Wrist Orthopaedic Test: Allen Test

The Allen test is a simple, non-invasive test that is used to evaluate the adequacy of blood flow to the hand. It is commonly performed before a radial artery cannulation or arterial line placement, to ensure that the radial artery is the best site for arterial access and to assess the presence of any arterial abnormalities or occlusions.

Here is a step-by-step procedure for performing the Allen test:

1. Position the patient's arm so that the wrist is extended and the hand is slightly elevated.
2. Identify the radial artery and the ulnar artery at the wrist crease. The radial artery is located on the thumb side of the wrist, and the ulnar artery is located on the pinky side of the wrist.
3. Using your thumb and index finger, compress both the radial and ulnar arteries simultaneously, cutting off blood flow to the hand. Instruct the patient to relax their hand and fingers.
4. Observe the patient's hand and note the time it takes for the hand to become pale or blanched.
5. Ask the patient to open and close their hand several times to allow blood flow to return to the hand.
6. While maintaining pressure on the radial and ulnar arteries, release the pressure on the ulnar artery only and observe the time it takes for the patient's hand to return to its normal color.
7. If the hand remains pale or takes longer than 10 seconds to regain its color, it may indicate an occlusion or stenosis of the ulnar artery, which could potentially cause ischemia to the hand if the radial artery is cannulated.
8. If the hand quickly regains its normal color within 5-10 seconds, it indicates that blood flow through the ulnar artery is adequate and the radial artery can be safely cannulated.
9. In some cases, the Allen test may be repeated on the other hand for comparison or confirmed with a Doppler ultrasound.

A normal result of the Allen test is rapid return of color to the hand after release of pressure on the ulnar artery, indicating that the ulnar artery is functioning properly and providing adequate blood flow to the hand. This suggests that the radial artery can be safely cannulated for procedures such as arterial blood gas sampling or continuous blood pressure monitoring.

If the hand remains pale or takes longer than 10 seconds to regain its color, it may indicate an occlusion or stenosis of the ulnar artery, which could potentially cause ischemia to the hand if the radial artery is cannulated. In such cases, other sites for arterial access may need to be considered.

References:

1. Allen, E. V. (1929). Thromboangiitis obliterans: Methods of diagnosis of chronic occlusive arterial lesions distal to the wrist with illustrative cases. The American journal of the medical sciences, 178(2), 237-244.
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