your health professional will make sure that both arteries are open and working correctly. A procedure called the Allen test may be used to find out if the blood flow to your hand is normal.

For the Allen test, the health professional drawing your blood will apply pressure to the arteries in your wrist for several seconds. This will stop the blood flow to your hand, and your hand will become cool and pale. Blood is then allowed to flow through the artery that will not be used to collect the blood sample. This is usually the ulnar artery, which is found on the outer (little finger side) of your wrist. Arterial blood gases are usually taken from the radial artery, which is found on the inner (thumb side) of the wrist.

If your hand remains pale and cold, the Allen test will then be performed on your other hand. If your other hand also remains pale, the blood often will be collected from another artery, usually in the groin or elbow crease.