The Allen’s Test is a quick physical examination used to evaluate blood flow in the hand, serving as a safety measure before procedures involving the wrist arteries. By assessing the hand’s vascular supply, healthcare providers can minimize the risk of complications. The original test has largely been replaced by a simplified version known as the Modified Allen’s Test.
Assessing Collateral Circulation
The primary purpose of the Modified Allen’s Test is to determine the adequacy of collateral circulation, or the backup blood supply, in the hand. The hand is uniquely supplied by two main arteries: the radial artery and the ulnar artery, which connect to form the palmar arches. This dual supply means that if one artery becomes blocked, the other can often supply enough blood to keep the entire hand healthy.
The test checks if the ulnar artery can provide sufficient blood flow if the radial artery is temporarily or permanently obstructed. This assessment is necessary because the radial artery is frequently used for common medical procedures. If the ulnar artery’s collateral circulation is insufficient, compromising the radial artery could lead to ischemia (a lack of blood flow), potentially causing tissue damage or loss of the hand.
The test is necessary before procedures such as Arterial Blood Gas (ABG) sampling, which involves puncturing the radial artery. It is also performed before inserting an arterial line, a catheter placed in the radial artery for continuous blood pressure monitoring. Furthermore, the test is performed when the radial artery is considered for harvesting to be used as a conduit in heart bypass surgery.
Step-by-Step Technique
The Modified Allen’s Test is a bedside procedure requiring patient cooperation and meticulous technique. The patient sits comfortably with their palm facing upward and wrist slightly extended. This position allows the examiner to easily locate the radial and ulnar arteries near the wrist crease.
The patient is instructed to clench their fist tightly for about 30 seconds, forcing the blood out and making the palm appear pale or blanched. While the fist is clenched, the examiner applies firm, occlusive pressure to both the radial and ulnar arteries simultaneously at the wrist. This dual compression stops blood flow into the hand.
While the examiner maintains pressure on both arteries, the patient opens their hand, keeping it relaxed and slightly flexed. The palm should appear noticeably pale because the blood supply has been temporarily cut off. The examiner must ensure the patient does not hyperextend their fingers, as this can lead to an inaccurate result.
The examiner then releases the pressure on the ulnar artery only, continuing to compress the radial artery. This assesses collateral circulation, as the ulnar artery is now the sole source of blood flow into the hand. The examiner observes the hand for the return of color, timing how long it takes for the palm and fingers to flush pink.
If the initial blanching is not achieved, the examiner must repeat the entire procedure. A pulse oximeter placed on the thumb can sometimes be used to confirm the loss of blood flow, especially if the patient is unable to cooperate fully. However, the visual assessment of color return remains the standard method.
Interpreting the Outcomes
The interpretation of the Modified Allen’s Test is based on the speed at which the pink color returns to the palm after releasing the ulnar artery. A result is considered normal, or “negative,” if the color returns rapidly, indicating sufficient collateral blood flow. This rapid reperfusion suggests the hand has a reliable backup supply if the radial artery is compromised.
A return of color to the entire hand within 5 to 15 seconds is generally considered a negative test, though some guidelines suggest a threshold of 5 to 7 seconds for an optimal result. If the hand flushes within this time frame, the ulnar artery is deemed patent (open), and the planned procedure on the radial artery is considered safe.
Conversely, if the hand remains pale or if the color return takes longer than the critical time limit, the result is considered abnormal, or “positive.” A positive test suggests that the ulnar circulation is inadequate to supply the entire hand. In this situation, performing a procedure that could compromise the radial artery poses a significant risk of hand ischemia.
When an abnormal result is observed, the clinical decision is to avoid using the radial artery in that arm for the planned procedure (e.g., arterial line placement or blood gas sampling). Instead, the healthcare team will test the opposite arm or choose an alternative artery, ensuring the safety of the patient’s hand. Persistent pallor in only patches of the hand may indicate insufficiency in the smaller vessels, which warrants caution.