Allen’s test is used to gauge the flow of blood in the hands. It determines whether one or both of the two arteries — the ulnar and radial arteries — that bring blood to the hands are functional. The test was developed by Edgar Van Nuys Allen, a physician from the United States of America.
This procedure temporarily stops blood flow to the hands so that the doctor can observe how long it takes for the hands to return to their normal color. The Allen’s test is typically conducted before the insertion of a cannula tube or radial arterial blood sampling in order to ensure that they will not disrupt blood flow to the hands.
The full Allen’s test takes about a minute to complete. First the patient raises the hand to be tested above heart level. Then the patient is asked to make a strong fist. This helps to remove blood from the hand. Then the ulnar and radial arteries are pressed hard enough so that blood cannot flow into the hand. After a few seconds, the arteries are released and the patient will be asked to relax his or her hand.
If the normal color of the hand returns within five to seven seconds, then both arteries are supplying blood to the hand and the test is negative. The test is positive if it takes over seven seconds for the hand to return to its normal color. A positive result indicates that there is only one artery delivering blood to the hand.
An Allen’s test with a positive result means that it is not safe to draw blood or insert a cannula in the area. The test may be done again in the other hand. It is rare for both hands to show positive results.
Most patients have negative results when they take Allen’s test. Those who have a positive result usually get sufficient blood flow from the one functioning artery. The purpose of the test is to ensure that a prick or other intrusion into an artery would not stop the flow of blood to the hand. It is also conducted so that the medical professional can avoid damaging the only functional artery in the hand.
Both the ulnar and radial arteries are branches of the brachial artery. The ulnar is the larger of the two. It runs from just below the elbow to the palm. The radial artery runs from the forearm and ends in the palm as well.