The Allen Test (AT) and Modified Allen Test (MAT) tests are used to ensure collateral circulation of the hand. The test can be used as a diagnostic tool for any numbers of disorders with a reduced vascular flow to the arm or after operations on the hand. More commonly, it is used to assess collateral flow to the hand through ulnar artery when the radial artery is going to be used for arterial punctures for drawing blood gases, cannulation for placement of arterial lines, catheterizations or radial artery harvest for bypass surgeries. These procedures are typically done involving the radial artery because its easily palpable nature at the bedside. One of the risks associated with the arterial punctures include ischemia distal to the puncture site which could lead compromise of the limb if there is not adequate collateral blood flow. Though ischemia is a rare complications associated with an arterial puncture, in most patients, in the clinical setting the Allen test is not often performed. There is weak evidence supporting the accuracy of this test in assesing ulnar artery patency and ensure collateral circulation.