Blood Type Test: About This Test

What is it?

Blood type tests are used to check what blood type you have. Your blood is "typed" according to whether certain markers, called antigens, are present on the surface of red blood cells.

The two most common blood type tests are the ABO and Rh tests. The ABO test shows which of four blood types you have: A, B, AB, or O. These letters stand for which kind of antigen you have on your red blood cells. If your red blood cells do not have either the A or B antigen, you have type O blood.

An Rh test checks for the Rh antigen (or Rh factor). It is done in early pregnancy to check a woman's blood type. Problems can occur when a woman who has Rh-negative blood becomes pregnant with a baby (fetus) that has Rh-positive blood. If this happens, treatment is available to help avoid these problems.

Why is this test done?

A blood type test is done:

• Before you get a blood transfusion.
• When a woman is planning to become pregnant or first becomes pregnant.
• Before you donate blood.
• Before you have surgery.
• Before a person donates an organ for transplantation.
• To show whether two people could be blood relatives.
• To check the identity of a person suspected of committing a crime.

How do you prepare for the test?

In general, there's nothing you have to do before this test, unless your doctor tells you to.

How is the test done?

A health professional uses a needle to take a blood sample, usually from the arm.

What happens after the test?

• You will probably be able to go home right away.
• You can go back to your usual activities right away.