The Allen’s Test is a medical procedure used to evaluate the circulation in the hand before certain procedures that might affect blood flow. The primary purpose of this quick, non-invasive assessment is to check for the presence of adequate collateral circulation to the hand. It determines if the blood supply from one artery can sufficiently maintain the health of the entire hand if the other main artery is temporarily or permanently compromised. This assessment is a standard safety measure in clinical settings to prevent complications related to a lack of blood flow, or ischemia, following an intervention.
The Dual Blood Supply to the Hand
The hand receives its blood supply from two main arteries that travel through the forearm: the radial artery and the ulnar artery. The radial artery runs along the thumb side of the wrist, while the ulnar artery is located on the little finger side. These two vessels are responsible for delivering oxygenated blood to the entire hand.
Hand circulation is unique because the radial and ulnar arteries connect with one another in the palm to form a crucial network called the palmar arch. Specifically, there are two arches, the superficial palmar arch and the deep palmar arch, which create a redundant system for blood delivery. This anatomical arrangement means that if blood flow through one artery is interrupted, the other artery can still supply blood to the entire hand through the connecting arch.
The Allen’s Test confirms the patency and functionality of this collateral circulation. If a person has an incomplete or insufficient palmar arch, the hand is at risk of ischemia if one of the main arteries is utilized or damaged during a medical procedure. Assessing this anatomical safeguard is a proactive step to ensure patient safety.
Performing the Standard Allen’s Test
The Allen’s Test is a straightforward physical examination maneuver that requires the cooperation of the patient. The procedure typically begins with the patient sitting or lying down with their palm facing upward and their arm stabilized. The examiner then locates the pulses of both the radial and ulnar arteries at the wrist and applies firm pressure with their fingers to temporarily stop the blood flow through both vessels simultaneously.
Next, the patient is instructed to clench their fist tightly for about 30 to 60 seconds, or to rapidly open and close their hand multiple times. This action forces the blood out of the hand, causing the palm to become pale or blanched due to the temporary occlusion of both arteries. The patient is then asked to open their hand, keeping it relaxed, revealing the paleness of the palm.
The examiner then maintains pressure on the radial artery while releasing the pressure on the ulnar artery alone. This step allows the ulnar artery to attempt to perfuse the entire hand through the palmar arch. The time it takes for the normal pink or “flush” color to return to the palm is then observed and timed. To assess the radial artery’s collateral flow, the test is repeated on the same hand, this time releasing the radial artery while keeping the ulnar artery compressed.
Interpreting Test Results and Clinical Application
The interpretation of the Allen’s Test result is based on how quickly the normal color returns to the palm after releasing the pressure on the tested artery. A “Normal” result, often referred to as a negative test, indicates that the collateral circulation is adequate, meaning the artery being tested can supply the entire hand. For the modified test, this is typically defined as the color returning to the hand within an acceptable timeframe, often cited as five to seven seconds. Some older studies suggest an average refill time of about 2.3 seconds for the ulnar artery in healthy individuals.
An “Abnormal” result, or a positive test, occurs if the palm remains blanched or if the color return takes longer than the accepted time limit, such as over five to ten seconds. This prolonged pallor suggests that the collateral circulation from the tested artery is insufficient. If the test is abnormal, the vessel that was not released is deemed unsafe for use in the planned procedure.
The Allen’s Test is considered mandatory before any medical procedure that involves cannulating or puncturing the radial artery. Common applications include:
- Prior to obtaining an Arterial Blood Gas (ABG) sample.
- Before placing a radial arterial line for continuous invasive blood pressure monitoring in hospitalized patients.
- When the radial artery is planned for harvesting, such as its use as a conduit in coronary artery bypass grafting (CABG) surgery.