The Allen’s test is a simple, non-invasive clinical maneuver used to assess the blood supply to the hand. This physical examination technique specifically evaluates the concept of collateral circulation, which is the alternative blood flow available should a primary vessel become obstructed. The test is a rapid screening tool designed to determine if the hand has a sufficient “backup” blood supply to prevent tissue damage. Its purpose is to ensure patient safety before certain medical procedures are performed.
The Purpose of Assessing Collateral Circulation
The hand receives its arterial blood supply primarily from two major vessels: the radial artery and the ulnar artery. These two arteries connect within the hand through the palmar arches, forming a circulatory bypass system. This dual supply means that if one artery is temporarily blocked, the other can usually sustain the hand’s perfusion.
The Allen’s test confirms the adequacy of this collateral flow, particularly the ulnar artery’s ability to supply the hand if the radial artery is compromised. This assessment is necessary before procedures involving the radial artery, such as drawing an arterial blood gas (ABG) sample or placing an arterial line. Ensuring robust collateral circulation is a precaution against the risk of hand ischemia should the radial artery clot after the procedure.
How the Allen’s Test is Performed
The test is commonly performed using the modified technique, focusing on one hand at a time. The patient is asked to elevate their hand and clench it tightly into a fist for approximately 30 seconds to force the blood out of the palm. While the fist remains clenched, the examiner applies firm, simultaneous pressure over both the radial and ulnar arteries at the wrist. This dual compression temporarily stops all arterial blood flow into the hand.
The patient is then instructed to open their hand, keeping it relaxed and slightly flexed. The palm should appear blanched, or noticeably pale, due to the lack of blood flow. While maintaining continuous pressure on the radial artery, the examiner releases only the compression on the ulnar artery. The subsequent observation of the color return to the palm determines the test’s result.
Defining a Positive and Negative Result
The interpretation of the Allen’s test focuses on the speed and completeness of the color returning to the blanched palm after the ulnar artery is released. The standard clinical interpretation is based on the adequacy of the blood flow. A negative Allen’s test is the desired, normal result, signifying a rapid flush of color back into the palm. This typically occurs within 5 to 7 seconds, indicating that the ulnar artery has sufficient collateral flow to safely perfuse the hand.
A positive Allen’s test is the abnormal finding, meaning that the color return is significantly delayed or absent. A return time exceeding 5 to 10 seconds is considered a positive result, indicating inadequate collateral circulation from the ulnar artery. In this scenario, the radial artery is considered unsafe for use, as its compromise could result in hand ischemia.
Actions Taken After a Positive Test
The primary clinical implication of a positive Allen’s test is that the radial artery in that specific hand must be avoided for the planned procedure. The risk of radial artery occlusion leading to permanent hand damage is too high given the lack of sufficient collateral blood flow. The healthcare provider must then seek an alternative site for the procedure.
The first alternative is typically to perform the Allen’s test on the opposite radial artery. If that test yields the desired negative result, that site may be safely used. Other potential alternative sites for arterial access include the femoral artery or the brachial artery. The determination of the safest alternative location is based on the patient’s condition and the requirements of the planned medical intervention.