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Allen's test is a test used to determine the integrity of the blood supply to the hand.
With the hand elevated both the ulnar and the radial arteries are occluded, which leads to blanching of the hand. Then, one of the arteries is released and, in the normal case the blanching disappears over the whole of the hand. This is repeated with both arteries - in theory the whole of the blood supply of the hand can come from either artery.