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The procedure for performing a modified Allen’s Test is as follows:

  1. Instruct the patient clench his/her fist, or if the patient is unable, you may close the hand tightly.

  2. Using your fingers, apply occlusive pressure to both the ulnar and radial arteries. This maneuver obstructs blood flow to the hand.

  3. While applying occlusive pressure to both the arteries, have the patient relax his/her hand. Blanching of the palm and fingers should occur. If it does not, you have not completely occluded the arteries with your fingers.

  4. Release the occlusive pressure on the ulnar artery. You should notice a flushing of the hand within 5 to 15 seconds. This denotes that the ulnar artery if patent and has good blood flow. This normal flushing of the hand is considered to be a positive modified Allen’s test. A negative modified Allen’s test is one in which the hand does not flush within the specified time period. This indicates that ulnar circulation is inadequate or nonexistence. The radial artery supplying arterial blood to that hand should not be punctured.

 

Testing for collateral circulation to the hand by evaluating the patency of the radial and ulner arteries.

The Allen Test

  • The hand is elevated and the patient/person is asked to make a fist for about 30 seconds.

  • Pressure is applied over the ulnar and the radial arteries so as to occlude both of them.

  • Still elevated, the hand is then opened. It should appear blanched (pallor can be observed at the finger nails).

  • Ulnar pressure is released and the color should return in 7 seconds.

Inference: Ulnar artery supply to the hand is sufficient and it is safe to cannulate/prick the radial

If color does not return or returns after 7 seconds, then the ulnar artery supply to the hand is not sufficient and the radial artery therefore cannot be safely pricked/cannulated.

 

Anatomical basis

The hand is normally supplied by blood from the ulnar and radial arteries. The arteries undergo anastomosis in the hand. Thus, if the blood supply from one of the arteries is cut off, the other artery can supply adequate blood to the hand. A minority of people lack this dual blood supply.

Significance

An uncommon complication of radial arterial blood sampling/cannulation is disruption of the artery (obstruction by clot), placing the hand at risk of ischemia. Those people who lack the dual supply are at much greater risk of ischemia. The risk can be reduced by performing Allen's test beforehand. People who have a single blood supply in one hand often have a dual supply in the other, allowing the practitioner to take blood from the side with dual supply.

The utility of the Allen's test is questionable, and no direct correlation with reduced ischemic complications of radial artery cannulation have ever been proven. In 1983, Slogoff and colleagues reviewed 1,782 radial artery cannulations and found that 25% of them resulted in complete radial artery occlusion, without apparent adverse effects. A number of reports have been published in which permanent ischemic sequelae occurred even in the presence of a normal Allen's test. In addition, the results of Allen's tests do not appear to correlate with distal blood flow as demonstrated by fluorescein dye injections or photoplethysmography.

 

 

Allen's test is not routinely used before radial arterial puncture

In his article on arterial blood gas analysis Williams repeats the common advice to perform a modified Allen's test before attempting radial artery puncture. My impression is that this advice is never carried out in practice, and a survey of anaesthetist colleagues confirmed that none of six specialist registrars and eight consultants (with a combined experience of several thousand radial artery punctures) used the test routinely.

Allen's test has a poor sensitivity and specificity for complications after radial artery cannulation. In a series of 1699 patients undergoing arterial cannulation for coronary artery surgery, 16 of 411 who had an Allen's test had abnormal results. None of these 16 had complications from radial arterial cannulation. Mandel and Dauchot have reported serious complications in 2 of 982 patients who had a normal result of an Allen's test before radial arterial cannulation.

The available evidence does not support the routine use of Allen's test before radial artery puncture. Nevertheless, because of the rare incidence of serious complications, common sense suggests that all patients should have regular clinical observation of their hand and finger blood supply after arterial puncture or cannulation.

Adrian Steele, Specialist registrar in anaesthesia. 
St Helier Hospital, Carshalton, Surrey SM5 1AA Spinder@sthelier.sghac.ukms

 

 

 

 

 

 
 
 
 

    

         

 

 

 

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