The Allen's test assesses collateral circulation in the hand, in 2 steps. Step 1 occludes the radial artery for several minutes and compares the hand color to the other hand. The hand is said to have sufficient collateral circulation through the ulnar artery if there is no change in color. Step 2 occludes the ulnar artery. A change in hand color means the potential for radial artery occlusion is high. That is a positive Allen's test, which contraindicates radial-artery puncture Modified Allen's Test The procedure for performing a modified Allen's Test is as follows: 1. Instruct the patient clench his/her fist, or if the patient is unable, you may close the hand tightly. 2. Using your fingers, apply occlusive pressure to both the ulnar and radial arteries. This maneuver obstructs blood flow to the hand. 3. While applying occlusive pressure to both the arteries, have the patient relax his/her hand. Blanching of the palm and fingers should occur. If it does not, you have not completely occluded the arteries with your finger. 4. Release the occlusive pressure on the ulnar artery. You should notice a flushing of the hand within 5 to 15 seconds. This denotes that the ulnar artery if patent and has good blood flow. This normal flushing of the hand is considered to be a positive modified Allen's test. A negative modified Allen's test is one in which the hand does not flush within the specified time period. This indicates that ulnar circulation is inadequate or nonexistence. The radial artery supplying arterial blood to that hand should not be punctured.