Blood typing is used to determine an individual's blood group, to establish whether a person is blood group A, B, AB, or O and whether he or she is Rh positive or Rh negative.

Blood typing may be used to:

• Ensure compatibility between the blood type of a person who requires a transfusion of blood or blood components and the ABO and Rh type of the unit of blood that will be transfused. Blood typing is often done in conjunction with other tests such as an RBC antibody screen and a crossmatch to determine what type of blood or blood components the person can safely receive. A potentially fatal transfusion reaction can occur if a unit of blood containing an ABO antigen to which the blood recipient has an antibody is transfused to the recipient. For example, people with blood group O have both anti-A and anti-B antibodies in their blood. If a unit of blood that is group A, B, or AB is transfused to this person, the antibodies in the recipient's blood will react with the red blood cells, destroying them and causing potentially serious complications.

  Similarly, if an Rh-negative individual is transfused with Rh-positive blood, it is likely that the person will produce antibodies against Rh-positive blood. Although this situation does not cause problems for the recipient during the current transfusion, a future transfusion with Rh-positive blood could result in a serious transfusion reaction.

• Determine compatibility between a pregnant woman and her developing baby (fetus). Rh typing is especially important during pregnancy because a mother and her fetus could be incompatible. If the mother is Rh negative but the father is Rh positive, the fetus may be positive for the Rh antigen. As a result, the mother's body could develop antibodies against the Rh antigen. The antibodies may cross the placenta and cause destruction of the baby's red blood cells, resulting in a condition known as hemolytic disease of the fetus and newborn (HDFN). To prevent development of Rh antibodies, an Rh-negative mother is treated with an injection of Rh immune globulin during her pregnancy and again after delivery if the baby is Rh-positive. The Rh immune globulin binds to and "masks" the fetus's Rh antigen during pregnancy and delivery and prevents the mother from developing antibodies against the Rh antigen.
• Determine the blood group of potential blood donors at a collection facility. Units of blood collected from donors are blood typed and then appropriately labeled so they can be used for people that require a specific ABO group and Rh type.
• Determine the blood group of potential donors and recipients of organs, tissues, or bone marrow, as part of a workup for a transplant procedure. Along with HLA testing, ABO blood typing is used to identify and match organ and tissue transplant donors with recipients who have the same or an acceptable number of matching HLA genes and antigens.

ABO grouping and Rh typing are performed on all donated blood. They are also performed when people require blood transfusion. Conditions or situations that may warrant a transfusion include:

• Severe anemia and conditions causing anemia such as sickle cell disease and thalassemia
• Bleeding during or after surgery
• Injury or trauma
• Excessive blood loss
• Cancer and the effects of chemotherapy
• Bleeding disorders such as hemophilia

Testing is ordered when a woman becomes pregnant to determine whether she is Rh negative or positive. All newborn babies of Rh-negative mothers are typed for ABO and Rh soon after birth to determine if the mother needs to receive Rh immune globulin.

Blood typing may be ordered when a person becomes a candidate for an organ, tissue, or bone marrow transplant, or when a person wishes to become a donor. It is one of the first of many tests used when determining whether a potential donor and recipient are compatible.

Sometimes blood typing may be done as part of the process for determining whether someone could be a blood relative. For more on this, see the article The Universe of Genetic Testing: The Basics, Identity Testing, and Parentage Testing.

The results of blood typing will determine if a person is type A, B, AB, or O and if he or she is Rh negative or positive. The results will tell the healthcare provider what blood or blood components will be safe for the person to receive.

The following table shows what types of blood patients can safely receive, based on their individual blood type.

*These apply for RBC transfusions only; when transfusing plasma products and platelets, the compatible choices are different. (See the article on Blood and Components for more on this.)

Blood typing results will show whether a pregnant woman is Rh positive or negative. This information will indicate whether she may be a candidate for receiving Rh immune globulin, which would prevent her from developing antibodies against her fetus' blood cells.

Typing of donated blood is important because this information allows health practitioners to determine which patients are compatible and can safely receive that blood.

Similarly, when a donor organ, tissue, or bone marrow is compatible with the intended recipient, it is less likely to be rejected in the immediate post-transplant period.

The following table summarizes the approximate distribution of blood groups and types in the U.S. population:

Although Rh incompatibility has more severe consequences, one of the most common causes of hemolytic disease of the fetus and newborn (HDFN) is actually an incompatibility between the mother's and baby's ABO blood groups, not the Rh factor. However, ABO grouping cannot be used to predict whether HDFN will occur because antibodies to the ABO blood groups are naturally occurring.

Besides A and B, many other antigens exist. Having a rare blood type is especially problematic if you need repeated transfusions, as sickle cell anemia and thalassemia patients do. If blood transfusions are not closely matched to blood types of patients, they may suffer transfusion reactions. Such reactions are less likely if donors and recipients are from the same racial or ethnic groups.