Allen Test

Anatomy and Physiology

The vascular supply to the upper extremities primarily comes from the left and right subclavian arteries, both of which arise from the aortic arch. The left subclavian artery originates directly from the arch, while the right subclavian artery branches off the brachiocephalic (innominate) artery. The subclavian artery travels beneath the clavicle and transitions into the axillary artery as it passes the lateral margin of the first rib. The axillary artery then continues past the teres major muscle and becomes the brachial artery, which supplies blood to the upper arm. As it continues distally past the antecubital fossa, the brachial artery divides into the radial and ulnar arteries, which provide the primary blood supply to the forearm and hand.

Radial Artery

The radial artery runs along the lateral (radial) side of the forearm, positioned between the flexor carpi radialis (FCR) and brachioradialis muscles. The artery then passes through the first extensor compartment of the wrist—between the abductor pollicis longus (APL) and extensor pollicis brevis (EPB)—and the FCR as it enters the dorsal carpus. From there, it divides into a superficial palmar branch, which connects to the superficial palmar arch, and a deep palmar branch, which courses between the heads of the first dorsal interosseous muscle.

Ulnar Artery

The ulnar artery runs along the medial (ulnar) side of the forearm beneath the flexor carpi ulnaris (FCU) muscle. At the wrist, it passes lateral to the ulnar nerve and enters the Guyon canal, where it divides into superficial and deep branches. The radial and ulnar artery branches together form the superficial and deep palmar arches.[6]

Superficial Palmar Arch

The superficial palmar arch runs distal to the deep palmar arch and is situated beneath the palmar fascia. This arch is primarily supplied by the ulnar artery, with a small contribution from the superficial branch of the radial artery.[7] The superficial palmar arch gives rise to 6 principal branches, as mentioned below. 

• First branch: This deep branch communicates with the deep palmar arch.
• Second branch: This is the digital (ulnar) artery of the little finger.
• Third to sixth branches: These branches are the common digital arteries.

In approximately 80% of patients, the superficial arch is complete.[8] 

Deep Palmar Arch 

The deep palmar arch runs proximal to the superficial palmar arch and lies beneath the flexor tendons near the base of the metacarpals. This is primarily supplied by the deep branch of the radial artery, with a small contribution from the deep branch of the ulnar artery. Similar to the superficial palmar arch, the deep palmar arch gives rise to several branches, as mentioned below.

• Princeps pollicis: This artery courses between the adductor pollicis and the first dorsal interosseous muscles.
• Radialis indicis (radial artery of index finger): This is the proper radial digital artery supplying the index finger.
• Common digital artery branches: These branches supply the second to fourth web spaces.

The deep palmar arch is complete in 97% of patients.[9]

Although the radial and ulnar arteries provide the primary vascular supply to the hand, the anterior and posterior interosseous arteries also contribute, supplying blood to the dorsal hand. The dorsal carpal arch further gives rise to the dorsal metacarpal arteries. 

Indications

The Allen test is necessary when considering procedures that may compromise the arterial patency of the hand. These procedures may include harvesting a radial forearm flap, where the radial artery is utilized to perfuse the flap. Additionally, radial artery cannulation or catheterization can result in thrombosis, compromising hand perfusion.[10][11] In such cases, the Allen test helps confirm that the hand can maintain adequate blood flow through the ulnar artery and collateral vessels if the radial artery becomes occluded.

A positive Allen test indicates that the patient likely has an adequate dual blood supply to the hand. Conversely, a negative Allen test suggests that the patient may not have sufficient dual blood supply, which could contraindicate catheterization, radial forearm flap harvesting, or any procedure that might lead to vessel occlusion.[12][13][14]

Contraindications

There are no absolute contraindications for performing the Allen test. 

Equipment

The Allen test can be conducted using only a clinician's hands, without the need for specific equipment. However, in certain situations, a Doppler probe placed on the thenar eminence or a pulse oximeter on the thumb may be beneficial, especially if the patient presents with Raynaud phenomenon and cool or pale hands at baseline.

Intraoperatively, an Acland microvascular clamp and a Doppler probe can be used to confirm the results of the preoperative Allen test before ligation of the radial artery or in cases of iatrogenic arterial injury.[15][16]

Personnel

The Allen test can be conducted by any trained healthcare professional.

Technique or Treatment

Original Allen Test

The original Allen test involves instructing the patient to elevate both arms above their head for 30 seconds to exsanguinate the hands. The patient then clenches their fists tightly for up to 60 seconds while the examiner simultaneously occludes the radial artery in both arms. Afterward, the patient quickly opens their hands, allowing the examiner to compare the color of the palms.

The initial pallor should be replaced by the hands' normal color as the ulnar artery restores perfusion. The test is then repeated with the ulnar artery occluded instead of the radial artery. The time taken for normal color to return indicates the degree of collateral blood flow. The test is considered positive when normal color is restored to both hands during the occlusion of either artery alone. Persistent pallor in the palm suggests inadequate collateral blood flow to the hand.

Modified Allen Test

The MAT differs from the original Allen test, which primarily assesses the radial or ulnar arteries one hand at a time.[17][18] The test is traditionally performed by initially having the patient flex their arm at the elbow with a tightly clenched fist to exsanguinate the hand. The examiner then compresses both the ulnar and radial arteries simultaneously. The elbow is extended to approximately 180 degrees, taking care to avoid overextension, which could result in a false-negative test. The fist is then unclenched, and the palm should appear white.

The compression on the ulnar artery is then released while maintaining pressure on the radial artery. Once the compression is released, the color should return to the palm within 5 to 15 seconds. The test is repeated on the same hand by releasing the radial artery first while continuing to compress the ulnar artery if evaluation of radial collateral blood flow is necessary (see Image. Modified Allen Test).

In a patient with normal, patent arteries, color should return to the palms relatively quickly (within 5-15 seconds in a normothermic patient) after the release of either artery, indicating a positive test. If the rubor of the hand does not return within 15 seconds, it is considered a negative test, suggesting inadequate circulation. For instance, if the radial artery is compressed and palmar pallor persists, this indicates compromised blood flow in the ulnar arterial circulation. Conversely, if the ulnar artery is compressed and palmar pallor persists, it suggests compromised blood flow in the radial arterial circulation.

Supplemental Instrumentation

The examiner can use additional tools that can improve the overall accuracy of the assessment. Digital plethysmography, duplex ultrasonography with dynamic testing, and pulse oximetry—typically with the sensor placed on the thumb tip—can all be utilized.[19] Using pulse oximetry concurrently with the Allen test is the simplest method. This technique involves placing a pulse oximeter on the thumb before compression to obtain baseline saturation and waveform readings.

The examiner compresses the radial and ulnar arteries until the waveform disappears and the oxygen saturation drops to zero. Next, pressure on the ulnar artery is released, and the waveform and saturation are recorded. If these values align with the baseline, adequate collateral flow is indicated.[19]

Complications

Although no specific complications are commonly associated with this simple physical examination maneuver, the most significant risk that can be mitigated by performing the Allen test is catastrophic hand ischemia and subsequent tissue loss if circulation to the hand is compromised. In such cases, either the ulnar or radial artery may be inadequate to maintain proper perfusion of the hand.

If the radial artery is compromised, tissue loss is most likely to occur in the thumb and thenar eminence, as these regions are furthest from the remaining ulnar blood supply. In cases where iatrogenic injury to radial arterial blood flow results in hand ischemia, reconstruction may be necessary, utilizing either primary anastomosis or a vein graft.

Clinical Significance

The Allen test and MAT assess adequate collateral circulation in hand and the presence of a complete palmar arch before performing any any procedure that could potentially disrupt blood flow in the radial or ulnar arteries. Additionally, these tests can serve as diagnostic tools for various disorders that lead to reduced vascularity in the upper extremities. The test is primarily used to evaluate collateral flow to the hand via the ulnar artery when the radial artery is needed for procedures such as blood gas analysis, cannulation for arterial line placement, cardiac catheterization, radial artery harvest for bypass surgeries, and radial forearm flap harvest for reconstructive surgeries.

The radial artery is often selected as a puncture or vascular access site because it is typically more easily palpable than the ulnar artery. However, one of the risks associated with arterial punctures is ischemia distal to the puncture site, which can jeopardize the extremity if collateral blood flow is insufficient. Although ischemia is a rare complication of arterial puncture, many healthcare providers do not perform the Allen test or MAT before accessing the radial artery due to conflicting evidence regarding the accuracy of these tests in assessing ulnar artery patency or collateral circulation adequacy. A study found that the MAT demonstrated a sensitivity of 73.2% and specificity of 97.1% for identifying circulatory deficits before radial artery harvest for coronary artery bypass grafting.[20] 

A 2017 meta-analysis reported a lower specificity of 93% and an interobserver agreement rate of only 71.5%, raising concerns about the overall utility of the test.[3] However, a more recent study demonstrated that MAT has a high negative predictive value.[4] Nonetheless, the authors recommended follow-up diagnostic testing using one of the previously mentioned objective methods if the Allen test result is negative.

Enhancing Healthcare Team Outcomes

Interprofessional healthcare team members, including physicians, physician assistants, nurse practitioners, nurses, and respiratory therapists, should perform an Allen test before cannulating or potentially compromising the radial artery. If the Allen test result is negative, further diagnostic evaluation may be necessary to assess circulation more comprehensively. Although uncommon, some patients may experience hand ischemia if blood flow is disrupted in the radial or ulnar arteries.

All members of the healthcare team should monitor for complications and promptly notify team leaders to facilitate a swift intervention, minimizing further ischemic effects. Ultimately, consistent use of the Allen test, along with coordinated efforts by all healthcare team members to surveil for post-procedural complications, can enhance patient safety and improve overall outcomes.