In medicine, the Modified Allen's test, also the Modified 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 colour should return in 7 seconds.
If color returns as described above, the Allen's test is considered to be "positive." If color fails to return, the test is considered "negative" and the ulnar artery supply to the hand is not sufficient. The radial artery therefore cannot be safely pricked/cannulated.
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.
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.
Allen's test is also performed prior to heart bypass surgery. The radial artery is occasionally used as a conduit for bypass surgery, and its patency lasts longer in comparison to the saphenous veins. Prior to heart bypass surgery, Allen's test is performed to assess the suitability of the radial artery to be used as a conduit. A result of less than 3 seconds is considered as good and suitable. A result of between 3-5 seconds is equivocal, whereas the radial artery will not be considered for grafting if the result is longer than 5 seconds.
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
Modifications to the test have been proposed to improve reliability.
- Jarvis MA, Jarvis CL, Jones PR, Spyt TJ (October 2000). "Reliability of Allen's test in selection of patients for radial artery harvest". Ann. Thorac. Surg. 70 (4): 1362–5. doi:10.1016/S0003-4975(00)01551-4. PMID 11081899.
- Slogoff, S; Keats AS, Arlund C (1983). "On the safety of radial artery cannulation.". Anesthesiology 59: 42–7.
- Thompson, SR; Hirschberg A (1988). "Allen's test re-examined.". Crit Care Med 16: 915.
- Wilkins, RG (1985). "Radial artery cannulation and ischaemic damage: A review.". Anaesthesia 40: 896–899.
- McGregor, AD (1987). "The Allen test – an investigation of its accuracy by fluorescein angiography.". J Hand Surg (BR) 12: 82–85.
- Stead, SW; Stirt JA (1985). "Assessment of digital blood flow and palmar collateral circulation.". Int J Clin Monit Comput 2: 29.
- Asif M, Sarkar PK (August 2007). "Three-digit Allen's test". Ann. Thorac. Surg. 84 (2): 686–7. doi:10.1016/j.athoracsur.2006.11.038. PMID 17643672.