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. You might need this if you are having certain types of surgery, if you lose a lot of blood from bleeding, or if you have certain blood conditions.

If you need a blood transfusion, the blood bank or transfusion center will make sure to avoid giving you blood your immune system will attack. This can happen if you get blood that has antigens your immune system sees as foreign.

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 do an "antibody screen." If your blood type is Rh-negative, your immune system might attack the fetus's blood. You might get shots during your pregnancy to help prevent this from happening. Your doctor will tell you if you should get these shots.

You might find out your blood type if you donate blood, although you should not give blood just to learn your blood type. The blood bank will check your 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 who share your blood type.

How do I prepare for a blood type 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 to your blood draw appointment. This makes it easier for the person drawing your blood to get to your arm.

What happens during a 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 have a bleeding problem – They will make sure 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 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.

When you need a transfusion, several tests are done. One is the blood type, and the other is called a "screen" or "antibody screen." Once the blood bank chooses a unit of blood to give you, they do a "crossmatch." These other tests help the blood bank know if the blood they want to give you will be attacked by your immune system. Together, the tests are called "type and screen" or "type and crossmatch."

If it has been more than a few days since your last blood type test, the tests will be done again see if there are any changes. Your blood type does not change, but your immune system might have changed. This can happen if you have been exposed to blood from other transfusions or have been pregnant.

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

More on this topic

Patient education: Red blood cell antibody screening (The Basics)
Patient education: Coombs test (The Basics)
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)