The Allen Test for Blood Flow

Original Editors - Alexander Ghyssels,Sheik Abdul Khadir

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Introduction 

• The Allen test is a worldwide standardised test used to determine whether the patency of the radial or ulnar artery is normal. It is performed prior to radial cannulation or catheterisation, as placement of such a catheter may result in thrombosis. The test is therefore used to reduce the risk of ischemia to the hand. The Allen test can also be used preceding the removal of the radial artery for a coronary bypass graft.
  
• A positive Allen Test shows that the patient does not have a dual blood supply to the hand, which is a negative indication for catheterisation or removal of the radial arteries.
• There is also a digital Allen test, which is used to assess the arterial supply to the fingers.
  

Clinically Relevant Anatomy

• The supply in the arm starts in the neck between the scalenus muscles. It is the subclavian artery who supply the arm from above. The subclavian artery becomes the arteria axillaris, who becomes after 3 inches an arteria brachialis. In the region af the elbow (fossa cubita) divides the arteria in two pieces: the ulnar artery and the radial artery.
• The hand has a complex and rich vascular network, which is mostly supplied by the radial and ulnar arteries.
• The radial artery runs between the brachioradialis and flexor carpi radialis muscles. At the wrist, it splits into a superficial branch to contribute to the superficial palmar arch. The other branch crosses dorsally deep to the tendons of the anatomic snuffbox to form the deep palmar arch.
• The ulnar artery lies under flexor carpi ulnaris. At the wrist it enters the Guyons canal, where it splits into a deep palmar branch and a superficial palmar branch. The superficial palmar branch forms the superficial palmar arch, while the deep branch contributes to the deep palmar arch.
• Since the radial as well as the ulnar arteries both form anastomosis (cfr. the deep and superficial palmar arch) the blood supply of the hand is ensured even if one of the arteries is occluded,

Epidemiology 

In 3% of the hospitalised patients an inadequate collateral blood supply was diagnosed by an Allen test. 

Differential Diagnosis

The Allen test can be used as a diagnostic tool for a number of disorders, such as any diseases with reduced vascularisation in the arm, i.e. thoracic outlet syndrome or compartment syndrome.

The Allen test is commonly used after operations to the hand to determine whether the blood supply is normalised:

Diagnostic Procedures

The Allen test is also used to asses whether the radial artery is patent enough for a coronary bypass graft. A negative test is where the first three digits become pale, indicating a lack of blood flow.

Examination 

The patient is asked to open and close the hand several times as quickly as possible and then to squeeze the hand tightly as possible . The examiner's thumb and index finger are placed over the radial and ulnar arteries, compressing them . Alternatively, the examiner may use both hands, compressing each artery and placing the fingers on the posterior aspect of the arm for stability. The patient then opens the hand while pressure is maintained on the arteries. One artery is tested at a time by releasing the pressure over that artery to see if the hand flushes. The other artery is then tested in a similar fashion. Both hands should be tested for comparison . 

Specificity & Sensitivity 

Martin et al (2000) reported a sensitivity of 73.2% and a specificity of 97.1%. Based on these findings the Allen test is a good and valid screening test for blood supply of the hand.