The Allen Test is a quick, non-invasive physical examination used in medicine. It assesses blood flow to the hand and helps medical professionals understand arterial supply before certain interventions.
Purpose of the Allen Test
The Allen Test assesses the patency of the ulnar artery and the adequacy of collateral blood flow to the hand. The hand receives its blood supply mainly from two arteries: the radial artery, located on the thumb side of the wrist, and the ulnar artery, found on the pinky finger side. These arteries connect in the hand through a network of vessels called the palmar arches, which allow for alternative blood supply if one artery is compromised.
The test determines if the ulnar artery can adequately supply blood to the entire hand if the radial artery’s flow is temporarily or permanently interrupted. This assessment is particularly important before procedures that might affect the radial artery, such as drawing arterial blood samples, inserting a catheter, or using the artery for surgical grafts. Ensuring sufficient collateral flow helps prevent complications like reduced blood supply to the hand.
Performing the Allen Test
To perform the Allen Test, the patient sits with their arm resting comfortably. The patient is then instructed to make a tight fist for about 30 seconds. This helps push blood out of the hand.
While the patient’s fist is clenched, a healthcare professional applies firm pressure to both the radial and ulnar arteries at the wrist, blocking blood flow to the hand. The patient then opens their hand, which should appear pale due to the temporary lack of blood supply. The examiner releases pressure from one artery (usually the ulnar first) while maintaining pressure on the other. The examiner observes how quickly the color returns to the palm and fingers. This process is repeated for the other artery. The hand should be kept partially open, avoiding hyperextension of the fingers or wrist, as this can lead to an inaccurate result.
Interpreting the Results
Interpreting Allen Test results involves observing how quickly color returns to the hand after releasing pressure on an artery. A “negative” Allen Test means the hand flushes and returns to its usual color within 5 to 15 seconds after releasing the ulnar artery. This rapid return signifies that the ulnar artery is providing adequate collateral circulation to the hand.
A “positive” Allen Test occurs if the hand remains pale or takes longer than 15 seconds to flush. This prolonged pallor suggests inadequate collateral circulation through the ulnar artery. In such cases, procedures involving the radial artery on that hand might carry a higher risk of complications due to insufficient alternative blood supply.
When and Why the Allen Test is Performed
The Allen Test is performed in various clinical scenarios to ensure patient safety before medical procedures. It is routinely conducted before arterial blood gas (ABG) sampling from the radial artery, which assesses oxygen and carbon dioxide levels. The test is also performed prior to radial artery catheterization, used for continuous blood pressure monitoring or repeated blood draws. Another application is before using the radial artery for coronary artery bypass grafting (CABG) surgery, where a section of the artery may be harvested. The test helps confirm the hand will receive sufficient blood supply from the ulnar artery if the radial artery is used for grafting, minimizing the risk of complications like ischemia to the hand.