Back Blood-tests Explained


Blood Typing


At a Glance

Why Get Tested?

To determine your ABO blood group and Rh type

When to Get Tested?

When you need a transfusion of blood or blood components; when you donate blood at a collection facility or donate an organ, tissue, or bone marrow for transplantation; before or during a woman's pregnancy to determine the risk of Rh incompatibility with the fetus

Sample Required?

A blood sample drawn from a vein in your arm or, for infants, from a heelstick

Test Preparation Needed?

None

The Test Sample

What is being tested?

Blood types are based on the markers (specific carbohydrates or proteins) or antigens on the surface of red blood cells (RBCs). Two major antigens or surface identifiers on human RBCs are the A and B antigens. Another important surface antigen is called Rh. Blood typing detects the presence or absence of these antigens to determine a person's ABO blood group and Rh type.

People whose red blood cells have A antigens are in blood group A, those with B antigens are group B, those with both A and B antigens are in group AB, and those who do not have either of these markers are in blood group O.

If the Rh protein is present on the red blood cells, a person's blood type is Rh+ (positive); if it is absent, the person's blood is type Rh- (negative).

Our bodies naturally produce antibodies against the A and B antigens that we do not have on our red blood cells. For example, a person who is blood type A will have anti-B antibodies directed against the B antigens on red blood cells and someone who is type B will have anti-A antibodies directed against the A antigens. People with type AB blood have neither of these antibodies, while those with type O blood have both.

The following table indicates the type of antibodies a person is expected to have based on their blood type.

a person with blood type ...will have antibodies to ...
A B antigen
B A antigen
AB Neither antigen
O A and B antigens

These antibodies are useful for determining a person's blood type and help determine the types of blood that he or she can safely receive (compatibility). If a person who is group A with antibodies directed against the B antigen, for example, were to be transfused with blood that is type B, his or her own antibodies would target and destroy the transfused red blood cells, causing severe, potentially fatal complications. Thus, it is critical to match a person's blood type with the blood that is to be transfused.

Unlike antibodies to A and B antigens, antibodies to Rh are not produced naturally. That is, Rh antibodies develop only after a person who does not have Rh factor on his or her red blood cells (Rh negative) is exposed to Rh positive red blood cells. This can happen during pregnancy or birth when an Rh-negative woman is pregnant with an Rh-positive baby, or sometimes when an Rh-negative person is transfused with Rh-positive blood. In either case, the first exposure to the Rh antigen may not result in a strong response against the Rh positive cells, but subsequent exposures may cause severe reactions.

How is the sample collected for testing?

A sample of blood is drawn from a vein in the arm or from the tip of the finger (fingerstick). In newborns, blood from the umbilical cord or a small amount of blood from a heelstick may be used for testing.

Is any test preparation needed to ensure the quality of the sample?

No special preparation is needed for this test.

The Test



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