The Allen’s Test is a quick, non-invasive clinical assessment used to evaluate blood circulation in the hand. Healthcare professionals perform this maneuver before any procedure requiring puncturing or cannulating the radial artery, such as drawing arterial blood gases or placing an arterial line. The test determines if the hand’s blood supply is robust enough to handle a temporary disruption to the radial artery. Knowing the status of the secondary blood supply is important for preventing serious complications like hand ischemia should the primary vessel become blocked.
Assessing Collateral Circulation
The forearm and hand receive their primary blood supply from the radial artery and the ulnar artery. These two vessels connect within the hand through a network called the palmar arches, forming the collateral circulation. This system ensures the hand has a backup source of blood flow.
The radial artery is often the preferred site for arterial access due to its superficial location. If a procedure compromises the radial artery, the hand’s survival depends on the ulnar artery’s capacity to carry the full blood supply. Therefore, the Allen’s Test specifically assesses the functional integrity of the ulnar artery and its ability to act as the sole provider of oxygenated blood.
Performing the Procedure
The patient should be positioned comfortably, with the arm supported and the palm facing upward. The examiner first locates the pulses of both the radial artery (thumb side) and the ulnar artery (pinky-finger side) at the wrist. Once confirmed, the patient is instructed to make a tight fist for approximately 30 seconds.
While the fist is clenched, the examiner applies firm, constant pressure to both the radial and ulnar arteries simultaneously, completely obstructing blood flow. This maneuver drains blood from the hand. The patient is then asked to open their hand, which should appear pale or blanched due to the cutoff of arterial supply.
Next, the examiner releases pressure from the ulnar artery while maintaining compression on the radial artery. This allows blood to flow into the hand only through the ulnar artery, testing its ability to perfuse the tissue. The examiner observes the palm and fingers, timing how long it takes for the hand’s normal color to return. Maintaining radial pressure isolates the ulnar artery’s circulation.
Understanding the Test Results
The results of the Allen’s Test are determined by the speed at which the hand’s color returns once the ulnar artery is released. A normal result, often called a “positive” Allen’s Test, is indicated by the palm flushing with color within 5 to 15 seconds. This rapid return confirms that the ulnar artery has sufficient collateral flow to supply the entire hand, making it safe to proceed with a procedure on the radial artery.
If the hand remains pale for longer than 15 seconds, or if the color fails to return at all, the result is considered abnormal, or a “negative” Allen’s Test. This outcome suggests that the collateral circulation provided by the ulnar artery is insufficient or non-existent. In this situation, the radial artery on that side must not be used for cannulation or puncture, as potential damage could result in irreversible tissue injury.
Certain physical factors can influence the accuracy of the test results. Excessive hyperextension of the wrist or applying too much pressure to the arteries can lead to inaccurate findings. Cold hands can also slow down the return of color, potentially leading to an equivocal result that requires further evaluation before a procedure can be performed.