What is an Allen test?

What is an Allen Test?

The Allen test is a clinical maneuver used to assess collateral circulation to the hand by evaluating the patency and adequacy of blood flow from both the radial and ulnar arteries through the palmar arch. 1

How to Perform the Test

The modified Allen test involves the following steps:

• The examiner occludes both the patient's radial and ulnar arteries while the patient makes a tight fist, causing the hand to blanch 2
• The patient then opens the hand (without hyperextending the fingers, as this can decrease perfusion to the palmar arch and cause false results) 2
• The examiner releases pressure on the ulnar artery while maintaining compression of the radial artery 2
• Normal color return to the hand within 5-6 seconds indicates adequate collateral circulation from the ulnar artery 3
• The test can be repeated by releasing the radial artery while maintaining ulnar compression to assess radial artery contribution 2

Traditional Clinical Indication

The ACC/AHA guidelines recommend performing Allen's test when knowledge of hand perfusion is needed, particularly as part of the vascular physical examination 1. Historically, it was used before:

• Radial artery cannulation for arterial blood gas sampling 4
• Radial artery harvest for coronary artery bypass grafting 3, 5
• Transradial catheterization procedures 1

Current Evidence and Controversy

However, recent high-quality evidence demonstrates that the Allen test is "only of historical interest" and should no longer be routinely performed before transradial access. 6 Here's why:

• The RADAR trial found no evidence of hand ischemia in patients undergoing transradial access, even in those with abnormal Allen test results 1, 6
• No differences in thumb capillary lactate, grip strength, or incidence of ischemia were found between patients with normal versus abnormal Allen test results 6
• Hand ischemia following transradial procedures is extraordinarily rare due to robust collateral circulation from the ulnar artery through the palmar arch 6
• Radial artery occlusion occurs in less than 5% of patients with current prevention strategies and is almost always clinically silent 1, 6

Test Performance Limitations

The Allen test has significant diagnostic limitations:

• At the conventional 6-second cutoff: sensitivity 54.5%, specificity 91.7%, diagnostic accuracy only 78.5% 3
• At the optimal 5-second cutoff: sensitivity 75.8%, specificity 81.7%, diagnostic accuracy 79.6% 3
• At no cutoff point does the Allen test perform satisfactorily as a discriminatory test 3

Current Best Practice Recommendation

An abnormal Allen test should NOT preclude transradial access or radial artery procedures. 6 The American Heart Association and American College of Cardiology consensus is that:

• Routine application of the Allen test is not a useful triage strategy for radial artery access 6
• Patients should not be denied a radial approach simply because of a "failed" Allen test 1, 6
• Cardiac specialists no longer routinely confirm collateral circulation with Allen's test as it does not predict hand ischemia 6

Alternative Assessment Methods

If vascular assessment is truly needed:

• Ultrasound imaging can identify an occluded radial artery that fills via retrograde collaterals 6
• Duplex ultrasonography predicts safe radial artery harvest even in patients with abnormal Allen tests 5
• The reverse Allen or Barbeau test may help identify occluded radial arteries 6

Common Pitfalls to Avoid

• Do not hyperextend the fingers when opening the hand during the test, as this decreases perfusion to the palmar arch and causes false-positive results 2
• Do not deny patients transradial procedures based solely on an abnormal Allen test result 1, 6
• Do not assume the test reliably predicts hand ischemia—documented cases of hand ischemia have occurred even with negative tests, and the test has poor sensitivity 7, 3