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The modified Allen test is used to evaluate the patency of the palmar collateral circulation through the radial and ulnar arteries before either of these vessels is punctured. The patient is asked to elevate the hand (or form a fist), after which pressure is applied to the radial and ulnar arteries simultaneously to occlude palmar perfusion. The palm should appear pale if the arterial occlusion is complete. Pressure over one of the arteries is then released while maintaining the occlusion of the other. Palmar collateral circulation is considered adequate if palmar hyperemia occurs. The procedure is repeated with the other artery.

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