What is the purpose and procedure of Allen's test (Arterial Blood Flow Assessment)?

Allen's Test: Purpose and Procedure for Arterial Blood Flow Assessment

Allen's test is performed to assess collateral circulation in the hand by evaluating the patency of the radial and ulnar arteries and the palmar arch, primarily used before procedures involving radial artery access or harvest. 1

Purpose of Allen's Test

• Allen's test evaluates the adequacy of collateral circulation to the hand, particularly when knowledge of hand perfusion is needed 2
• It helps determine if the ulnar artery can adequately perfuse the hand if the radial artery becomes occluded 1
• The test is historically used before procedures such as radial artery cannulation, transradial cardiac catheterization, or radial artery harvest for coronary bypass grafting 3

Procedure for Performing Allen's Test

Traditional Allen's Test

• Have the patient elevate their hand and make a tight fist to exsanguinate the hand 4
• Compress both the radial and ulnar arteries at the wrist 4
• Ask the patient to open their hand, which should appear blanched 4
• Release pressure on the ulnar artery while maintaining compression of the radial artery 4
• Observe for return of normal color to the palm and fingers 4

Interpretation

• Normal (negative) test: Hand flushes within 5-6 seconds, indicating adequate ulnar collateral circulation 3
• Abnormal (positive) test: Delayed return of color (>6 seconds), suggesting inadequate ulnar collateral circulation 3

Modified Approaches

• Quantitative Allen's test: Measures digital systolic pressure during compression of either the radial or ulnar artery, with normal being less than 25% reduction in pressure 4
• Barbeau test: Uses plethysmography to objectively assess hand perfusion during radial artery compression 1
• Ultrasound-guided assessment: Provides direct visualization of arterial flow and collateral circulation 1

Clinical Relevance and Current Guidelines

• Recent guidelines suggest that Allen's test is now considered "only of historical interest" for transradial access procedures 1
• The RADAR trial found no evidence of hand ischemia in patients undergoing transradial procedures, even in those with abnormal Allen's test results 2, 1
• Current evidence indicates that patients should not be denied transradial access based solely on an abnormal Allen's test 2, 1

Limitations and Considerations

• At a conventional cut-off of 6 seconds, Allen's test has limited sensitivity (54.5%) but good specificity (91.7%) 3
• Maximum diagnostic accuracy (79.6%) occurs at a 5-second cut-off, with 75.8% sensitivity and 81.7% specificity 3
• More objective tests like Doppler ultrasound may be more reliable for assessing hand circulation 3, 5
• Radial artery occlusion occurs in less than 5% of patients with current prevention strategies and is almost always clinically silent due to collateral circulation from the palmar arch via the ulnar artery 2

Applications Beyond Hand Assessment

• A modified version called the Lower-Extremity Allen Test (LEAT) has been developed to assess the relative contributions of the posterior tibial, dorsalis pedis, and peroneal arteries to foot perfusion 6
• This adaptation uses handheld Doppler to evaluate distal perfusion of the lower extremity 6

Despite its historical importance, current evidence suggests that Allen's test should not be the sole determinant for proceeding with radial artery procedures, as the robust collateral circulation in the hand makes ischemic complications extraordinarily rare 1.