Overview of Allen Test
The Allen test is a method used to check the blood flow to the hand. It specifically looks for a full blood supply path, known as the palmar arch, created by two arteries, the radial and ulnar arteries. This procedure was first introduced in 1929 by Dr. Edgar Van Nuys Allen, who was a professor specializing in diseases involving blood flow to the limbs at Mayo Clinic.
A revised version of this test, called the modified Allen test (MAT), was later introduced by Dr. Irving Wright in 1952. Unlike the original Allen test, the MAT checks each hand separately and can assess the blood flow in either the radial or ulnar artery. That’s why it’s generally the preferred method.
Anatomy and Physiology of Allen Test
The blood supply to the arms mainly comes from the left and right subclavian arteries, both of which extend from the aorta, the main blood vessel that supplies oxygenated blood to the body. The left subclavian artery comes directly from the aorta, while the right subclavian artery stems from a major artery named the brachiocephalic artery. The subclavian arteries run under the collarbone, transition into the axillary arteries after crossing the first rib, then become the brachial arteries that supply blood to the upper arm.
When the brachial artery moves further down the arm, it splits into the radial and ulnar arteries to provide blood to your forearm and hand.
The radial artery runs along the side of the forearm that’s on the same side as your thumb. It’s located between two muscles, the flexor carpi radialis and the brachioradialis. The radial artery continues to the back of the wrist, where it splits into two branches supplying the palm of the hand.
The ulnar artery runs along the side of the forearm that’s on the same side as your little finger and is located beneath a muscle called the flexor carpi ulnaris. The ulnar artery also splits into two branches which help provide blood to the palm of the hand. Together, the branches of the radial and ulnar arteries form two loops in the palm — the deep and superficial palmar arches.
The superficial palmar arch is situated beneath a connective tissue called the palmar fascia and is primarily supplied by the ulnar artery with a small contribution from a branch of the radial artery. This arch gives rise to smaller arteries:
- The first branch connects with the deep palmar arch.
- The second branch supplies blood to the little finger.
- The other branches, third to sixth, supply blood to the fingers.
About 80% of people have a complete superficial arch, which means it runs a full circle.
The deep palmar arch runs above the superficial arch and is mainly provided with blood by a branch of the radial artery, again with a small contribution from a branch of the ulnar artery. This arch also gives rise to several branches:
- The first artery courses between two muscles in the thumb, supplying it with blood.
- The second artery specifically supplies the index finger with blood.
- The other branches supply the spaces between the fingers with blood.
About 97% of people have a complete deep arch.
While the radial and ulnar arteries mainly supply the hand with blood, the anterior and posterior interosseous arteries also contribute by supplying blood to the back of the hand. Here, another loop – the dorsal carpal arch – arises and gives rise to further arteries supplying the hand.
Why do People Need Allen Test
The Allen test is a simple exercise performed by medical professionals to evaluate the blood flow to your hand. It’s particularly important when considering certain procedures that might affect the normal functioning of blood vessels (arteries) in the hand. These procedures can include taking a piece of tissue with its blood vessels from the wrist area (known as harvesting a radial forearm flap), or inserting a thin tube into a vessel in the hand for various medical treatments – a process known as radial artery cannulation or catheterization. Both of these procedures might lead to blood clots (thrombosis), which can compromise the blood supply to the hand.
The Allen test checks if your hand can still get enough blood from the ulnar artery and nearby vessels if the radial artery gets blocked. If the test is positive, it means you probably have a good back-up blood supply to the hand. However, a negative result indicates that the back-up blood supply to the hand may not be enough. This is an important consideration, as it could be risky to proceed with procedures like catheterization or radial forearm flap harvesting, or any procedure that might block a vessel.
When a Person Should Avoid Allen Test
There are no definitive reasons for which the Allen test cannot be done.
Equipment used for Allen Test
The Allen test is a simple exam that your doctor can do using only his or her hands, and doesn’t need any special medical tools. However, in some cases, a tool called a Doppler probe might be placed on the thick pad of your thumb, or a device called a pulse oximeter might be put on your thumb. These tools can be helpful, especially if you have a condition called Raynaud’s phenomenon which can make your hands feel cold or look pale.
If you need to have surgery, your doctor may use a special type of clamp known as an Acland microvascular clamp and a Doppler probe. These tools can be used to double-check the results of the Allen test before they tie off your radial artery, which is one of the main blood vessels in your arm. They may also be used in cases where the artery has been accidentally damaged during a procedure.
Who is needed to perform Allen Test?
The Allen test is a simple medical procedure that can be performed by any healthcare professional who has the right training. This could be a nurse, doctor, or other medical worker. They have learned how to do the test properly to make sure it provides correct and useful results.
How is Allen Test performed
The Original Allen Test is used to verify how well blood flows through the arteries in your hands. Here’s how it’s done:
First, the patient raises both arms above their head for about half a minute to drain the blood from the hands. Then, the patient tightly clenches their fists for up to a minute while the doctor blocks the main artery (radial artery) in both arms. Once this is done, the patient quickly opens their hands and the doctor checks the color of the palms.
Initially, the palms will look pale, but should regain their normal color as blood flow is restored through the secondary artery (ulnar artery). The test is then repeated, but this time the doctor blocks the ulnar artery instead of the radial artery. The time it takes for the color to return to normal gives a measure of how well blood can flow to the hand through other channels when the main artery is blocked. If the color returns to normal when only one artery is blocked, this suggests that blood flow is adequate. However, a pale palm can indicate poor blood flow to the hand.
The Modified Allen Test is a little bit different as it examines one artery at a time in a single hand. The patient performs it by bending their arm at the elbow and clenching their fist tightly to drain blood from their hand. The doctor then presses on both the ulnar artery and radial artery at the same time, blocking blood flow. Then, the elbow is straightened out and the palm should turn pale.
After this, pressure on the ulnar artery is released while the radial artery remains blocked, and the color should return to the palm within 5 to 15 seconds. This test can be repeated by releasing the radial artery first while the ulnar artery remains blocked. If the color returns quickly (within 15 seconds), this means that blood flow is adequate. If it takes longer, it may suggest that the blood circulation is not good. For example, if the palm remains pale when the radial artery is blocked, this could mean that blood flow through the ulnar artery is poor, and the other way around.
To improve the accuracy of the test, additional tools can be used, such as digital plethysmography (a method of measuring volume changes), duplex ultrasonography (an ultrasound technique that shows how blood moves through your arteries and veins), or pulse oximetry (a test used to measure your oxygen level). Using pulse oximetry during the Allen test is the simplest method. This involves placing a pulse oximeter on the thumb before the arteries are blocked to measure the level of oxygen and displaying the result as a graph. After that, when pressure is released from the ulnar artery, these measurements are repeated and compared to the initial ones. If they are the same, this suggests that blood flow is adequate.
Possible Complications of Allen Test
While the Allen test, a basic physical examination, doesn’t have any common complications associated with it, it can be critical in preventing severe damage and loss of tissue in the hand. This can happen when the blood circulation to the hand is not sufficient if either the ulnar or radial arteries – blood vessels in your arm that carry blood to your hand – become compromised.
If the radial artery – the one running along the thumb side of your arm – is compromised, tissue loss is more likely to happen in the thumb and the palm muscle near the thumb (the thenar eminence). This is because these areas are farthest from the ulnar artery – the one running along the little finger side of the arm – which would be the remaining supplier of blood. If an inadvertent injury to the radial artery affects the hand’s blood flow resulting in inadequate blood supply (ischemia), surgical treatment may be necessary. This treatment could involve directly connecting the ends of the blood vessel (a primary anastomosis) or using a vein graft, where a piece of vein from elsewhere in the body is used to bypass the blockage.
What Else Should I Know About Allen Test?
The Allen test and the modified Allen’s test (MAT) are used to check if the blood flow in your hand is normal. This is important before any medical procedure that could affect the blood flow in the main blood vessels of your hand, called the radial and ulnar arteries. These tests can also help diagnose conditions that could decrease blood flow to your hands.
These tests are often used when doctors need to use the radial artery, one of the main blood vessels in your wrist, for a medical procedure. For example, they might need to take a blood sample, put in a line to monitor blood pressure, do a heart test, or use part of the artery for heart bypass surgery or reconstructive surgery.
The radial artery is often chosen for these procedures because it’s easier to feel under the skin than the ulnar artery. But one of the risks is that the area below where the procedure is done might not get enough blood, which could harm the hand or arm. This risk is small, but it’s one of the reasons the Allen test or MAT is done first.
However, not all doctors do these tests before using the radial artery because there’s some debate about how accurate they are. One study found that the MAT is pretty good at finding problems with blood flow before using the radial artery for heart bypass surgery. But another review of studies found that the tests might not be as reliable.
A recent study suggested that the MAT is very good at ruling out problems if the test is negative. However, doctors may need to do other tests if the Allen test or MAT is positive, just to be sure.
