If arterial blood is drawn from the wrist, a procedure called the Allen test may be done before the blood sample is taken. The Allen test helps ensure that the blood flow to your hand is normal. The Allen test is done as follows:

• Two arteries normally supply blood to the hand. The blood flow in both arteries is stopped until your hand becomes pale.
• Blood is then allowed to flow through the artery that will not be used to collect the blood sample.
• If this artery is working well, your hand will quickly regain its normal color. This means that an accidental injury to the artery used for collecting the blood sample will not completely block all blood flow to your hand.
• If the artery that will not be used to collect the blood sample is not working well, your hand will remain pale. The Allen test will then be performed on your other hand. If it also remains pale, the blood often will be collected from another artery, usually in the groin or elbow crease.