What Is an Allen Test?

Medically Reviewed by Nayana Ambardekar, MD on April 06, 2024
3 min read

Also called a modified Allen test, this is a simple way to measure how well blood flows in your hand. Your doctor may need to check your circulation before they operate on your wrist or a spot nearby, or before other surgeries.

Two arteries supply blood to your hand. One is the radial artery, and the other is the ulnar artery. A healthy hand needs both arteries open and working. Otherwise, you could get permanent hand damage.

Some reasons why your doctor may want to perform an Allen test include:

Arterial blood gas test. You may need this if you have asthma or other breathing or lung problems. The blood sample is often taken from an artery in your wrist. Because there’s a chance the needle puncture could cause problems to the artery, your doctor may order an Allen test to make sure that the second artery is in good shape.

Wrist surgery. If you have a growth in your wrist near an artery, your doctor will want to make sure each of your arteries can supply your hand with enough blood on its own before the surgery. That way, if one artery gets nicked or damaged, the other artery can do the job.

Heart bypass surgery. Sometimes doctors use the radial artery from the wrist in heart bypass surgery.

Your doctor would take the artery from your wrist to help create a new pipeline for the blood to the heart.

Kidney dialysis. If your kidneys don’t work well enough to filter waste and fluids from your body, a machine called a dialyzer with tubes connected to your blood vessels can do it for you. An Allen test can help find the best location of the artery for the dialysis.

Your doctor will ask you to put your hand palm side up and make a tight fist. They’ll then press on two spots on your wrist to stop blood flow to your hand: A spot on the thumb side of your wrist stops blood flow through your radial artery, and a spot on the pinky-finger side of your wrist stops blood flow through your ulnar artery.

You’ll be told to open your hand. The palm and fingers should look pale, since the blood supply to your hand has been cut off.

Your doctor will stop pressing on the ulnar artery only, to check how quickly color returns to your hand as blood flows through a single artery. They can then check the radial artery flow by pressing on the ulnar artery.

If normal color comes back to your hand within 5-15 seconds, it means that one artery is healthy enough to supply blood to your hand all by itself.

If not, your doctor may not be able to go ahead with a surgery or procedure in case they damage the single working artery.