Allen's test

Overview
In medicine, Allen's test, also Allen test, is used to test blood supply to the hand. It is performed prior to radial arterial blood sampling or cannulation.

Etymology
It is named after Edgar Van Nuys Allen.

The Allen Test
1) The hand is elevated and the patient/person is asked to make a fist for about 30 secs.

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

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

4) Ulnar pressure is released and the color should return in 7 secs.

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.

Modifications to the test have been proposed to improve reliability.