​What is the Allen’s test?

​What is the Allen’s test? 
Allen’s test it the test of the patency of the ulnar artery. Mostly doe prior to performing Arterial Blood Gas or placing radial arterial line. 
Patient is required to make a fist while the examiner occludes both the ulnar and radial arteries with fingers.

Patient then will open the fist and the examiner will release the occlusion on the ulnar artery to assess the flow of blood to the hand.