Patient education: Blood type test (The Basics)

What is a blood type test? — This is a blood test to find out your blood "type." Your basic blood type has 2 parts: your "ABO blood group" and your "Rh blood group." There are many other blood groups as well. But these are not always tested.

Your ABO blood group is based on whether your red blood cells ("RBCs") have A or B antigens. Antigens are proteins on the outside of the cell. There are 2 antigens in the ABO blood group: A and B. Based on which antigens you have, your ABO blood type can be:

●Type A – Your RBCs have A antigens only.

●Type B – Your RBCs have B antigens only.

●Type AB – Your RBCs have both A and B antigens.

●Type O – Your RBCs have neither A nor B antigens.

Your Rh blood group is based on whether your RBCs have the "RhD" antigen. You can be either "positive" or "negative":

●Rh-positive (or RhD-positive) – Your RBCs have the RhD antigens. This might be shown with a "+" symbol.

●Rh-negative (or RhD-negative) – Your RBCs do not have the RhD antigens. This might be shown with a "–" symbol.

So, for example, if your RBCs have the B antigen and the RhD antigen, your blood type is B+ or "B positive." If your RBCs have both A and B antigens, but no RhD antigen, your blood type is AB– or "AB negative."

Depending on why your blood is being tested, the test might check for other antigens, too.

Why might I get a blood type test? — The main reasons are if you:

●Need a blood transfusion – This means receiving donated blood. If you need a blood transfusion, the blood bank or transfusion center will make sure to give you blood that your immune system will not attack. This can happen if you get blood that has antigens that your blood does not have. An immune system attack can cause a very serious problem called a "hemolytic transfusion reaction." The table shows which blood types you can receive based on your blood type (table 1).

●Are pregnant – Your doctor or nurse will check your blood type and the fetus's blood type. If your blood type is Rh-negative and the fetus's blood type is Rh-positive, your immune system might attack the fetus's blood. If this is the case, you can get shots during your pregnancy to help prevent an immune system attack.

You might find out your blood type if you donate blood. You do not need to know your blood type to donate blood. The blood bank will check the blood type and keep track of it. They will also give you a card with your blood type listed. If you have a very rare blood type, you might be able to help people by donating blood when there is a shortage.

How do I prepare for a blood type test? — For this test, you need to get a "blood draw." Your doctor or nurse will tell you where to go for this.

It might help to wear a short-sleeve shirt. This makes it easier for the person drawing your blood to get to your arm.

What happens during a blood draw? — For the blood draw, a needle is used to take a small amount of blood from your arm or hand (figure 1). Collecting the blood only takes a few minutes. The blood is then tested in the blood bank.

Tell the person who takes your blood:

●If you take "blood thinner" medicines or if you have a bleeding problem – They will make sure that your bleeding is under control before you leave.

●If you have a latex allergy – Some of the supplies used for blood draws might contain latex.

●If you have a preferred arm to use

If you are donating blood, they will collect about 1 pint (500 mL) of blood. Besides the questions above, the person taking your blood will ask you many other important questions. These are to make sure that the blood is safe to give to other people. Answer the questions honestly. You can also call the blood bank after you leave if you need to change your answers or give more information.

Usually, getting blood taken does not cause problems. You might have a little soreness or bruising where the needle went in.

What do my results mean? — If you need a blood transfusion, your blood type can be used by the blood bank to decide which blood to give you. If it has been more than a few days since your last blood type test, they will do the test again to see if there are any changes. Your blood type does not change, but it is possible for your immune system to have new reactions to other antigens. This can happen if you have been exposed to blood from other transfusions or have been pregnant.

If you are pregnant, your blood type can be used to decide if you or your fetus need more tests or treatments.

More on this topic

Patient education: Blood donation (giving blood) (The Basics)
Patient education: Blood transfusion (The Basics)
Patient education: Pregnancy in Rh-negative people (The Basics)

Patient education: Blood donation and transfusion (Beyond the Basics)