A modified Allen test measures arterial competency, and should be performed before taking an arterial sample. The procedure for performing the test is as follows (see , below).
Allen test. Source: http://fitsweb.uchc.edu/student/selectives/TimurGraham/Modified_Allen%27s_Test.html
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.
Positive 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.
Negative 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.
