How to Perform a Modified Allen Test Illustration

How to Perform a Modified Allen Test: Step-by-Step (2024)

by | Updated: Apr 9, 2024

The modified Allen test serves as a vital diagnostic tool to evaluate the patency of the radial and ulnar arteries in the wrist.

Primarily utilized before arterial blood gas (ABG) sampling or radial artery catheterization, the test ensures adequate collateral blood flow to the hand.

The process, though simple, requires precision and an understanding of its significance to ensure patient safety and accurate results.

This article breaks down the steps of the modified Allen test, outlining the benefits of this modification and providing clear, concise instructions on its execution.

What is the Modified Allen Test?

The modified Allen test is a simple, noninvasive clinical procedure used to assess the patency of the radial and ulnar arteries, the two primary arteries of the hand. This test is particularly important before invasive procedures such as radial artery catheterization or arterial blood gas (ABG) sampling from the radial artery, ensuring there’s adequate collateral circulation to the hand in case one artery becomes occluded.

How to Perform the Modified Allen Test

Performing the modified Allen test involves a sequence of carefully executed steps to assess the patency of the radial and ulnar arteries.

Here’s a streamlined guide:

  1. Explain the Procedure: Always inform the patient about what you are going to do, so they are aware and can cooperate effectively.
  2. Instruct the Patient to Make a Fist: Ask the patient to make a tight fist to temporarily restrict blood flow in the hand’s arteries. If they’re unable to do so, gently help them close their fist.
  3. Identify the Radial and Ulnar Arteries: Position yourself facing the patient’s wrist. Detect the radial artery on the thumb side of the wrist and the ulnar artery towards the pinky side. It’s crucial to familiarize yourself with the pulse points of both these arteries.
  4. Hold the Patient’s Hand: Grasp the patient’s left hand with your right hand or vice versa, based on comfort and accessibility for both you and the patient.
  5. Position Your Fingers on the Pulses: Place your index finger on the ulnar pulse and your middle finger on the radial pulse.
  6. Occlude Both Arteries: Once you feel both pulses, apply firm pressure to simultaneously occlude the ulnar and radial arteries. Ensure the patient’s hand remains relaxed.
  7. Instruct the Patient to Open Their Hand: Observe for blanching, signifying successful occlusion. The hand should exhibit a pallor or whitish hue.
  8. Release Pressure on the Ulnar Artery: Gently lift the finger from the ulnar artery while maintaining pressure on the radial artery. The hand should redden or “flush” within 5 to 15 seconds, signifying that the ulnar artery is patent and circulation is intact.

Note: Although the modified Allen test is widely used in clinical practice, it’s essential to understand that it’s a qualitative assessment, and results may vary based on subjective interpretation. Some practitioners may opt for more objective methods, such as Doppler ultrasound, to assess arterial blood flow if there are concerns about vascular patency.

Modified Allen Test Results

The results of a modified Allen test are categorized as either positive or negative, though it’s essential to note that the terminology can be counterintuitive:

Positive vs Negative Modified Allen Test Illustration

Positive Modified Allen Test

The hand returns to its natural color within 5-15 seconds after releasing the ulnar artery. This suggests that the ulnar artery provides adequate blood flow, implying good collateral circulation.

It’s generally safe to proceed with procedures involving the radial artery.

Negative Modified Allen Test

The hand does not regain its color within 5-15 seconds after releasing the ulnar artery. This indicates potentially compromised or absent ulnar circulation.

It’s a warning that the radial artery should not be punctured or cannulated, as there might not be sufficient collateral blood flow to maintain hand viability if the radial artery becomes occluded.

Note: In the clinical setting, a clear understanding of these results is critical, especially if considering procedures like arterial blood gas sampling from the radial artery or radial artery catheterization. If a negative Modified Allen Test is obtained, alternative sites or methods might be needed for the intended procedure.

FAQs About the Modified Allen Test

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.

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 as the modified Allen test does. For this reason, the modified Allen test is a more accurate test for assessing radial and ulnar artery patency.

Is a Positive Modified Allen Test Good or Bad?

A positive modified Allen test indicates that the ulnar artery provides adequate blood flow, suggesting good collateral circulation to the hand.

In other words, a positive result is good as it implies that it’s generally safe to proceed with procedures involving the radial artery.

When Performing a Modified Allen Test, Which Artery is Released First?

When performing a modified Allen test, after occluding both the radial and ulnar arteries simultaneously, the ulnar artery is released first to assess its ability to provide adequate blood flow and collateral circulation to the hand.

Final Thoughts

Accurately evaluating the vascular sufficiency of the hand by using the modified Allen test is paramount to ensuring patient safety in radial artery procedures.

By meticulously following the outlined steps, medical professionals can make informed decisions about the viability of the radial artery for intervention.

It’s imperative, however, to remember that while this test provides valuable insights, it remains a qualitative assessment. When in doubt, opting for more objective assessments may be required.

John Landry, BS, RRT

Written by:

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.

References

  • 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.

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