Allen's test and systolic arterial pressure in the thumb.

The result of Allen's test for arterial patency was compared with systolic arterial pressure in the thumb following manual occlusion of the radial artery in 8 patients aged 11-72 yr undergoing cardiovascular surgery. In 19 of 235 hands (8.1%), manual compression of the radial artery reduced the systolic arterial pressure in the thumb to less than 40 mm Hg, indicating inadequacy of the collateral ulnar arterial supply. The predictive value of a negative Allen's test was 0.992. Thus, in only 0.8% of cases would a negative Allen's test falsely indicate adequacy of the collateral ulnar supply. These results justify general application of Allen's test before radial artery cannulation.