The Allen test

In the Allen test, the patient is instructed to make a fist, which will empty the blood from the hand and fingers (A). The examiner's thumbs are then pressed down across the thenar and hypothenar eminences to the wrist to occlude the radial and ulnar arteries. The patient then opens the hand, making sure not to overextend the fingers. The pressure on the ulnar artery is then released while the radial artery is still compressed (B). The hand does not fill with blood. Note the paleness of the hand on the right compared with the hand on left, indicating occlusion of the ulnar artery distal to the wrist (abnormal test result). If there is prompt return of color to the hand (indicating a normal test result), the test is repeated except this time pressure on the radial artery is released while the ulnar artery remains compressed.