Modified Allen Test Steps

This review will cover the modified Allen test.

The modified Allen test is performed before collecting an arterial blood gas (ABG) on the radial artery. An arterial blood gas, sometimes referred to as an ABG, is a blood test collected from an artery that assesses the oxygenation and acid-base balance of a patient.

The blood sample for this test can be obtained through an arterial line, also known as an art line, which provides direct access to a patient’s radial artery. Many patients in the ICU have an arterial line, but the sample can also be obtained through a needle stick via an artery. The radial artery is the most commonly used for this type of test, although the femoral and brachial arteries can be used as well.

Now, why do we perform a modified Allen test before collecting an arterial blood gas sample from the radial artery?

Let’s talk about the radial artery for a moment. Locate your thumb, and just below it where your radial bone is, is your radial artery, which you can feel pulsating. Next to it, on the other side where your ulna bone is located, is your ulnar artery. These two arteries work together to deliver fresh, oxygenated blood to your hand and its structures, and we always want to ensure these arteries can do that.

The modified Allen test checks blood flow to the hand to ensure there is good collateral blood flow to the hand in case the radial artery develops problems after ABG collection. In other words, it ensures that your ulnar artery can work as a backup if needed.

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Modified Allen Test Steps

Before performing the test, turn the patient’s hand so the palm is facing up. Have the patient make a clenched fist, then find the radial and ulnar arteries.

To help remember the steps of this test, remember the word “Allen.“

Apply firm pressure to the radial and ulnar arteries at the same time using your thumbs or fingertips, temporarily stopping blood flow to the hand.

Let the patient open and close their hand several times. The hand will start to lose its color, appearing lighter or blanched.

Loosen the pressure on the ulnar artery only, while keeping pressure on the radial artery.

Evaluate the return of blood flow from the ulnar artery to the hand.

Normal response is when the hand returns to its normal color, appearing flushed within less than 5 seconds. If this occurs, you know the radial artery is good to go for collecting an arterial blood gas.