1. Put on clean, but not necessarily sterile gloves.
2. Have patient hold wrist in supine position.
3. Perform Allen test for collateral flow.

Allen test:
The most common site for arterial puncture is the radial artery at the wrist. Because of the potential for formation of an obstructing thrombus in the artery after puncture it is recommended that the operator demonstrate the presence of good collateral flow through the ulnar artery prior to proceeding. The Allen test is performed by having the patient clench their fist several times while the operator occludes the radial and ulnar artery at the wrist. The patient then extends their fingers, palm up, which should show a "blanched" hand. The operator then releases the pressure on the ulnar artery and the hand is observed for "blushing". If the color of the hand does not return in 5-10 seconds the Allen test is considered positive and arterial puncture should not be attempted at that site. It should be noted that the risks of hand ischemia after arterial puncture are very low and in fact many clinicians do not routinely perform an Allen test. Having said this it is a simple and quick test to perform and it is our recommendation that students do an Allen test prior to attempting arterial puncture at the wrist.