The modified Allen test is used to check for collateral circulation of the arteries in the wrist. It is often performed prior to an arterial puncture to ensure that adequate blood flow is present in both arteries.
This article will discuss the procedure, indications, and steps on how to properly perform a modified Allen test before sticking an arterial blood gas (ABG).
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What is a Modified Allen Test?
The Allen test for assessment of blood flow was originally developed by Edgar V. Allen in 1929 as a noninvasive method of assessing arterial patency.
In modern medicine, it has been adopted as the modified Allen test and is used to check for collateral circulation of the radial and ulnar arteries in the wrist.
The difference between the modified Allen test and the original Allen test is that the modified version efficiently evaluates the adequacy of blood circulation on one hand at a time. It is used to measure the competency and quality of the artery and should be performed prior to collecting an arterial blood sample.
How to Perform a Modified Allen Test
The procedure of performing a modified Allen test requires several steps, including the following:
1. Have the Patient Make a Fist
Instruct the patient to clench their fist in order to enhance circulation within the arteries. If the patient lacks the ability to do so, you can assist by manually closing the fist for them.
2. Locate the Radial and Ulnar Arteries
Face the patient and locate the radial and ulnar arteries. The radial artery is located on the thumb side of the wrist on the underside of the forearm. The ulnar artery is located on the pinky side of the wrist.
During this step, make sure to locate both the radial and ulnar pulses.
3. Grab the Patient’s Hand
Using your right hand, slowly grab the patient’s left hand. You can also use your left hand to grab the patient’s right hand if that is your preference. Do whichever is more comfortable for both you and the patient.
4. Locate the Pulse
Place your middle finger on top of the radial pulse and your pointer finger on the ulnar pulse of the patient’s wrist.
5. Apply Pressure to Both Arteries
When both pulses can be felt, apply occlusive pressure to both the ulnar and radial arteries. This will temporarily stop blood circulation to the hand. Be sure that the patient’s hand is relaxed during this step.
6. Have the Patient Open Their Hand
This step is performed to check whether the palm and fingers have blanched. Blanching means that you have completely occluded the radial and ulnar arteries with your fingers. At this time, the hand should appear to have a whitish color.
7. Slowly Release the Pressure on the Ulnar Artery
You can release pressure on the ulnar artery by removing your finger while keeping the radial artery occluded. After doing so, the patient’s hand should flush within 5 to 15 seconds.
When flushing occurs, the hand turns pink in color, which indicates that blood is flowing into the hand. This indicates that the ulnar artery is patent and circulation is normal.
Modified Allen Test Results
There are two possible results that can be seen after performing the test:
- Positive modified Allen test
- Negative modified Allen test
Each result depends on the amount of time it takes for the patient’s hand to flush after releasing the pressure on the ulnar artery.
Positive Modified Allen Test
A positive modified Allen test occurs when blood immediately flushes into the hand after releasing pressure on the ulnar artery. This result indicates that both the radial and ulnar arteries are patent, and circulation is normal.
Negative Modified Allen Test
A negative modified Allen test occurs when blood does not flush into the hand within 5 to 15 seconds after releasing pressure on the ulnar artery. This result is an indication that the ulnar artery is occluded and collateral circulation is not present.
In this case, you should not puncture the radial artery in the hand that was tested. Instead, you can perform a separate modified fallen test on the other hand to check for collateral circulation.
If a negative modified Allen test is obtained in both hands, you should use an alternative site for an arterial puncture, such as the brachial artery.
What is the Purpose of the Modified Allen Test?
The purpose of the modified Allen test is to check for the patency of both the radial and ulnar arteries. Additionally, the test can be used to check for collateral circulation in the event that one of the arteries is occluded.
What is an Arterial Blood Gas?
An arterial blood gas (ABG) is a test that involves the collection of an arterial blood sample to measure the oxygen and carbon dioxide levels in the blood. The test is used to assess lung function and check for respiratory problems.
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What Arteries are in the Wrist?
There are two arteries in the wrist: the radial artery and ulnar artery. The radial artery is located on the thumb side of the wrist, while the ulnar artery is located on the pinky side of the wrist.
During a modified Allen test, each artery is compressed in order to temporarily stop blood flow to the hand. Then one is released to check for collateral circulation.
What is Collateral Circulation?
Collateral circulation is a term that refers to the flow of blood through alternative routes when the primary route is blocked.
In the case of the modified Allen test, collateral circulation occurs when blood flows through the ulnar artery in the event that the radial artery is occluded.
What is Blanching?
Blanching is a term that refers to the whitish color of the skin when blood flow has been cut off. During a modified Allen test, blanching occurs when both the radial and ulnar arteries are occluded, and blood flow to the hand has been stopped.
What is the Difference Between the Allen and Modified Allen Test?
The Allen test is similar to the modified Allen test in that it involves the compression of both the radial and ulnar arteries. However, in the Allen test, both arteries are released at the same time instead of one at a time.
Additionally, the Allen test does not check for collateral circulation like the modified Allen test does. For this reason, the modified Allen test is a more accurate test for assessing radial and ulnar artery patency.
The modified Allen test is a quick and easy way to check for radial and ulnar artery patency. Additionally, the test can be used to check for collateral circulation in the event that one of the arteries is occluded.
It is a requirement that respiratory therapists perform this test before collecting an ABG sample. Thanks for reading and, as always, breathe easy, my friend.
John Landry, BS, RRT
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
- Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
- Faarc, Heuer Al PhD Mba Rrt Rpft. Wilkins’ Clinical Assessment in Respiratory Care. 8th ed., Mosby, 2017.
- “Modified Allen Test – WHO Guidelines on Drawing Blood – NCBI Bookshelf.” NCBI, 30 July 2020, www.ncbi.nlm.nih.gov/books/NBK138652.