Photo illustration of Allen’s Test

Simply Allen`s test "named for Edgar Van Nuys Allen" is used to check the collateral circulation of the hand by evaluating the patency of the radial and ulnar arteries prior to perform radial arterial blood sampling or cannulation.

+ve Allen test :

When you occlude the radial artery for several minutes and compares the hand color to the other hand and you find that there is no change in color------>This hand is said to have sufficient collateral circulation through the ulnar artery .

Step 1 occlusion

Step 2 releasing

Modified Allen’s test :

Step 1 occlusion : With the same idea, Ask the patient to clench the hand into a tight fist and then apply pressure to both the radial and ulnar arteries, and ask the patient to relax their hand .The hand should appear somewhat white & blanched (if not....you haven`t completely occluded the arteries with your fingers).

Step 2 releasing : When you Release the occlusive pressure on the ulnar artery it will result in the hand regaining its colour "flushing of the hand within 5 to 15 seconds"------> the ulnar collateral circulation is adequate, and it`s called positive modified Allen’s test
The test can be repeated, this time releasing the pressure on the radial artery, to assess radial collateral circulation.