The Allen’s Test is a quick, non-invasive method used by healthcare professionals to evaluate blood flow in the hands. It assesses the patency of the two main arteries supplying the hand: the radial artery and the ulnar artery. By temporarily blocking and releasing one vessel, the test determines if the other artery can provide sufficient blood supply to the entire hand. This procedure is performed at the wrist, using observation of skin color changes to gauge the adequacy of the arterial network. Its primary function is to screen for sufficient collateral circulation before procedures that might affect these major arteries.
The Purpose of the Test
The hand receives its blood supply from two separate arteries, the radial and the ulnar, which connect in the palm via a network called the palmar arches. This dual supply is known as collateral circulation, a natural backup system that ensures the hand remains perfused even if one artery is compromised. Assessing this collateral flow is necessary before any medical procedure that risks damaging or blocking one of the arteries, most often the radial artery.
The radial artery is frequently used for obtaining blood samples for arterial blood gas analysis or for placing an arterial line to monitor blood pressure. It is also sometimes harvested for use as a bypass graft during heart surgery. If a procedure on the radial artery causes it to become temporarily or permanently blocked, the hand must rely on the ulnar artery for its entire blood supply.
If the Allen’s Test indicates inadequate collateral circulation, there is an increased risk of ischemia, or lack of oxygen, to the hand should the radial artery be compromised. The test helps healthcare providers decide if it is safe to proceed with a procedure on a specific artery. While historically routine, recent studies have questioned its reliability as the sole determinant for radial artery access, though the principle of assessing collateral flow remains important.
Step-by-Step Procedure
The modified Allen’s Test focuses on assessing one hand at a time. The patient is typically seated with their arm resting comfortably, palm facing upward. The patient is asked to clench their fist tightly for about 30 seconds.
The healthcare professional then applies firm pressure to the wrist, simultaneously compressing both the radial and ulnar arteries. This manual compression temporarily stops blood flow to the hand from both vessels. While pressure is maintained, the patient is instructed to open their hand, which should appear blanched, or pale, due to the lack of blood.
The next step involves releasing the pressure on only one artery, typically the ulnar artery, while keeping the radial artery compressed. The examiner observes the palm, noting how quickly the normal pink color returns. This observation indicates the patency of the released artery and the adequacy of the collateral blood flow through the palmar arch. The entire process is then repeated to assess the collateral circulation from the radial artery, if necessary.
Understanding the Test Results
The interpretation of the Allen’s Test is based on the time it takes for the hand’s color to return to normal after the pressure on one artery is released. A rapid return of color is considered a normal or “positive” result, suggesting the released artery and the connecting palmar arch can adequately supply the hand. Normal perfusion is typically indicated by a color return time of less than 5 to 7 seconds.
If the hand remains pale for an extended time, or if the color never fully returns, the result is considered abnormal or “negative,” suggesting insufficient collateral flow. A return time exceeding 5 to 15 seconds indicates that the hand’s backup circulation is not adequate. This finding means that the procedure on the other compressed artery should not be performed due to the high risk of hand tissue damage.
Factors such as the patient’s body temperature or the degree of hand hyperextension can influence the result, potentially leading to a false reading. Due to subjectivity in interpreting the visual color return time, objective methods like Doppler ultrasound or pulse oximetry are sometimes used to confirm findings, especially in patients with pre-existing vascular conditions. A clear, prompt flush of color is the goal, demonstrating a robust and safe dual arterial supply to the hand.