Allen test

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 1929. 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.

The original test proposed by Allen is performed as follows:

Allen's test looks for abnormal circulation. If color returns quickly as described above, Allen's test is considered to demonstrate normal circulation. If the pallor persists for some time after the patient opens their fingers, this suggests a degree of occlusion of the uncompressed artery.

In the modified Allen test, one hand is examined at a time:

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.

The hand is normally supplied by blood from both the ulnar and radial arteries. The arteries undergo [anastomosis] in the hand. Thus, if the blood supply from one of the arteries is cut off, the other artery can supply adequate blood to the hand. A minority of people lack this dual blood supply.

An uncommon complication of radial arterial blood sampling/cannulation is disruption of the artery (obstruction by clot), placing the hand at risk of ischemia. Those people who lack the dual supply are at much greater risk of ischemia. The risk can be reduced by performing the modified Allen's test beforehand. People who have a single blood supply in one hand often have a dual supply in the other, allowing the practitioner to take blood from the side with dual supply.

The modified Allen's test is also performed prior to heart bypass surgery. The radial artery is occasionally used as a conduit for bypass surgery, and its patency lasts longer in comparison to the . Prior to heart bypass surgery, the test is performed to assess the suitability of the radial artery to be used as a conduit. A result of less than 3 seconds is considered as good and suitable. A result of between 3–5 seconds is equivocal, whereas the radial artery will not be considered for grafting if the result is longer than 5 seconds.

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