In medicine, Allen's test or the Allen test is a medical sign used in physical examination of arterial blood flow to the hands. It was named for Edgar Van Nuys Allen, who described the original version of the test in 1942.

An altered test, first suggested by Irving S Wright in 1952, has almost universally replaced the original method in contemporary medical practice. The alternative method is often referred to as the modified Allen's test or modified Allen test.

Method
Original test
The original Allen test is performed by asking the patient to elevate both arms above the head for thirty seconds in order to exsanguinate the hands. Next, the patient squeezes their hands into tight fists, and the examiner occludes the radial artery simultaneously on both hands. The patient then opens both hands rapidly, and the examiner compares the color of the palms. The initial pallor should be replaced with the hands' normal color as the ulnar arteries restore perfusion. The test is then repeated while occluding the ulnar arteries rather than the radial arteries. The time that it takes for the normal color to return should indicate the degree of collateral blood flow. The test is positive when there is a return of normal color to both hands during occlusion of either artery alone. Persistent pallor in the palm indicates inadequate collateral blood flow to the hand.

Modified test
In the modified Allen test, one hand is examined at a time:
The hand is elevated and the patient is asked to clench their fist for about 30 seconds. Pressure is applied over the ulnar and the radial arteries so as to occlude both of them. The hand is then opened. It should appear blanched. Ulnar pressure is released while radial pressure is maintained, and the color should return within 5 to 15 seconds. If color returns as described, Allen's test is considered to be normal. If color fails to return, the test is considered abnormal and it suggests that the ulnar artery supply to the hand is not sufficient. This indicates that it may not be safe to cannulate or needle the radial artery. There is still some confusion as to whether a normal Allen test should be referred to as negative or positive. In preoperative surgical notes it is perhaps best to avoid the words negative or positive and document the result as normal or abnormal

For more detail please visit:
https://www.ncbi.nlm.nih.gov/books/NB...

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