Prior to insertion of an a-line into the radial artery, the Allen’s test is performed to confirm adequate collateral circulation to the hand provided by the ulnar artery. This involves applying pressure to both the radial and ulnar arteries, observing the hand blanch, then removing pressure from the ulnar artery to confirm that the ulnar artery alone can perfuse the hand, demonstrated by the hand becoming pink. If the perfusion is inadequate, the radial artery is contraindicated for a-line insertion.