Allen test or the modified Allen test is a sign that is used to determine the flow of the blood to the hands. The Allen Test was first described by Edgar Van Nuys Allen in 1929.
In 1952, Irving S Wright suggested a modification of the test which has replaced the original test universally.
The alternative method is often referred to as the modified Allen’s test or modified Allen test. As this is the test now mostly used, the term Allen test may suggest this only.
The Allen test is used to assess the arterial blood flow to the hand and determine the patency of the radial or ulnar artery.
It is reported to have a sensitivity of 73% and a specificity of 97%.
The original test as proposed by Allen is performed as follows
- The patient is asked to clench both fists tightly for 1 minute at the same time.
- Pressure is applied over both radial arteries simultaneously so as to occlude them.
- The patient then opens the fingers of both hands rapidly, and the examiner compares the color of both. The initial pallor should be replaced quickly by rubor [redness]
- The test may be repeated, this time occluding the ulnar arteries.
The 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.
Modified Allen Test
- Instruct the patient to clench his or her fist; if the patient is unable to do this, close the person’s hand tightly.
- Using your fingers, apply occlusive pressure to both the ulnar and radial arteries, to obstruct blood flow to the hand.
- While applying occlusive pressure to both arteries, have the patient relax his or her hand, and check whether the palm and fingers have blanched. If this is not the case, you have not completely occluded the arteries with your fingers.
Release the occlusive pressure on the ulnar artery only to determine whether the modified Allen test is positive or negative.
Negative modified Allen test
If the hand flushes within 5-15 seconds it indicates that the ulnar artery has good blood flow. This normal flushing of the hand is considered to be a positive test.
Positive modified Allen test
If the hand does not flush within 5-15 seconds, it indicates that ulnar circulation is inadequate or nonexistent. In this situation, the radial artery supplying arterial blood to that hand should not be punctured.
Basis of Test
The hand has a complex and rich vascular network, which is mostly supplied by the radial and ulnar arteries.
The radial artery runs between the brachioradialis and flexor carpi radialis muscles and splits into a superficial branch to contribute to the superficial palmar arch and the other branch crosses dorsally deep to the tendons of the anatomic snuffbox to form the deep palmar arch.
The ulnar artery lies under flexor carpi ulnaris. At the wrist, it enters the Guyon’s canal, where it splits into a deep palmar branch and a superficial palmar branch. The superficial palmar branch forms the superficial palmar arch, while the deep branch contributes to the deep palmar arch.
Thus the hand is supplied by two arteries. If the blood supply from one of the arteries is cut off, the other artery can supply adequate blood to the hand.
While most of the people have this dual arterial supply, a minority of people lack this.
The test aims to establish ts fact especially before an intervention involving the arteries. This could be a simple sampling for arterial blood gas or an arm surgery. [See Uses]
Video of Allen Test
Following video explains well the performance of Allen test.
Uses of Allen Test
Arterial Blood Gas Analysis
Arterial blood gas analysis requires a sample from an arterial blood. The radial artery is one of the common arteries used for this purpose. An uncommon complication of radial arterial blood sampling/cannulation is disruption of the artery (obstruction by a clot), placing the hand at risk of ischemia where the people lack dual blood supply.
Using the Allen test before sampling may help to screen this issue.
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.
Heart Bypass Surgery
The radial artery is occasionally used as a conduit for bypass surgery as its patency lasts longer in comparison to the saphenous veins. Before removal of radial artery removal, Allen test is performed to assess the suitability of the radial artery to be used as a conduit.
A result of fewer than 3 seconds is considered as good and suitable. A result of between 3–5 seconds is equivocal.
The artery is not considered for grafting if the result is longer than 5 seconds.
Before operating the wrist it is important to ensure if both arteries supply the hand especially in situations where an artery has a chance of getting damaged.