The Allen’s Test is a quick, non-invasive physical examination maneuver used to assess the adequacy of blood flow to the hand. Its primary function is to confirm the hand has a reliable, alternate blood supply before a medical procedure that might compromise a main artery in the wrist. This test is a safety measure designed to prevent hand ischemia, a complication where the hand does not receive enough oxygenated blood.
Understanding Collateral Circulation
The hand receives its blood supply from two major arteries: the radial artery (thumb side) and the ulnar artery (pinky side). These arteries connect in the palm to form a redundant network of vessels called the palmar arches. This anatomical connection is known as collateral circulation.
The superficial palmar arch is primarily formed by the ulnar artery, while the deep palmar arch is mainly formed by the radial artery. This dual supply means that if one artery is blocked or used for a procedure, the other can usually maintain perfusion to the entire hand. The Allen’s Test confirms this redundancy, ensuring the ulnar artery can adequately supply the hand if the radial artery’s flow is interrupted.
The ulnar artery is the vessel most often assessed for its ability to provide this backup circulation because it is the dominant contributor to the major superficial palmar arch. Testing this collateral flow is important because in a small percentage of people, the palmar arches are incomplete. If the arches are incomplete, the ulnar artery cannot fully compensate if the radial artery is occluded, which could lead to tissue damage.
Executing the Standard Allen’s Test
The procedure begins with the patient sitting comfortably, with their arm stabilized and the palm facing upward. The examiner locates the pulse points of both the radial artery (near the base of the thumb) and the ulnar artery (near the pinky finger side of the wrist).
The patient is instructed to clench their fist tightly for approximately 30 seconds to force blood out of the hand, causing the palm to become pale or blanched. While the fist remains clenched, the examiner applies firm, sustained pressure to simultaneously occlude both the radial and ulnar arteries at the wrist.
With the arteries still compressed, the patient opens their hand, which should appear pale due to the lack of blood flow. The hand must not be hyperextended or the fingers spread too widely, as this can affect muscle tension and potentially lead to a false result. The hand should be open and relaxed for the next step.
While maintaining pressure on the radial artery, the examiner releases pressure on the ulnar artery only. This allows blood to rush back into the hand exclusively through the ulnar artery and the palmar arches. The time it takes for the palm and fingers to return to their normal color is then observed and measured.
The return of color, known as flushing, should be uniformly distributed across the palm, thumb, and fingers. This process isolates the collateral blood supply, demonstrating the ulnar artery’s ability to perfuse the entire hand. Maintaining consistent pressure on the radial artery throughout the observation period is essential for an accurate reading.
Analyzing the Test Results
Interpretation of the Allen’s Test relies on the speed at which color returns to the blanched hand after the ulnar artery is released. A normal result, sometimes referred to as a “positive” test due to historical nomenclature, indicates adequate collateral circulation. This occurs if the hand flushes and color returns within 5 to 15 seconds.
This rapid return of color confirms that the ulnar artery can satisfactorily supply the entire hand, meaning the radial artery can be safely used for a procedure. Conversely, an “abnormal” result, sometimes termed a “negative” test, suggests insufficient collateral flow. This occurs if the hand remains pale or color return takes longer than 15 seconds.
A prolonged or absent flush indicates that the ulnar artery cannot provide enough blood flow to compensate for the blocked radial artery. In this situation, proceeding with any procedure that might compromise the radial artery carries a high risk of hand ischemia, and the radial artery should not be used.
When and How the Test Is Used
The Allen’s Test is a standard precaution before any procedure involving the radial artery, such as drawing an arterial blood gas (ABG) sample. It is also performed prior to radial artery cannulation for continuous blood pressure monitoring. Furthermore, it is a necessary screening tool when the radial artery is considered for harvest, such as for a coronary artery bypass graft (CABG) surgery.
Because the original test had some limitations, the Modified Allen’s Test (MAT) is the version almost universally adopted in clinical practice today. The modified test focuses solely on one hand at a time, making it more practical and efficient. While the steps are similar to the standard test, the MAT assesses the patency of the ulnar artery’s contribution to the palmar arch before using the radial artery.
The crucial difference in the Modified Allen’s Test is the focus on a unilateral hand assessment for efficiency. However, the modified version has limitations, and some clinicians prefer more objective assessments like pulse oximetry or Doppler ultrasonography to confirm collateral flow before complex surgical procedures. Despite these alternatives, the Modified Allen’s Test remains a widely used bedside screening tool.