It is possible for the Allen test to return a false negative if a physician performs it incorrectly. With false results, doctors may move forward with cannulation, and the hand may not receive enough blood. This is largely the reason for the three-digit modification. Certain sections of the radial and ulnar arteries may be in slightly different locations in some people. This can result in doctors not properly blocking the arteries during the test if the thumb is not large enough. The three-digit Allen test provides more surface area for better accuracy.

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