The radial artery is easily palpable. The need for this test arises from inabiliity to palpate the ulnar artery. Because the ulnar artery cannot be palpated its patency cannot be assessed, and therefore, collateral circulation of the hand cannot be confirmed. If the radial artery needs to be canulated or catheterized and there is a risk for thrombosis, then the Allen test help to confirm that the hand will maintain adequate blood flow through the ulnar artery and collaterals in the event it becomes occluded. A positive Allen test means that the patient does not have an adequate dual blood supply to the hand which would be a negative indication for catheterization, removal of radial artery or any procedure which may result in occlusion of the vessels.